GO Term	Evidence	Action	Reason
GO:0015979 photosynthesis	IEA GO_REF:0000043	REMOVE	Summary: Keyword-derived (SPKW, GO_REF:0000043) annotation that existed in the Sept 2025 GOA snapshot and was removed from the current GOA release. It originates solely from the UniProt keyword "Photosynthesis", which is applied across the whole PEPC family because the family is famous for its C4/CAM photosynthetic CO2-fixation role. PPC16/Q02909 is the housekeeping (anaplerotic) PEPC isozyme of soybean, a C3 plant; it has no role in photosynthesis. Reason: GOA's removal of this annotation was JUSTIFIED; it was a family-level, pathway-context over-annotation, not a correct annotation that was lost. GO:0015979 (photosynthesis) is defined as light-driven synthesis of organic compounds from CO2. Soybean is a C3 plant, and the original cloning paper states this protein is a "C3-type" PEPC that does not resemble C4/CAM photosynthetic isoforms; it is the ubiquitously expressed housekeeping isozyme. The light-driven CO2-fixing role of PEPC is restricted to the dedicated C4/CAM isozymes, which evolved separately from non-photosynthetic C3 progenitors. The housekeeping C3 PEPC performs anaplerotic carboxylation, not photosynthesis. Keeping retired: true; this is correctly removed. Supporting Evidence: PMID:1450389 The soybean encoded protein tends to resemble other 'C3-type' PEPC proteins more closely than those implicated in C4 or crassulacean acid metabolism. PMID:1450389 A full-length cDNA encoding a subunit of phosphoenolpyruvate carboxylase (PEPC) was isolated from a developing seed expression library of the C3 plant Glycine max. PMID:21524275 The critical role of PEPC in assimilating atmospheric CO(2) during C(4) and Crassulacean acid metabolism photosynthesis has been studied extensively. PEPC also fulfils a broad spectrum of non-photosynthetic functions, particularly the anaplerotic replenishment of tricarboxylic acid cycle intermediates consumed during biosynthesis and nitrogen assimilation. PMID:1450389 The corresponding mRNA is present at similar levels in leaf, stem, root and developing seed.
GO:0005829 cytosol	IBA GO_REF:0000033	ACCEPT	Summary: IBA annotation from the PEPC phylogenetic group (PANTHER PTHR30523). Plant-type PEPCs are cytosolic Class-1 enzymes, and UniProt records the subcellular location of Q02909 as Cytoplasm. The cytosolic localization is a conserved, well-established property of this enzyme class. Reason: Correct and consistent with UniProt's curated subcellular location (Cytoplasm) and with the established biology of plant-type PEPCs as cytosolic homotetramers. IBA is appropriate and the term is at the right level of specificity. Supporting Evidence: PMID:21524275 PTPC genes encode ~110-kDa polypeptides containing conserved serine-phosphorylation and lysine-mono-ubiquitination sites, and typically exist as homotetrameric Class-1 PEPCs.
GO:0005737 cytoplasm	IEA GO_REF:0000044	ACCEPT	Summary: IEA annotation derived from the UniProtKB subcellular-location vocabulary mapping (UniProt records "Cytoplasm" for Q02909). This is correct but less specific than the cytosol annotation. Reason: Consistent with the UniProt subcellular location and with the IBA cytosol annotation. The term is broader than cytosol (GO:0005829) but not incorrect; an IEA being broader than an experimentally/phylogenetically supported finer term is acceptable. Supporting Evidence: PMID:21524275 PTPC genes encode ~110-kDa polypeptides containing conserved serine-phosphorylation and lysine-mono-ubiquitination sites, and typically exist as homotetrameric Class-1 PEPCs.
GO:0006099 tricarboxylic acid cycle	IEA GO_REF:0000002	MODIFY	Summary: InterPro2GO annotation mapping the PEPC InterPro signatures (IPR021135, IPR022805) to the tricarboxylic acid cycle. PEPC is not one of the eight enzymes of the TCA cycle; it is an anaplerotic enzyme that supplies oxaloacetate to the cycle from PEP. The mapping conflates anaplerosis with cycle membership. Reason: The TCA cycle (GO:0006099) is defined as the cyclic oxidation of acetyl-CoA via citrate, isocitrate, 2-oxoglutarate, succinyl-CoA, succinate, fumarate, malate and oxaloacetate. PEPC catalyses none of these steps; it is the canonical ANAPLEROTIC enzyme that replenishes TCA-cycle C4 intermediates consumed by biosynthesis and N assimilation. Annotating PEPC as involved_in the TCA cycle is too coarse and asserts cycle membership it does not have. A biologically accurate replacement is oxaloacetate metabolic process (GO:0006107), which captures PEPC's role in producing the OAA that feeds the cycle without asserting that the protein is part of the cycle. Proposed replacements: oxaloacetate metabolic process Supporting Evidence: PMID:21524275 PEPC also fulfils a broad spectrum of non-photosynthetic functions, particularly the anaplerotic replenishment of tricarboxylic acid cycle intermediates consumed during biosynthesis and nitrogen assimilation. PMID:1450389 Through the carboxylation of phosphoenolpyruvate (PEP) it forms oxaloacetate, a four-carbon dicarboxylic acid source for the tricarboxylic acid cycle. [paraphrasing the UniProt FUNCTION text drawn from this entry's primary reference]
GO:0008964 phosphoenolpyruvate carboxylase activity	IEA GO_REF:0000120	ACCEPT	Summary: IEA annotation (combined automated methods; ARBA/InterPro/RHEA/EC) for the catalytic activity of the enzyme. This directly matches the UniProt CATALYTIC ACTIVITY entry (oxaloacetate + phosphate = phosphoenolpyruvate + hydrogencarbonate; RHEA:28370; EC 4.1.1.31) and is the core molecular function of the protein. Reason: This is the defining, core molecular function of PPC16. The annotation matches the curated catalytic activity, the EC number, the Rhea reaction, and the PEPCase type 1 family assignment in UniProt. The active-site residues His-172 and Lys-602 are annotated in UniProt, consistent with canonical PEPC catalysis. Supporting Evidence: PMID:21524275 PEPC [PEP (phosphoenolpyruvate) carboxylase] is a tightly controlled enzyme located at the core of plant C-metabolism that catalyses the irreversible beta-carboxylation of PEP to form oxaloacetate and Pi.
GO:0015977 carbon fixation	IEA GO_REF:0000002	MODIFY	Summary: InterPro2GO annotation (IPR021135, IPR022805) and UniProt keyword "Carbon dioxide fixation" mapping the PEPC family to carbon fixation. GO:0015977 is an autotrophic concept: all of its subtypes are autotrophic CO2-fixation pathways (C4 photosynthesis, CAM photosynthesis, reductive TCA cycle, acetyl-CoA pathway, 3-hydroxypropionate cycle). The housekeeping C3 PEPC does not perform autotrophic carbon fixation. Reason: GO:0015977 (carbon fixation) is defined as a metabolic process in which carbon (usually from CO2) is incorporated into organic compounds, usually carbohydrates; every child term is an autotrophic, net-carbon-gain pathway. Housekeeping PEPC carboxylates PEP to oxaloacetate for anaplerosis; it does not produce carbohydrate and does not perform autotrophic carbon fixation. As with the retired photosynthesis annotation, this is a family-level keyword/InterPro over-annotation: the carbon-fixing role of PEPC belongs to the dedicated C4/CAM photosynthetic isozymes, not to the C3 housekeeping isozyme. The biologically accurate process term is oxaloacetate metabolic process (GO:0006107), reflecting the anaplerotic carboxylation that produces OAA. Proposed replacements: oxaloacetate metabolic process Supporting Evidence: PMID:1450389 The soybean encoded protein tends to resemble other 'C3-type' PEPC proteins more closely than those implicated in C4 or crassulacean acid metabolism. PMID:36309625 all the genes of the C4 photosynthetic pathway are present in C3 plants, although they are involved in diverse non-photosynthetic functions. Non-photosynthetic isoforms of carbonic anhydrase (CA), phosphoenolpyruvate carboxylase (PEPC), malate dehydrogenase (MDH) ... catalyze reactions that are essential for major plant metabolism pathways, such as the tricarboxylic acid (TCA) cycle, maintenance of cellular pH, uptake of nutrients and their assimilation.

Deep Research

Falcon

(PPC16-deep-research-falcon.md)

this with annotations you find in gene/protein databases, but these can be outdated or inaccurate. Falcon Edison Scientific Literature 49 citations 2 artifacts 2026-05-21T10:36:58.822497

Edison artifact artifact-00 (text/markdown)

## Context ID: pqac-00000018 Table 1 provides a comprehensive list of soybean GmPEPC genes (GmPEPC1 to GmPEPC10), including their gene model names (IDs) and pre (image/png)

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Research report: Soybean PPC16 (UniProt Q02909) — phosphoenolpyruvate carboxylase (housekeeping/C3-type)

0) Target verification and scope (critical)

Target identity: The requested UniProt entry Q02909 is described as phosphoenolpyruvate carboxylase (PEPC), housekeeping isozyme (EC 4.1.1.31) from Glycine max (soybean) with gene name PPC16.

Literature match to PPC16/gmppc16: Primary soybean literature uses the gene name gmppc16 (also referenced as GmPEPC16 in later sources) to denote a C3/housekeeping PEPC isoform that is expressed broadly across soybean tissues (leaf/stem/root/developing seed) and resembles other C3/housekeeping PEPCs more than C4/CAM PEPCs. (sugimoto1992cdnasequenceand pages 1-2, sugimoto1992cdnasequenceand pages 4-5, sugimoto1992cdnasequenceand pages 2-4, hata1998expressionofa pages 1-2)

Important limitation: In the full texts retrieved here, an explicit database cross-reference linking gmppc16 to the UniProt accession Q02909 was not found. Therefore, the mapping to Q02909 is treated as consistent by organism + enzyme + “PPC16/gmppc16” naming, but not directly demonstrated within the retrieved papers. (sugimoto1992cdnasequenceand pages 1-2, sugimoto1992cdnasequenceand pages 2-4)

Section	Name / Topic	Evidence type	Key findings	Identifiers / notes	Supporting citations
Soybean nomenclature relevant to PPC16	gmppc16	Full-length cDNA clone	Soybean PEPC cDNA cloned from developing seed; authors concluded the encoded protein more closely resembles other C3/housekeeping PEPCs than C4/CAM-associated forms	GenBank/DDBJ/EMBL accession D10717; direct UniProt Q02909 mapping not found in retrieved texts	(sugimoto1992cdnasequenceand pages 1-2, sugimoto1992cdnasequenceand pages 2-4)
Soybean nomenclature relevant to PPC16	gmppc16	Expression analysis (Northern blot / tissue RNA)	mRNA detected at similar levels in leaf, stem, root, and developing seed, consistent with a broad constitutive/housekeeping role	Broad tissue expression reported; no Glyma model given in these primary papers; direct UniProt Q02909 mapping not found in retrieved texts	(sugimoto1992cdnasequenceand pages 1-2, sugimoto1992cdnasequenceand pages 4-5)
Soybean nomenclature relevant to PPC16	gmppc16 / GmPEPC16	Comparative sequence similarity	Nodule-enhanced GmPEPC7 is highly similar to gmppc16 (92.5% aa identity over 967 aa; 86.6% nucleotide identity over 2901 bp), supporting gmppc16 as a closely related non-nodule/housekeeping isoform	No direct UniProt mapping in retrieved texts	(hata1998expressionofa pages 1-2)
Soybean nomenclature relevant to PPC16	GmPEPC16	Expression survey in soybean C3 plants	Reported as broadly but low-level expressed across tissues examined, distinct from nodule-enhanced GmPEPC7	No accession given in excerpt; direct UniProt Q02909 mapping not found in retrieved texts	(sullivan2004identificationandexpression pages 84-87)
Soybean nomenclature relevant to PPC16	Gmppc16	Later soybean PEPC family literature	Later studies explicitly list Gmppc16 among soybean PEPC isogenes analyzed in seed-focused phylogenetic/expression work, confirming continued use of this nomenclature	Presence of Gmppc16 recognized, but no direct UniProt mapping shown	(yamamoto2020theplanttypephosphoenolpyruvate pages 1-3, yamamoto2020rapidlyevolvingphosphoenolpyruvate pages 3-4)
Soybean nomenclature relevant to PPC16	Soybean PEPC family context	Genome-wide family analysis	Soybean contains 10 PEPC genes classified into plant-type and bacterial-type subfamilies; PTPCs are the typical ~100–110 kDa conserved plant PEPCs predicted mainly cytosolic	Table reports GmPEPC1–10 and predicted localization; does not directly map PPC16 to a Glyma ID or UniProt Q02909	(wang2016genomewideanalysisof pages 9-10, wang2016genomewideanalysisof pages 4-5, wang2016genomewideanalysisof pages 1-2, wang2016genomewideanalysisof media d68602a1)
PEPC reaction/regulation summary	Catalytic reaction	Biochemical mechanism	Plant PEPC catalyzes PEP + HCO3− → oxaloacetate + Pi; reaction is described as highly exergonic and effectively irreversible in vivo	Requires divalent cation, especially Mg2+	(shi2015phosphoenolpyruvatecarboxylasein pages 1-6, sullivan2004identificationandexpression pages 26-29, aldous2014evolutionofthe pages 1-2)
PEPC reaction/regulation summary	Core metabolic role	Functional annotation	In C3/nonphotosynthetic tissues, PEPC serves an anaplerotic role, replenishing TCA intermediates, supporting malate production, carbon–nitrogen balance, and legume nodule metabolism	Cytosolic plant enzyme; relevant to nitrate/ammonium assimilation and organic acid supply	(sullivan2004identificationandexpression pages 29-33, shi2015phosphoenolpyruvatecarboxylasein pages 1-6, sullivan2004identificationandexpression pages 23-26, aldous2014evolutionofthe pages 2-3)
PEPC reaction/regulation summary	Allosteric regulation	Enzyme regulation	Activated by glucose-6-phosphate and other hexose/triose phosphates; inhibited by malate, aspartate, glutamate (and OAA in some contexts)	Phosphorylation shifts effector sensitivity	(sullivan2004identificationandexpression pages 29-33, shi2015phosphoenolpyruvatecarboxylasein pages 1-6, sullivan2004identificationandexpression pages 33-36, aldous2014evolutionofthe pages 2-3)
PEPC reaction/regulation summary	Phosphorylation by PPCK	Post-translational regulation	Reversible phosphorylation of a conserved N-terminal serine by PEPC kinase (PPCK) decreases malate inhibition and increases activation by Glc-6-P; example values reported for C3 PEPC include Ki malate ~0.17 mM (dephosphorylated) vs ~1.2 mM (phosphorylated) and Ka Glc-6-P ~1.3 mM vs ~0.28 mM	Supports dynamic regulation by light, nitrogen, and metabolic status	(sullivan2004identificationandexpression pages 29-33, sullivan2004identificationandexpression pages 33-36, aldous2014evolutionofthe pages 2-3, aldous2014evolutionofthe pages 1-2)

Table: This table summarizes how soybean PPC16/gmppc16 is described in the literature and what evidence supports its annotation as a housekeeping/C3-type phosphoenolpyruvate carboxylase. It also condenses the core catalytic reaction and major regulatory features of plant-type PEPC needed for functional annotation.

1) Key concepts and definitions (current understanding)

1.1 Enzyme class and reaction (primary function)

Plant phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) catalyzes the β-carboxylation of phosphoenolpyruvate (PEP) using bicarbonate:

PEP + HCO3− → oxaloacetate (OAA) + Pi

This reaction is described as highly exergonic (ΔG° ≈ −30 kJ/mol) and effectively irreversible under physiological conditions. (sullivan2004identificationandexpression pages 26-29, shi2015phosphoenolpyruvatecarboxylasein pages 1-6)

Cofactors/requirements: activity requires a divalent metal, with Mg2+ most effective (Mn2+/Co2+ can substitute; Ca2+ inhibitory). (sullivan2004identificationandexpression pages 26-29)

1.2 “Housekeeping” (C3-type) PEPC vs other PEPC types

In higher plants, multiple PEPC isoforms exist. The “housekeeping” (C3-type, non-photosynthetic) PEPCs function mainly in anaplerosis—replenishing tricarboxylic acid (TCA) cycle intermediates—and in coordinating carbon–nitrogen (C/N) metabolism. (shi2015phosphoenolpyruvatecarboxylasein pages 1-6, aldous2014evolutionofthe pages 2-3, sullivan2004identificationandexpression pages 33-36)

Soybean contains a small multigene PEPC family; genome-wide analysis identified 10 PEPC genes in soybean (GmPEPC1–GmPEPC10) belonging to plant-type and bacterial-type PEPC subfamilies. (wang2016genomewideanalysisof pages 2-4, wang2016genomewideanalysisof pages 9-10, wang2016genomewideanalysisof pages 1-2)

1.3 Subcellular localization (where the reaction occurs)

Housekeeping/C3 PEPC is widely treated as a plant cytosolic enzyme; this is explicitly stated in soybean nodule context (“PEPCs in nodules are generally thought to be plant cytosolic enzymes”). (hata1998expressionofa pages 1-2)

A soybean genome-wide analysis also predicts predominantly cytosolic localization for soybean PEPC family members (Table 1). (wang2016genomewideanalysisof pages 1-2, wang2016genomewideanalysisof media d68602a1)

1.4 Regulation: allostery and phosphorylation

PEPC integrates metabolic state through:

Allosteric regulation
- Activated by glucose-6-phosphate (G6P) and other sugar phosphates (e.g., hexose/triose phosphates). (shi2015phosphoenolpyruvatecarboxylasein pages 1-6, sullivan2004identificationandexpression pages 33-36)
- Inhibited by malate and amino acids such as aspartate and glutamate (and other feedback inhibitors reported). (shi2015phosphoenolpyruvatecarboxylasein pages 1-6, sullivan2004identificationandexpression pages 33-36, sullivan2004identificationandexpression pages 36-39)

Post-translational modification: phosphorylation
- A conserved N-terminal Ser is reversibly phosphorylated by PEPC kinase (PPCK), which decreases malate inhibition and increases activation by G6P. (aldous2014evolutionofthe pages 2-3, sullivan2004identificationandexpression pages 29-33, shi2015phosphoenolpyruvatecarboxylasein pages 1-6)
- Example quantitative effect: at pH ~7.3, Ki(L-malate) ~0.17 mM (dephosphorylated) vs ~1.2 mM (phosphorylated), and Ka(G6P) ~1.3 mM vs ~0.28 mM after phosphorylation. (sullivan2004identificationandexpression pages 29-33)

2) Soybean PPC16/gmppc16: gene-level functional annotation

2.1 Sequence identity and classification as housekeeping/C3-type

The soybean gene gmppc16 was cloned as a full-length PEPC cDNA from a developing-seed library. The authors conclude the encoded PEPC resembles other “C3-type/housekeeping” PEPCs more than C4/CAM PEPCs. (Sugimoto et al., 1992-11, Plant Molecular Biology; URL: https://doi.org/10.1007/bf00046459) (sugimoto1992cdnasequenceand pages 1-2, sugimoto1992cdnasequenceand pages 2-4)

A soybean nodule-enhanced PEPC gene (GmPEPC7) was reported to be highly similar to gmppc16 (92.5% amino-acid identity over 967 aa; 86.6% nucleotide identity over 2901 bp), consistent with gmppc16 being a close paralog representing a broadly expressed/housekeeping lineage. (Hata et al., 1998-01, The Plant Journal; URL: https://doi.org/10.1046/j.1365-313x.1998.00022.x) (hata1998expressionofa pages 1-2)

2.2 Expression pattern (tissues and developmental context)

Broad tissue expression: Northern blotting detected gmppc16 mRNA in leaf, stem, root, and developing seed, with similar levels across tissues, consistent with a housekeeping role rather than strong tissue specialization. (Sugimoto et al., 1992-11; https://doi.org/10.1007/bf00046459) (sugimoto1992cdnasequenceand pages 4-5, sugimoto1992cdnasequenceand pages 2-4)

Seed development: A seed-focused legume metabolism paper notes that in soybean, gmppc16 (and gmppc1) are expressed in seeds, and gmppc16 is also expressed in vegetative organs (leaf/stem/root). This supports a model in which gmppc16 contributes to anaplerotic carbon flow needed for amino-acid biosynthesis during seed filling. (Golombek et al., 1999-03, Planta; https://doi.org/10.1007/s004250050535) (golombek1999phosphoenolpyruvatecarboxylasein pages 1-2)

A later soybean RNA-seq/proteomics-oriented paper analyzing PEPC isogenes reports that Gmppc16 corresponds to Glyma.12g161300 and shows peak expression at the late maturation stage in seed development (with relative expression described vs other isogenes). (Yamamoto et al., 2020-03, Bioscience, Biotechnology, and Biochemistry; https://doi.org/10.1080/09168451.2019.1696179) (yamamoto2020theplanttypephosphoenolpyruvate pages 4-6)

2.3 Biochemical pathway placement in soybean (inferred + supported context)

Given the reaction (PEP → OAA) and its cytosolic localization, PPC16/gmppc16 most plausibly feeds into:
- TCA anaplerosis (OAA replenishment) and provision of carbon skeletons for amino-acid biosynthesis (aspartate family amino acids), consistent with “C3/housekeeping” PEPC roles. (shi2015phosphoenolpyruvatecarboxylasein pages 1-6, sullivan2004identificationandexpression pages 33-36, sullivan2004identificationandexpression pages 23-26)
- Seed nitrogen assimilation and storage-protein accumulation: PEPC activity in soybean has been reported to correlate with seed protein concentration (cited in the legume seed metabolism literature), supporting a role in balancing carbon supply with imported nitrogen during seed development. (golombek1999phosphoenolpyruvatecarboxylasein pages 1-2)

3) Soybean PEPC family context (helps interpret PPC16)

A genome-wide soybean PEPC analysis identified 10 PEPC genes (GmPEPC1–GmPEPC10) classified into 7 plant-type PEPCs (PTPC) and 3 bacterial-type PEPCs (BTPC), and provides predicted subcellular localization (mostly cytosolic). (Wang et al., 2016-12, Scientific Reports; https://doi.org/10.1038/srep38448) (wang2016genomewideanalysisof pages 2-4, wang2016genomewideanalysisof pages 9-10, wang2016genomewideanalysisof pages 1-2, wang2016genomewideanalysisof media d68602a1)

Visual evidence (Table): The following table image (Table 1) lists soybean GmPEPC gene models and predicted localization. (wang2016genomewideanalysisof media d68602a1)

Stress response (family-level, not PPC16-specific): In controlled seedling assays, certain GmPEPC paralogs (notably GmPEPC6/8/9) are induced by Al toxicity, cold, salt, and osmotic stress, and PEPC enzymatic activity increases in leaves under salt/PEG and in roots under Al/cold. (Wang et al., 2016-12; https://doi.org/10.1038/srep38448) (wang2016genomewideanalysisof pages 9-10, wang2016genomewideanalysisof pages 7-9, wang2016genomewideanalysisof pages 10-11)

4) Recent developments (prioritizing 2023–2024)

Direct 2023–2024 studies specifically on soybean PPC16 were not retrieved here; recent advances most relevant to PPC16 are cross-plant mechanistic/engineering and systems-biology developments that sharpen interpretation of “housekeeping PEPC” roles.

4.1 2024: PEPC-centered engineering to improve drought tolerance/photosynthetic performance

A 2024 primary study overexpressing C4-type PEPC (SaPEPC1) in Arabidopsis (C3) reports improved drought tolerance and photosynthetic electron transport performance, supporting the idea that tuning PEPC capacity/regulation can modify stress resilience. Quantitatively, under intense light, SaPEPC1 transgenic lines reached rETR maxima of 56.38 and 46.25, while an AtPEPC1 overexpression line had a lower maximum rETR of 27.48; osmotic stress assays used 200–300 mM mannitol and showed improved stomatal closure/oxidative stress markers in SaPEPC1 lines. (Li et al., 2024-08, Frontiers in Plant Science; https://doi.org/10.3389/fpls.2024.1443691) (li2024enhanceddroughttolerance pages 5-6, li2024enhanceddroughttolerance pages 1-2)

4.2 2024: Expert reviews on installing C4/CAM components (including PEPC) into C3 crops

A 2024 review of photosynthesis engineering highlights introduction of C4-specific enzymes such as PEPC into C3 plants as a strategy to increase CO2 concentration around Rubisco; it summarizes transgenic examples (e.g., rice overexpressing maize PEPC showing improved photosynthesis/biomass under some conditions) while emphasizing that benefits are often modest and require careful multi-gene/regulatory tuning to avoid metabolic imbalance. (Nazari et al., 2024-08, Cells; https://doi.org/10.3390/cells13161319) (nazari2024enhancingphotosynthesisand pages 12-14)

A second 2024 review on C4 induction feasibility catalogs engineering approaches including high-level expression of maize PEPC in rice and multi-enzyme stacking strategies, framing PEPC as a central biochemical component but not sufficient alone for full “C4 syndrome.” (Mukundan et al., 2024-06, Plant Biotechnology Reports; https://doi.org/10.1007/s11816-024-00908-2) (mukundan2024investigatingphotosyntheticevolution pages 12-13, mukundan2024investigatingphotosyntheticevolution pages 7-9)

4.3 2023: Systems biology and metabolic modeling of PEPC-driven CO2 recycling under drought

A 2023 genome-scale metabolic model of cassava leaves proposes that PEPC enables CO2 recycling during stomatal closure by fixing inorganic carbon into OAA and releasing CO2 via decarboxylation (including PEPCK), sustaining RuBisCO fixation when external CO2 diffusion is limited. Quantitatively, under simulated 50% reduced CO2 uptake, PEPC flux was ~6× higher and RuBisCO fixation retained at ~66% of normal; 10× PEPC perturbation increased the intracellular CO2 pool by ~1.88% and shifted carbon allocation, with biomass production rates reduced up to 79% in simulations. (Punyasu et al., 2023-05, Frontiers in Plant Science; https://doi.org/10.3389/fpls.2023.1159247) (punyasu2023co2recyclingby pages 4-6, punyasu2023co2recyclingby pages 6-8, punyasu2023co2recyclingby pages 10-12)

5) Current applications and real-world implementations

5.1 Crop improvement concepts using PEPC

Recent reviews treat PEPC as a candidate engineering target for improving photosynthetic efficiency in C3 crops by introducing C4/CAM-like features, including documented transgenic efforts (e.g., rice expressing maize PEPC) and the broader need for multi-component engineering (enzyme stacking, regulatory networks, possibly anatomy) to achieve robust yield benefits. (nazari2024enhancingphotosynthesisand pages 12-14, mukundan2024investigatingphotosyntheticevolution pages 7-9)

5.2 Practical soybean relevance

For soybean, PPC16/gmppc16 is best interpreted as a core central-metabolism enzyme supporting:
- C/N balance and provision of OAA/aspartate for amino-acid synthesis across multiple tissues (housekeeping expression). (sugimoto1992cdnasequenceand pages 4-5, sullivan2004identificationandexpression pages 33-36, sullivan2004identificationandexpression pages 23-26)
- Seed filling metabolism, where anaplerotic flux supports assimilation of imported nitrogen (amides) into amino acids and storage proteins; soybean PEPC activity has been linked to seed protein concentration in the seed-metabolism literature. (golombek1999phosphoenolpyruvatecarboxylasein pages 1-2)

6) Expert opinion / authoritative analysis (interpreting PPC16)

Across authoritative mechanistic and review sources, a consensus view is that C3/housekeeping PEPC is a major metabolic “valve” connecting glycolytic carbon (PEP) to organic-acid pools (OAA/malate/aspartate), enabling:
- Anaplerotic replenishment of TCA intermediates and carbon skeleton supply for nitrogen assimilation. (shi2015phosphoenolpyruvatecarboxylasein pages 1-6, sullivan2004identificationandexpression pages 33-36, aldous2014evolutionofthe pages 2-3)
- Regulatory integration via strong allosteric control (malate inhibition; G6P activation) and reversible phosphorylation by PPCK to tune flux in response to light/nutrient/metabolic cues. (sullivan2004identificationandexpression pages 29-33, aldous2014evolutionofthe pages 2-3)

Soybean-specific primary data support that gmppc16 fits precisely into this “housekeeping/anaplerotic” category via broad expression and C3-type sequence similarity. (sugimoto1992cdnasequenceand pages 4-5, hata1998expressionofa pages 1-2)

7) Key statistics and data (selected)

Reaction thermodynamics: ΔG° ≈ −30 kJ/mol; reaction described as strictly irreversible. (sullivan2004identificationandexpression pages 26-29)
Allostery + phosphorylation (example parameters): Ki malate 0.17 mM (dephosphorylated) vs 1.2 mM (phosphorylated); Ka(G6P) 1.3 mM vs 0.28 mM after phosphorylation. (sullivan2004identificationandexpression pages 29-33)
Soybean gene family size: 10 PEPC genes (7 PTPC, 3 BTPC). (wang2016genomewideanalysisof pages 2-4, wang2016genomewideanalysisof pages 9-10)
Soybean PPC16 identifier in a later seed study: Gmppc16 = Glyma.12g161300 (reported in 2020 seed-development analysis). (yamamoto2020theplanttypephosphoenolpyruvate pages 4-6)
Drought CO2-recycling modeling (cassava, 2023): at 50% CO2 uptake, PEPC flux ~6× and RuBisCO fixation ~66% retained; 10× PEPC perturbation increased intracellular CO2 ~1.88% and reduced biomass flux up to 79%. (punyasu2023co2recyclingby pages 4-6, punyasu2023co2recyclingby pages 6-8, punyasu2023co2recyclingby pages 10-12)
Transgenic PEPC engineering (Arabidopsis, 2024): SaPEPC1 lines under intense light had rETR maxima 56.38 and 46.25; an AtPEPC1 line had max rETR 27.48; mannitol stress 200–300 mM used in germination/osmotic assays. (li2024enhanceddroughttolerance pages 5-6)

8) Practical functional annotation summary for PPC16/Q02909 (soybean)

Molecular function: cytosolic phosphoenolpyruvate carboxylase catalyzing PEP + HCO3− → OAA + Pi (irreversible), providing OAA for conversion to malate/aspartate and supporting anaplerosis and amino-acid biosynthesis. (shi2015phosphoenolpyruvatecarboxylasein pages 1-6, sullivan2004identificationandexpression pages 26-29)

Biological processes (most supported):
- Housekeeping central carbon metabolism / anaplerotic replenishment of TCA intermediates. (shi2015phosphoenolpyruvatecarboxylasein pages 1-6, sullivan2004identificationandexpression pages 33-36)
- Carbon–nitrogen coordination, including provision of carbon skeletons for N assimilation (aspartate/asparagine pathways), with particular relevance to legumes and seed filling. (sullivan2004identificationandexpression pages 23-26, golombek1999phosphoenolpyruvatecarboxylasein pages 1-2)

Expression/localization:
- Broad tissue expression consistent with housekeeping role (leaf, stem, root, developing seed). (sugimoto1992cdnasequenceand pages 4-5, sugimoto1992cdnasequenceand pages 2-4)
- Cytosolic localization is the dominant plant PEPC localization in relevant contexts and is predicted for soybean PEPC family members in genome-wide analysis. (hata1998expressionofa pages 1-2, wang2016genomewideanalysisof media d68602a1)

Regulation: likely regulated by malate inhibition, G6P activation, and PPCK-mediated phosphorylation (conserved N-terminal Ser motif discussed for plant PEPCs; soybean gmppc16 contains relevant N-terminal phosphorylation motif context in early sequence analyses). (sullivan2004identificationandexpression pages 29-33, sugimoto1992cdnasequenceand pages 4-5, aldous2014evolutionofthe pages 2-3)

9) URLs and publication dates (most relevant sources used)

Sugimoto et al. Plant Molecular Biology (1992-11). “cDNA sequence and expression of a phosphoenolpyruvate carboxylase gene from soybean.” https://doi.org/10.1007/bf00046459 (sugimoto1992cdnasequenceand pages 1-2, sugimoto1992cdnasequenceand pages 4-5, sugimoto1992cdnasequenceand pages 2-4)
Hata et al. The Plant Journal (1998-01). “Expression of a soybean nodule-enhanced phosphoenolpyruvate carboxylase gene that shows striking similarity to another gene for a house-keeping isoform.” https://doi.org/10.1046/j.1365-313x.1998.00022.x (hata1998expressionofa pages 1-2)
Golombek et al. Planta (1999-03). “Phosphoenolpyruvate carboxylase in developing seeds of Vicia faba…” (cites soybean gmppc16 expression and seed roles). https://doi.org/10.1007/s004250050535 (golombek1999phosphoenolpyruvatecarboxylasein pages 1-2)
Wang et al. Scientific Reports (2016-12). “Genome-wide Analysis of Phosphoenolpyruvate Carboxylase Gene Family and Their Response to Abiotic Stresses in Soybean.” https://doi.org/10.1038/srep38448 (wang2016genomewideanalysisof pages 2-4, wang2016genomewideanalysisof pages 9-10, wang2016genomewideanalysisof pages 1-2, wang2016genomewideanalysisof media d68602a1)
Yamamoto et al. Bioscience, Biotechnology, and Biochemistry (2020-03). “The plant-type PEPC Gmppc2…” (reports Gmppc16 = Glyma.12g161300 and seed-stage expression). https://doi.org/10.1080/09168451.2019.1696179 (yamamoto2020theplanttypephosphoenolpyruvate pages 4-6)
Punyasu et al. Frontiers in Plant Science (2023-05). “CO2 recycling by PEPC enables cassava leaf metabolism to tolerate low water availability.” https://doi.org/10.3389/fpls.2023.1159247 (punyasu2023co2recyclingby pages 4-6, punyasu2023co2recyclingby pages 6-8, punyasu2023co2recyclingby pages 10-12)
Mukundan et al. Plant Biotechnology Reports (2024-06). “Investigating photosynthetic evolution and the feasibility of inducing C4 syndrome in C3 plants.” https://doi.org/10.1007/s11816-024-00908-2 (mukundan2024investigatingphotosyntheticevolution pages 12-13, mukundan2024investigatingphotosyntheticevolution pages 7-9)
Nazari et al. Cells (2024-08). “Enhancing Photosynthesis and Plant Productivity through Genetic Modification.” https://doi.org/10.3390/cells13161319 (nazari2024enhancingphotosynthesisand pages 12-14)
Li et al. Frontiers in Plant Science (2024-08). “Enhanced drought tolerance and photosynthetic efficiency in Arabidopsis by overexpressing PEPC…” https://doi.org/10.3389/fpls.2024.1443691 (li2024enhanceddroughttolerance pages 5-6, li2024enhanceddroughttolerance pages 1-2)

References

(sugimoto1992cdnasequenceand pages 1-2): Toshio Sugimoto, Tsutomu Kawasaki, Tomohiko Kato, Robert F. Whittier, Daisuke Shibata, and Yukio Kawamura. Cdna sequence and expression of a phosphoenolpyruvate carboxylase gene from soybean. Plant Molecular Biology, 20:743-747, Nov 1992. URL: https://doi.org/10.1007/bf00046459, doi:10.1007/bf00046459. This article has 62 citations and is from a peer-reviewed journal.
(sugimoto1992cdnasequenceand pages 4-5): Toshio Sugimoto, Tsutomu Kawasaki, Tomohiko Kato, Robert F. Whittier, Daisuke Shibata, and Yukio Kawamura. Cdna sequence and expression of a phosphoenolpyruvate carboxylase gene from soybean. Plant Molecular Biology, 20:743-747, Nov 1992. URL: https://doi.org/10.1007/bf00046459, doi:10.1007/bf00046459. This article has 62 citations and is from a peer-reviewed journal.
(sugimoto1992cdnasequenceand pages 2-4): Toshio Sugimoto, Tsutomu Kawasaki, Tomohiko Kato, Robert F. Whittier, Daisuke Shibata, and Yukio Kawamura. Cdna sequence and expression of a phosphoenolpyruvate carboxylase gene from soybean. Plant Molecular Biology, 20:743-747, Nov 1992. URL: https://doi.org/10.1007/bf00046459, doi:10.1007/bf00046459. This article has 62 citations and is from a peer-reviewed journal.
(hata1998expressionofa pages 1-2): Shingo Hata, Katsura Izui, and Hiroshi Kouchi. Expression of a soybean nodule-enhanced phosphoenolpyruvate carboxylase gene that shows striking similarity to another gene for a house-keeping isoform. The Plant journal : for cell and molecular biology, 13 2:267-73, Jan 1998. URL: https://doi.org/10.1046/j.1365-313x.1998.00022.x, doi:10.1046/j.1365-313x.1998.00022.x. This article has 49 citations.
(sullivan2004identificationandexpression pages 84-87): JS Sullivan. Identification and expression analysis of phosphoenolpyruvate carboxylase (pepc) and pepc kinase genes in c3 plants. Unknown journal, 2004.
(yamamoto2020theplanttypephosphoenolpyruvate pages 1-3): Naoki Yamamoto, Toshio Sugimoto, Tomoyuki Takano, Ai Sasou, Shigeto Morita, Kentaro Yano, and Takehiro Masumura. The plant-type phosphoenolpyruvate carboxylase gmppc2 is developmentally induced in immature soy seeds at the late maturation stage: a potential protein biomarker for seed chemical composition. Bioscience, Biotechnology, and Biochemistry, 84:552-562, Mar 2020. URL: https://doi.org/10.1080/09168451.2019.1696179, doi:10.1080/09168451.2019.1696179. This article has 8 citations.
(yamamoto2020rapidlyevolvingphosphoenolpyruvate pages 3-4): Naoki Yamamoto, Tomoyuki Takano, Takehiro Masumura, Ai Sasou, Shigeto Morita, Toshio Sugimoto, and Kentaro Yano. Rapidly evolving phosphoenolpyruvate carboxylase gmppc1 and gmppc7 are highly expressed in the external seed coat of immature soybean seeds. Gene, 762:145015, Dec 2020. URL: https://doi.org/10.1016/j.gene.2020.145015, doi:10.1016/j.gene.2020.145015. This article has 4 citations and is from a peer-reviewed journal.
(wang2016genomewideanalysisof pages 9-10): Ning Wang, Xiujuan Zhong, Yahui Cong, Tingting Wang, Songnan Yang, Yan Li, and Junyi Gai. Genome-wide analysis of phosphoenolpyruvate carboxylase gene family and their response to abiotic stresses in soybean. Scientific Reports, Dec 2016. URL: https://doi.org/10.1038/srep38448, doi:10.1038/srep38448. This article has 55 citations and is from a peer-reviewed journal.
(wang2016genomewideanalysisof pages 4-5): Ning Wang, Xiujuan Zhong, Yahui Cong, Tingting Wang, Songnan Yang, Yan Li, and Junyi Gai. Genome-wide analysis of phosphoenolpyruvate carboxylase gene family and their response to abiotic stresses in soybean. Scientific Reports, Dec 2016. URL: https://doi.org/10.1038/srep38448, doi:10.1038/srep38448. This article has 55 citations and is from a peer-reviewed journal.
(wang2016genomewideanalysisof pages 1-2): Ning Wang, Xiujuan Zhong, Yahui Cong, Tingting Wang, Songnan Yang, Yan Li, and Junyi Gai. Genome-wide analysis of phosphoenolpyruvate carboxylase gene family and their response to abiotic stresses in soybean. Scientific Reports, Dec 2016. URL: https://doi.org/10.1038/srep38448, doi:10.1038/srep38448. This article has 55 citations and is from a peer-reviewed journal.
(wang2016genomewideanalysisof media d68602a1): Ning Wang, Xiujuan Zhong, Yahui Cong, Tingting Wang, Songnan Yang, Yan Li, and Junyi Gai. Genome-wide analysis of phosphoenolpyruvate carboxylase gene family and their response to abiotic stresses in soybean. Scientific Reports, Dec 2016. URL: https://doi.org/10.1038/srep38448, doi:10.1038/srep38448. This article has 55 citations and is from a peer-reviewed journal.
(shi2015phosphoenolpyruvatecarboxylasein pages 1-6): Jianghua Shi, Keke Yi, Yu Liu, Li Xie, Zhongjing Zhou, Yue Chen, Zhang-hua Hu, T. Zheng, Renhu Liu, Yunlong Chen, and Jinqing Chen. Phosphoenolpyruvate carboxylase in arabidopsis leaves plays a crucial role in carbon and nitrogen metabolism. Plant Physiology, 167:671-681, Jan 2015. URL: https://doi.org/10.1104/pp.114.254474, doi:10.1104/pp.114.254474. This article has 165 citations and is from a highest quality peer-reviewed journal.
(sullivan2004identificationandexpression pages 26-29): JS Sullivan. Identification and expression analysis of phosphoenolpyruvate carboxylase (pepc) and pepc kinase genes in c3 plants. Unknown journal, 2004.
(aldous2014evolutionofthe pages 1-2): Sophia H. Aldous, Sean E. Weise, Thomas D. Sharkey, Daniel M. Waldera-Lupa, Kai Stühler, Julia Mallmann, Georg Groth, Udo Gowik, Peter Westhoff, and Borjana Arsova. Evolution of the phosphoenolpyruvate carboxylase protein kinase family in c3 and c4 flaveria spp. Plant Physiology, 165(3):1076-1091, May 2014. URL: https://doi.org/10.1104/pp.114.240283, doi:10.1104/pp.114.240283. This article has 34 citations and is from a highest quality peer-reviewed journal.
(sullivan2004identificationandexpression pages 29-33): JS Sullivan. Identification and expression analysis of phosphoenolpyruvate carboxylase (pepc) and pepc kinase genes in c3 plants. Unknown journal, 2004.
(sullivan2004identificationandexpression pages 23-26): JS Sullivan. Identification and expression analysis of phosphoenolpyruvate carboxylase (pepc) and pepc kinase genes in c3 plants. Unknown journal, 2004.
(aldous2014evolutionofthe pages 2-3): Sophia H. Aldous, Sean E. Weise, Thomas D. Sharkey, Daniel M. Waldera-Lupa, Kai Stühler, Julia Mallmann, Georg Groth, Udo Gowik, Peter Westhoff, and Borjana Arsova. Evolution of the phosphoenolpyruvate carboxylase protein kinase family in c3 and c4 flaveria spp. Plant Physiology, 165(3):1076-1091, May 2014. URL: https://doi.org/10.1104/pp.114.240283, doi:10.1104/pp.114.240283. This article has 34 citations and is from a highest quality peer-reviewed journal.
(sullivan2004identificationandexpression pages 33-36): JS Sullivan. Identification and expression analysis of phosphoenolpyruvate carboxylase (pepc) and pepc kinase genes in c3 plants. Unknown journal, 2004.
(wang2016genomewideanalysisof pages 2-4): Ning Wang, Xiujuan Zhong, Yahui Cong, Tingting Wang, Songnan Yang, Yan Li, and Junyi Gai. Genome-wide analysis of phosphoenolpyruvate carboxylase gene family and their response to abiotic stresses in soybean. Scientific Reports, Dec 2016. URL: https://doi.org/10.1038/srep38448, doi:10.1038/srep38448. This article has 55 citations and is from a peer-reviewed journal.
(sullivan2004identificationandexpression pages 36-39): JS Sullivan. Identification and expression analysis of phosphoenolpyruvate carboxylase (pepc) and pepc kinase genes in c3 plants. Unknown journal, 2004.
(golombek1999phosphoenolpyruvatecarboxylasein pages 1-2): Sabine Golombek, Ute Heim, Christian Horstmann, Ulrich Wobus, and Hans Weber. Phosphoenolpyruvate carboxylase in developing seeds of vicia faba l.: gene expression and metabolic regulation. Planta, 208:66-72, Mar 1999. URL: https://doi.org/10.1007/s004250050535, doi:10.1007/s004250050535. This article has 82 citations and is from a peer-reviewed journal.
(yamamoto2020theplanttypephosphoenolpyruvate pages 4-6): Naoki Yamamoto, Toshio Sugimoto, Tomoyuki Takano, Ai Sasou, Shigeto Morita, Kentaro Yano, and Takehiro Masumura. The plant-type phosphoenolpyruvate carboxylase gmppc2 is developmentally induced in immature soy seeds at the late maturation stage: a potential protein biomarker for seed chemical composition. Bioscience, Biotechnology, and Biochemistry, 84:552-562, Mar 2020. URL: https://doi.org/10.1080/09168451.2019.1696179, doi:10.1080/09168451.2019.1696179. This article has 8 citations.
(wang2016genomewideanalysisof pages 7-9): Ning Wang, Xiujuan Zhong, Yahui Cong, Tingting Wang, Songnan Yang, Yan Li, and Junyi Gai. Genome-wide analysis of phosphoenolpyruvate carboxylase gene family and their response to abiotic stresses in soybean. Scientific Reports, Dec 2016. URL: https://doi.org/10.1038/srep38448, doi:10.1038/srep38448. This article has 55 citations and is from a peer-reviewed journal.
(wang2016genomewideanalysisof pages 10-11): Ning Wang, Xiujuan Zhong, Yahui Cong, Tingting Wang, Songnan Yang, Yan Li, and Junyi Gai. Genome-wide analysis of phosphoenolpyruvate carboxylase gene family and their response to abiotic stresses in soybean. Scientific Reports, Dec 2016. URL: https://doi.org/10.1038/srep38448, doi:10.1038/srep38448. This article has 55 citations and is from a peer-reviewed journal.
(li2024enhanceddroughttolerance pages 5-6): Caixia Li, Juan Wang, Haiyan Lan, and Qinghui Yu. Enhanced drought tolerance and photosynthetic efficiency in arabidopsis by overexpressing phosphoenolpyruvate carboxylase from a single-cell c4 halophyte suaeda aralocaspica. Frontiers in Plant Science, Aug 2024. URL: https://doi.org/10.3389/fpls.2024.1443691, doi:10.3389/fpls.2024.1443691. This article has 9 citations.
(li2024enhanceddroughttolerance pages 1-2): Caixia Li, Juan Wang, Haiyan Lan, and Qinghui Yu. Enhanced drought tolerance and photosynthetic efficiency in arabidopsis by overexpressing phosphoenolpyruvate carboxylase from a single-cell c4 halophyte suaeda aralocaspica. Frontiers in Plant Science, Aug 2024. URL: https://doi.org/10.3389/fpls.2024.1443691, doi:10.3389/fpls.2024.1443691. This article has 9 citations.
(nazari2024enhancingphotosynthesisand pages 12-14): Mansoureh Nazari, Mojtaba Kordrostami, Ali Akbar Ghasemi-Soloklui, Julian J. Eaton-Rye, Pavel Pashkovskiy, Vladimir Kuznetsov, and Suleyman I. Allakhverdiev. Enhancing photosynthesis and plant productivity through genetic modification. Aug 2024. URL: https://doi.org/10.3390/cells13161319, doi:10.3390/cells13161319. This article has 38 citations.
(mukundan2024investigatingphotosyntheticevolution pages 12-13): Nidhi S. Mukundan, Kapaettu Satyamoorthy, and Vidhu Sankar Babu. Investigating photosynthetic evolution and the feasibility of inducing c4 syndrome in c3 plants. Plant Biotechnology Reports, 18:449-463, Jun 2024. URL: https://doi.org/10.1007/s11816-024-00908-2, doi:10.1007/s11816-024-00908-2. This article has 14 citations and is from a peer-reviewed journal.
(mukundan2024investigatingphotosyntheticevolution pages 7-9): Nidhi S. Mukundan, Kapaettu Satyamoorthy, and Vidhu Sankar Babu. Investigating photosynthetic evolution and the feasibility of inducing c4 syndrome in c3 plants. Plant Biotechnology Reports, 18:449-463, Jun 2024. URL: https://doi.org/10.1007/s11816-024-00908-2, doi:10.1007/s11816-024-00908-2. This article has 14 citations and is from a peer-reviewed journal.
(punyasu2023co2recyclingby pages 4-6): Nattharat Punyasu, Saowalak Kalapanulak, and Treenut Saithong. Co2 recycling by phosphoenolpyruvate carboxylase enables cassava leaf metabolism to tolerate low water availability. Frontiers in Plant Science, May 2023. URL: https://doi.org/10.3389/fpls.2023.1159247, doi:10.3389/fpls.2023.1159247. This article has 12 citations.
(punyasu2023co2recyclingby pages 6-8): Nattharat Punyasu, Saowalak Kalapanulak, and Treenut Saithong. Co2 recycling by phosphoenolpyruvate carboxylase enables cassava leaf metabolism to tolerate low water availability. Frontiers in Plant Science, May 2023. URL: https://doi.org/10.3389/fpls.2023.1159247, doi:10.3389/fpls.2023.1159247. This article has 12 citations.
(punyasu2023co2recyclingby pages 10-12): Nattharat Punyasu, Saowalak Kalapanulak, and Treenut Saithong. Co2 recycling by phosphoenolpyruvate carboxylase enables cassava leaf metabolism to tolerate low water availability. Frontiers in Plant Science, May 2023. URL: https://doi.org/10.3389/fpls.2023.1159247, doi:10.3389/fpls.2023.1159247. This article has 12 citations.

Artifacts

Edison artifact artifact-00

Citations

hata1998expressionofa pages 1-2
sullivan2004identificationandexpression pages 84-87
sullivan2004identificationandexpression pages 26-29
sullivan2004identificationandexpression pages 29-33
golombek1999phosphoenolpyruvatecarboxylasein pages 1-2
yamamoto2020theplanttypephosphoenolpyruvate pages 4-6
nazari2024enhancingphotosynthesisand pages 12-14
li2024enhanceddroughttolerance pages 5-6
sugimoto1992cdnasequenceand pages 1-2
sugimoto1992cdnasequenceand pages 4-5
sugimoto1992cdnasequenceand pages 2-4
yamamoto2020theplanttypephosphoenolpyruvate pages 1-3
yamamoto2020rapidlyevolvingphosphoenolpyruvate pages 3-4
wang2016genomewideanalysisof pages 9-10
wang2016genomewideanalysisof pages 4-5
wang2016genomewideanalysisof pages 1-2
shi2015phosphoenolpyruvatecarboxylasein pages 1-6
aldous2014evolutionofthe pages 1-2
sullivan2004identificationandexpression pages 23-26
aldous2014evolutionofthe pages 2-3
sullivan2004identificationandexpression pages 33-36
wang2016genomewideanalysisof pages 2-4
sullivan2004identificationandexpression pages 36-39
wang2016genomewideanalysisof pages 7-9
wang2016genomewideanalysisof pages 10-11
li2024enhanceddroughttolerance pages 1-2
mukundan2024investigatingphotosyntheticevolution pages 12-13
mukundan2024investigatingphotosyntheticevolution pages 7-9
https://doi.org/10.1007/bf00046459
https://doi.org/10.1046/j.1365-313x.1998.00022.x
https://doi.org/10.1007/s004250050535
https://doi.org/10.1080/09168451.2019.1696179
https://doi.org/10.1038/srep38448
https://doi.org/10.3389/fpls.2024.1443691
https://doi.org/10.3390/cells13161319
https://doi.org/10.1007/s11816-024-00908-2
https://doi.org/10.3389/fpls.2023.1159247
https://doi.org/10.1007/bf00046459,
https://doi.org/10.1046/j.1365-313x.1998.00022.x,
https://doi.org/10.1080/09168451.2019.1696179,
https://doi.org/10.1016/j.gene.2020.145015,
https://doi.org/10.1038/srep38448,
https://doi.org/10.1104/pp.114.254474,
https://doi.org/10.1104/pp.114.240283,
https://doi.org/10.1007/s004250050535,
https://doi.org/10.3389/fpls.2024.1443691,
https://doi.org/10.3390/cells13161319,
https://doi.org/10.1007/s11816-024-00908-2,
https://doi.org/10.3389/fpls.2023.1159247,

📚 Additional Documentation

Notes

(PPC16-notes.md)

PPC16 (Q02909) — research notes

Gene: PPC16 / soybean (Glycine max, NCBITaxon:3847)
UniProt: Q02909 (CAPP1_SOYBN) — "Phosphoenolpyruvate carboxylase, housekeeping isozyme", EC 4.1.1.31, 967 aa.
Reviewed for the SPKW-PLANTS retrospective validation subproject.

1. What this protein is

Q02909 is a soybean phosphoenolpyruvate carboxylase (PEPC). UniProt explicitly names it
the housekeeping isozyme ("PEPC 1"). It catalyzes the irreversible β-carboxylation of
phosphoenolpyruvate (PEP) using bicarbonate to form oxaloacetate (OAA) + Pi
(Rhea:RHEA:28370). The UniProt FUNCTION text states the role plainly:

"Through the carboxylation of phosphoenolpyruvate (PEP) it forms oxaloacetate, a
four-carbon dicarboxylic acid source for the tricarboxylic acid cycle."
[UniProt Q02909 FUNCTION]

Key UniProt facts: homotetramer; cytoplasmic (Cytoplasm subcellular location);
regulated by light-reversible (Ser-11) phosphorylation; allosteric enzyme; Mg2+ cofactor;
"Belongs to the PEPCase type 1 family" — i.e. a plant-type PEPC (PTPC), not a
bacterial-type PEPC (BTPC). PE level 2 (evidence at transcript level).

2. The defining literature: original cloning paper

The single primary reference cited by UniProt is the cDNA cloning paper:

PMID:1450389

This paper directly settles two of the central biological questions:

Soybean is a C3 plant — stated explicitly: "the C3 plant Glycine max".
This isozyme is a C3-type / housekeeping PEPC, NOT a photosynthetic C4 PEPC:
PMID:1450389
Ubiquitous, non-photosynthesis-specific expression:
PMID:1450389 — a housekeeping expression pattern, not the mesophyll-specific,
light-induced pattern of a C4 photosynthetic PEPC.
The paper also notes two potential start codons and that the protein could be subject
to regulation by protein kinase (consistent with the UniProt phosphoserine site).

3. Plant PEPC isozyme families (background)

The authoritative review is O'Leary, Park & Plaxton 2011 PMID:21524275:

Plant genomes encode a small multigene family: several plant-type PEPCs (PTPCs) and
one distantly related bacterial-type PEPC (BTPC).
PMID:21524275
PTPCs are ~110 kDa, carry the conserved N-terminal seryl-phosphorylation site, and form
homotetrameric Class-1 PEPCs. Q02909 is 967 aa / ~110 kDa with the Ser-11 phosphosite —
a textbook PTPC.
PMID:21524275
The well-studied role of PEPC in photosynthesis is restricted to the dedicated C4/CAM
isoforms; PEPC ALSO does a broad set of NON-photosynthetic jobs, dominated by
anaplerosis:
PMID:21524275

C4/CAM photosynthetic PEPC isoforms evolved repeatedly and independently from
non-photosynthetic C3 progenitor genes by gene duplication and acquisition of
mesophyll-specific, light-responsive expression plus kinetic adaptations (e.g. the
C4-diagnostic Ser-775 residue) [PMID:14644912; PMID:16136331; PMID:24913627]. A C3 plant
like soybean has only the ancestral, non-photosynthetic-type PTPCs — it has no C4 CO2-pump
machinery.

4. Anaplerotic / housekeeping role of PEPC in C3 plants

The "housekeeping" PEPC (= anaplerotic PEPC) in C3 plants performs:

Anaplerosis: refilling TCA-cycle C4 intermediates (oxaloacetate, malate) drained for
biosynthesis. PEPC is not a TCA-cycle enzyme; it feeds OAA into the cycle from outside.
PMID:21524275
Nitrogen assimilation / amino-acid biosynthesis: OAA → aspartate and glutamate-family
amino acids; provides carbon skeletons for NH4+ assimilation.
Cellular pH regulation / charge balance, osmotic regulation (e.g. guard-cell malate
for stomatal opening), and seed/fruit organic-acid accumulation.

Singh et al. 2022 PMID:36309625 reviews how, in C3 plants, the non-photosynthetic
isoforms of "C4 enzymes" (including PEPC) serve general metabolism rather than
photosynthesis:
PMID:36309625

Ting, She & Plaxton 2017 PMID:29240945 similarly frames PTPC function as anaplerotic in
biosynthetically active sink tissues:
PMID:29240945 (PTPCs such as Q02909 are the catalytic backbone; note BTPC is a
separate gene, not Q02909.)

5. PEPC in soybean nodules / N fixation

In legume root nodules PEPC activity is induced and supplies OAA/malate as carbon skeletons
for the assimilation of fixed nitrogen and as a respiratory substrate for bacteroids — an
anaplerotic, non-photosynthetic role. Importantly, the contribution of nodule
PEPC-mediated CO2 fixation to whole-plant carbon is minor: a field 13C/15N study of
nodulating vs non-nodulating soybean concluded
PMID:10938812
This reinforces that PEPC carboxylation in soybean is anaplerotic (replenishing organic
acids), not net autotrophic "carbon fixation". PPC16/Q02909 is the ubiquitously expressed
housekeeping isozyme (leaf/stem/root/seed) and is the kind of PTPC that operates in roots
and nodules.

6. Family/domain evidence

InterPro/PANTHER place Q02909 in PTHR30523 (PHOSPHOENOLPYRUVATE CARBOXYLASE), subfamily
PTHR30523:SF33 ("PHOSPHOENOLPYRUVATE CARBOXYLASE 3"). HAMAP MF_00595 = PEPcase type 1.
InterPro signatures: IPR021135 (PEP_COase), IPR022805 (PEP_COase bacterial/plant-type),
IPR018129 / IPR033129 (Lys and His active-site signatures). Active-site His-172 and
Lys-602 (UniProt FT ACT_SITE) are consistent with canonical PEPC catalysis.

7. Assessment of the GO annotations under review

Retired SPKW annotation — photosynthesis (GO:0015979), GO_REF:0000043

GO:0015979 def: "The synthesis by organisms of organic chemical compounds, especially
carbohydrates, from carbon dioxide (CO2) using energy obtained from light..."
This annotation came purely from the UniProt keyword "Photosynthesis" (KW line), which is
applied broadly to all PEPCs because the family is famous for its C4/CAM role. But:
(a) soybean is C3 PMID:1450389; (b) this isozyme is explicitly the housekeeping/C3-type
PEPC [PMID:1450389; UniProt RecName]; (c) C3 plants have no light-driven CO2-fixation by
PEPC. The protein has no role in photosynthesis. GOA's removal of this annotation was
JUSTIFIED — it was a keyword-driven, pathway-context over-annotation. Action: REMOVE
(keep retired: true).

carbon fixation (GO:0015977), IEA UniProtKB-KW / InterPro

GO:0015977 def: "A metabolic process in which carbon (usually derived from carbon dioxide)
is incorporated into organic compounds (usually carbohydrates)." Every child of GO:0015977
is an autotrophic CO2-fixation pathway: C4 photosynthesis, CAM photosynthesis, reductive
TCA cycle, carbon fixation by acetyl-CoA pathway, 3-hydroxypropionate cycle. The term is an
autotrophic / net-carbon-gain concept. Housekeeping PEPC carboxylates PEP to OAA for
anaplerosis — it does not perform autotrophic carbon fixation and does not produce
carbohydrate. So carbon fixation is an over-annotation/mis-mapping for the housekeeping
isozyme. The UniProt keyword "Carbon dioxide fixation" was again applied at family level.
The most biologically accurate replacement for the actual process is
oxaloacetate metabolic process (GO:0006107) — def: "The chemical reactions and pathways
involving oxaloacetate ... an important intermediate in metabolism, especially as a
component of the TCA cycle." Action: MODIFY → GO:0006107.

tricarboxylic acid cycle (GO:0006099), IEA InterPro

GO:0006099 def describes the cyclic oxidation of acetyl-CoA via citrate → isocitrate →
2-oxoglutarate → succinyl-CoA → succinate → fumarate → malate → oxaloacetate. PEPC is NOT
one of the eight TCA-cycle enzymes. It is an anaplerotic enzyme that feeds OAA into
the cycle from PEP PMID:21524275. Annotating PEPC involved_in tricarboxylic acid cycle
conflates anaplerosis with the cycle itself. This InterPro2GO mapping (IPR021135/IPR022805
→ GO:0006099) is too coarse. Better: oxaloacetate metabolic process (GO:0006107), which
captures the anaplerotic provision of the TCA intermediate without asserting cycle
membership. Action: MODIFY → GO:0006107.

phosphoenolpyruvate carboxylase activity (GO:0008964), IEA

Catalytic activity directly matching UniProt CATALYTIC ACTIVITY / EC 4.1.1.31 / Rhea
RHEA:28370. Correct and core. Action: ACCEPT.

cytosol (GO:0005829), IBA GO_REF:0000033

PTPCs are cytosolic enzymes; UniProt subcellular location = Cytoplasm. IBA from the PEPC
phylogenetic group. Correct and core. Action: ACCEPT.

cytoplasm (GO:0005737), IEA GO_REF:0000044

From UniProt subcellular-location mapping (Cytoplasm). Correct but less specific than
cytosol. Action: ACCEPT (consistent, broader term).

8. Core function summary

PPC16/Q02909 is the cytosolic, homotetrameric housekeeping (anaplerotic) plant-type
PEPC of soybean (a C3 legume). Its core molecular function is phosphoenolpyruvate
carboxylase activity (EC 4.1.1.31), carboxylating PEP with bicarbonate to oxaloacetate.
Its core biological role is anaplerotic provision of oxaloacetate to replenish
TCA-cycle C4-dicarboxylic-acid intermediates consumed by biosynthesis and nitrogen
assimilation. It is NOT a photosynthetic CO2-fixing enzyme and is NOT a TCA-cycle enzyme.

9. Over-annotation pattern identified

This gene is a clean example of family-level / pathway-context keyword over-annotation:
because the PEPC enzyme family is famous for C4/CAM photosynthesis, family-wide UniProt
keywords ("Photosynthesis", "Carbon dioxide fixation") and coarse InterPro2GO mappings
(→ photosynthesis, carbon fixation, TCA cycle) get propagated onto every PEPC, including
the housekeeping C3 isozymes that do none of those things. GOA's retirement of the SPKW
"photosynthesis" annotation correctly removed one instance of this pattern; the surviving
IEA "carbon fixation" and "tricarboxylic acid cycle" annotations are the same error class
and should be modified to the anaplerotic-accurate term GO:0006107.

📄 View Raw YAML

id: Q02909
gene_symbol: PPC16
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:3847
  label: Glycine max
description: >
  PPC16 (Q02909, CAPP1_SOYBN) is the cytosolic housekeeping (anaplerotic) phosphoenolpyruvate
  carboxylase (PEPC; EC 4.1.1.31) of soybean (Glycine max), a C3 legume. It is a plant-type
  PEPC (PTPC): a ~110 kDa, 967-residue polypeptide that assembles into a homotetrameric
  Class-1 PEPC and is regulated by light-reversible N-terminal (Ser-11) phosphorylation and
  by allosteric effectors. The enzyme catalyses the irreversible beta-carboxylation of
  phosphoenolpyruvate (PEP) with bicarbonate to form oxaloacetate (OAA) plus phosphate.
  Its core biological role is anaplerotic: it replenishes the C4-dicarboxylic-acid
  intermediates (oxaloacetate, malate) of the tricarboxylic acid cycle that are drained for
  biosynthesis, nitrogen assimilation, amino-acid synthesis, cellular pH/charge balance and
  organic-acid accumulation. PPC16 is the ubiquitously expressed housekeeping isozyme, with
  mRNA present at similar levels in leaf, stem, root and developing seed. It is explicitly a
  C3-type PEPC and is distinct from the dedicated C4/CAM photosynthetic PEPC isozymes found
  in C4 and CAM plants; because soybean is a C3 plant it has no C4-type photosynthetic
  CO2-fixation machinery, and PPC16 has no role in photosynthesis or in autotrophic carbon
  fixation.
existing_annotations:
# --- SPKW keyword-mapping annotation (GO_REF:0000043) ---
# Present in the Sept 2025 goa_uniprot_gcrp snapshot (go-db plant.ddb); REMOVED
# from the current (2026) GOA release when GOA retired the keyword2GO pipeline
# for cellular organisms. Reviewed retrospectively to assess whether removal was
# justified. TRUE SPKW-unique (closure-filtered).
- term:
    id: GO:0015979
    label: photosynthesis
  evidence_type: IEA
  original_reference_id: GO_REF:0000043
  retired: true
  review:
    summary: >
      Keyword-derived (SPKW, GO_REF:0000043) annotation that existed in the Sept 2025 GOA
      snapshot and was removed from the current GOA release. It originates solely from the
      UniProt keyword "Photosynthesis", which is applied across the whole PEPC family
      because the family is famous for its C4/CAM photosynthetic CO2-fixation role.
      PPC16/Q02909 is the housekeeping (anaplerotic) PEPC isozyme of soybean, a C3 plant;
      it has no role in photosynthesis.
    action: REMOVE
    reason: >
      GOA's removal of this annotation was JUSTIFIED; it was a family-level, pathway-context
      over-annotation, not a correct annotation that was lost. GO:0015979 (photosynthesis)
      is defined as light-driven synthesis of organic compounds from CO2. Soybean is a C3
      plant, and the original cloning paper states this protein is a "C3-type" PEPC that
      does not resemble C4/CAM photosynthetic isoforms; it is the ubiquitously expressed
      housekeeping isozyme. The light-driven CO2-fixing role of PEPC is restricted to the
      dedicated C4/CAM isozymes, which evolved separately from non-photosynthetic C3
      progenitors. The housekeeping C3 PEPC performs anaplerotic carboxylation, not
      photosynthesis. Keeping retired: true; this is correctly removed.
    supported_by:
    - reference_id: PMID:1450389
      supporting_text: "The soybean encoded protein tends to resemble other 'C3-type' PEPC
        proteins more closely than those implicated in C4 or crassulacean acid metabolism."
    - reference_id: PMID:1450389
      supporting_text: "A full-length cDNA encoding a subunit of phosphoenolpyruvate
        carboxylase (PEPC) was isolated from a developing seed expression library of the
        C3 plant Glycine max."
    - reference_id: PMID:21524275
      supporting_text: "The critical role of PEPC in assimilating atmospheric CO(2) during
        C(4) and Crassulacean acid metabolism photosynthesis has been studied extensively.
        PEPC also fulfils a broad spectrum of non-photosynthetic functions, particularly
        the anaplerotic replenishment of tricarboxylic acid cycle intermediates consumed
        during biosynthesis and nitrogen assimilation."
    - reference_id: PMID:1450389
      supporting_text: "The corresponding mRNA is present at similar levels in leaf, stem,
        root and developing seed."
# --- Current GOA annotations (2026 release) ---
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: >
      IBA annotation from the PEPC phylogenetic group (PANTHER PTHR30523). Plant-type PEPCs
      are cytosolic Class-1 enzymes, and UniProt records the subcellular location of
      Q02909 as Cytoplasm. The cytosolic localization is a conserved, well-established
      property of this enzyme class.
    action: ACCEPT
    reason: >
      Correct and consistent with UniProt's curated subcellular location (Cytoplasm) and
      with the established biology of plant-type PEPCs as cytosolic homotetramers. IBA is
      appropriate and the term is at the right level of specificity.
    supported_by:
    - reference_id: PMID:21524275
      supporting_text: "PTPC genes encode ~110-kDa polypeptides containing conserved
        serine-phosphorylation and lysine-mono-ubiquitination sites, and typically exist
        as homotetrameric Class-1 PEPCs."
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  review:
    summary: >
      IEA annotation derived from the UniProtKB subcellular-location vocabulary mapping
      (UniProt records "Cytoplasm" for Q02909). This is correct but less specific than the
      cytosol annotation.
    action: ACCEPT
    reason: >
      Consistent with the UniProt subcellular location and with the IBA cytosol annotation.
      The term is broader than cytosol (GO:0005829) but not incorrect; an IEA being broader
      than an experimentally/phylogenetically supported finer term is acceptable.
    supported_by:
    - reference_id: PMID:21524275
      supporting_text: "PTPC genes encode ~110-kDa polypeptides containing conserved
        serine-phosphorylation and lysine-mono-ubiquitination sites, and typically exist
        as homotetrameric Class-1 PEPCs."
- term:
    id: GO:0006099
    label: tricarboxylic acid cycle
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  review:
    summary: >
      InterPro2GO annotation mapping the PEPC InterPro signatures (IPR021135, IPR022805) to
      the tricarboxylic acid cycle. PEPC is not one of the eight enzymes of the TCA cycle;
      it is an anaplerotic enzyme that supplies oxaloacetate to the cycle from PEP. The
      mapping conflates anaplerosis with cycle membership.
    action: MODIFY
    reason: >
      The TCA cycle (GO:0006099) is defined as the cyclic oxidation of acetyl-CoA via
      citrate, isocitrate, 2-oxoglutarate, succinyl-CoA, succinate, fumarate, malate and
      oxaloacetate. PEPC catalyses none of these steps; it is the canonical ANAPLEROTIC
      enzyme that replenishes TCA-cycle C4 intermediates consumed by biosynthesis and N
      assimilation. Annotating PEPC as involved_in the TCA cycle is too coarse and
      asserts cycle membership it does not have. A biologically accurate replacement is
      oxaloacetate metabolic process (GO:0006107), which captures PEPC's role in producing
      the OAA that feeds the cycle without asserting that the protein is part of the cycle.
    proposed_replacement_terms:
    - id: GO:0006107
      label: oxaloacetate metabolic process
    supported_by:
    - reference_id: PMID:21524275
      supporting_text: "PEPC also fulfils a broad spectrum of non-photosynthetic functions,
        particularly the anaplerotic replenishment of tricarboxylic acid cycle
        intermediates consumed during biosynthesis and nitrogen assimilation."
    - reference_id: PMID:1450389
      supporting_text: "Through the carboxylation of phosphoenolpyruvate (PEP) it forms
        oxaloacetate, a four-carbon dicarboxylic acid source for the tricarboxylic acid
        cycle. [paraphrasing the UniProt FUNCTION text drawn from this entry's primary
        reference]"
      full_text_unavailable: true
- term:
    id: GO:0008964
    label: phosphoenolpyruvate carboxylase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  review:
    summary: >
      IEA annotation (combined automated methods; ARBA/InterPro/RHEA/EC) for the catalytic
      activity of the enzyme. This directly matches the UniProt CATALYTIC ACTIVITY entry
      (oxaloacetate + phosphate = phosphoenolpyruvate + hydrogencarbonate; RHEA:28370;
      EC 4.1.1.31) and is the core molecular function of the protein.
    action: ACCEPT
    reason: >
      This is the defining, core molecular function of PPC16. The annotation matches the
      curated catalytic activity, the EC number, the Rhea reaction, and the PEPCase type 1
      family assignment in UniProt. The active-site residues His-172 and Lys-602 are
      annotated in UniProt, consistent with canonical PEPC catalysis.
    supported_by:
    - reference_id: PMID:21524275
      supporting_text: "PEPC [PEP (phosphoenolpyruvate) carboxylase] is a tightly
        controlled enzyme located at the core of plant C-metabolism that catalyses the
        irreversible beta-carboxylation of PEP to form oxaloacetate and Pi."
- term:
    id: GO:0015977
    label: carbon fixation
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  review:
    summary: >
      InterPro2GO annotation (IPR021135, IPR022805) and UniProt keyword "Carbon dioxide
      fixation" mapping the PEPC family to carbon fixation. GO:0015977 is an autotrophic
      concept: all of its subtypes are autotrophic CO2-fixation pathways (C4 photosynthesis,
      CAM photosynthesis, reductive TCA cycle, acetyl-CoA pathway, 3-hydroxypropionate
      cycle). The housekeeping C3 PEPC does not perform autotrophic carbon fixation.
    action: MODIFY
    reason: >
      GO:0015977 (carbon fixation) is defined as a metabolic process in which carbon
      (usually from CO2) is incorporated into organic compounds, usually carbohydrates;
      every child term is an autotrophic, net-carbon-gain pathway. Housekeeping PEPC
      carboxylates PEP to oxaloacetate for anaplerosis; it does not produce carbohydrate
      and does not perform autotrophic carbon fixation. As with the retired photosynthesis
      annotation, this is a family-level keyword/InterPro over-annotation: the carbon-fixing
      role of PEPC belongs to the dedicated C4/CAM photosynthetic isozymes, not to the C3
      housekeeping isozyme. The biologically accurate process term is oxaloacetate metabolic
      process (GO:0006107), reflecting the anaplerotic carboxylation that produces OAA.
    proposed_replacement_terms:
    - id: GO:0006107
      label: oxaloacetate metabolic process
    supported_by:
    - reference_id: PMID:1450389
      supporting_text: "The soybean encoded protein tends to resemble other 'C3-type' PEPC
        proteins more closely than those implicated in C4 or crassulacean acid metabolism."
    - reference_id: PMID:36309625
      supporting_text: "all the genes of the C4 photosynthetic pathway are present in C3
        plants, although they are involved in diverse non-photosynthetic functions.
        Non-photosynthetic isoforms of carbonic anhydrase (CA), phosphoenolpyruvate
        carboxylase (PEPC), malate dehydrogenase (MDH) ... catalyze reactions that are
        essential for major plant metabolism pathways, such as the tricarboxylic acid
        (TCA) cycle, maintenance of cellular pH, uptake of nutrients and their
        assimilation."
core_functions:
- description: >
    PPC16 catalyses the irreversible beta-carboxylation of phosphoenolpyruvate with
    bicarbonate to form oxaloacetate plus phosphate (EC 4.1.1.31). This is the defining
    molecular activity of the protein and matches the curated UniProt catalytic activity
    and Rhea reaction RHEA:28370.
  molecular_function:
    id: GO:0008964
    label: phosphoenolpyruvate carboxylase activity
  directly_involved_in:
  - id: GO:0006107
    label: oxaloacetate metabolic process
  locations:
  - id: GO:0005829
    label: cytosol
  supported_by:
  - reference_id: PMID:21524275
    supporting_text: "PEPC [PEP (phosphoenolpyruvate) carboxylase] is a tightly controlled
      enzyme located at the core of plant C-metabolism that catalyses the irreversible
      beta-carboxylation of PEP to form oxaloacetate and Pi."
  - reference_id: PMID:1450389
    supporting_text: "A full-length cDNA encoding a subunit of phosphoenolpyruvate
      carboxylase (PEPC) was isolated from a developing seed expression library of the C3
      plant Glycine max."
  - reference_id: file:SOYBN/PPC16/PPC16-deep-research-falcon.md
    supporting_text: "providing OAA for conversion to malate/aspartate and supporting
      anaplerosis and amino-acid biosynthesis"
- description: >
    As the soybean housekeeping (anaplerotic) plant-type PEPC, PPC16 produces oxaloacetate
    that replenishes the C4-dicarboxylic-acid intermediates of the tricarboxylic acid cycle
    consumed by biosynthesis and nitrogen assimilation. This anaplerotic role is its core
    biological function and is distinct from the autotrophic CO2-fixation performed by
    dedicated C4/CAM photosynthetic PEPC isozymes.
  molecular_function:
    id: GO:0008964
    label: phosphoenolpyruvate carboxylase activity
  directly_involved_in:
  - id: GO:0006107
    label: oxaloacetate metabolic process
  locations:
  - id: GO:0005829
    label: cytosol
  supported_by:
  - reference_id: PMID:21524275
    supporting_text: "PEPC also fulfils a broad spectrum of non-photosynthetic functions,
      particularly the anaplerotic replenishment of tricarboxylic acid cycle intermediates
      consumed during biosynthesis and nitrogen assimilation."
  - reference_id: PMID:36309625
    supporting_text: "Non-photosynthetic isoforms of carbonic anhydrase (CA),
      phosphoenolpyruvate carboxylase (PEPC), malate dehydrogenase (MDH) ... catalyze
      reactions that are essential for major plant metabolism pathways, such as the
      tricarboxylic acid (TCA) cycle, maintenance of cellular pH, uptake of nutrients and
      their assimilation."
  - reference_id: file:SOYBN/PPC16/PPC16-deep-research-falcon.md
    supporting_text: "In C3/nonphotosynthetic tissues, PEPC serves an anaplerotic role,
      replenishing TCA intermediates, supporting malate production, carbon–nitrogen
      balance, and legume nodule metabolism"
proposed_new_terms: []
suggested_questions:
- question: >
    Does PPC16 expression and activity respond to nitrogen status, organic-acid demand, or
    nodulation in soybean roots, consistent with a dedicated anaplerotic / N-assimilation
    role for this housekeeping isozyme?
- question: >
    What is the full PEPC gene family structure in the soybean genome, and how do PPC16 and
    its paralogs partition the anaplerotic, seed-storage, and BTPC-associated Class-2 PEPC
    functions?
suggested_experiments:
- description: >
    Quantify PPC16 transcript and PEPC enzyme activity across soybean tissues (leaf, stem,
    root, nodule, developing seed) and under varied nitrogen supply, then measure flux of
    label from 13C-bicarbonate into oxaloacetate-derived organic acids and amino acids.
  hypothesis: >
    PPC16 functions as a constitutive anaplerotic enzyme supplying oxaloacetate for
    TCA-cycle replenishment and nitrogen assimilation, rather than for net photosynthetic
    carbon fixation.
- description: >
    Generate PPC16 knockdown/knockout soybean lines and assay growth, amino-acid pools,
    organic-acid content, and stomatal/pH phenotypes, with attention to root and nodule
    metabolism.
  hypothesis: >
    Loss of the housekeeping PEPC isozyme impairs anaplerotic OAA supply and N assimilation
    but does not affect photosynthetic CO2 fixation, which in this C3 plant is performed by
    Rubisco.
references:
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO terms
  findings:
  - statement: >
      InterPro2GO mapped the PEPC InterPro signatures (IPR021135, IPR022805) to
      tricarboxylic acid cycle and carbon fixation; both mappings are too coarse for the
      anaplerotic housekeeping isozyme and are recommended for modification to oxaloacetate
      metabolic process.
- id: GO_REF:0000043
  title: Gene Ontology annotation based on UniProtKB/Swiss-Prot keyword mapping
  findings:
  - statement: >
      SwissProt keyword-derived (SPKW) annotations present in the Sept 2025
      goa_uniprot_gcrp snapshot but removed from the current GOA release after GOA retired
      the keyword2GO pipeline for cellular organisms.
  - statement: >
      The SPKW "photosynthesis" annotation for PPC16 derived from the family-level UniProt
      keyword "Photosynthesis"; its removal by GOA was justified because soybean is a C3
      plant and PPC16 is the non-photosynthetic housekeeping PEPC isozyme.
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings:
  - statement: >
      IBA annotation from the PEPC phylogenetic group correctly assigns cytosolic
      localization, consistent with plant-type PEPCs being cytosolic Class-1 homotetramers.
- id: GO_REF:0000044
  title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location
    vocabulary mapping, accompanied by conservative changes to GO terms applied by UniProt
  findings:
  - statement: >
      Subcellular-location mapping assigned cytoplasm, consistent with the curated UniProt
      location and with the IBA cytosol annotation.
- id: GO_REF:0000120
  title: Combined Automated Annotation using Multiple IEA Methods
  findings:
  - statement: >
      Combined IEA methods (ARBA, InterPro, RHEA, EC) correctly assigned phosphoenolpyruvate
      carboxylase activity, the core molecular function of the protein.
- id: PMID:1450389
  title: cDNA sequence and expression of a phosphoenolpyruvate carboxylase gene from soybean.
  findings:
  - statement: >
      The PEPC cDNA was isolated from soybean, explicitly described as a C3 plant.
  - statement: >
      The encoded protein resembles C3-type PEPC proteins, not the C4 or CAM
      photosynthetic isoforms.
  - statement: >
      The PEPC mRNA is present at similar levels in leaf, stem, root and developing seed,
      a ubiquitous housekeeping expression pattern.
  - statement: >
      Two potential start codons exist and the protein may be subject to regulation by
      protein kinase (consistent with the conserved N-terminal phosphoserine site).
- id: PMID:21524275
  title: The remarkable diversity of plant PEPC (phosphoenolpyruvate carboxylase); recent
    insights into the physiological functions and post-translational controls of
    non-photosynthetic PEPCs.
  findings:
  - statement: >
      PEPC catalyses the irreversible beta-carboxylation of PEP to form oxaloacetate and Pi
      and sits at the core of plant carbon metabolism.
  - statement: >
      Beyond the C4/CAM photosynthetic role, PEPC fulfils non-photosynthetic functions,
      particularly anaplerotic replenishment of TCA-cycle intermediates consumed during
      biosynthesis and nitrogen assimilation.
  - statement: >
      Plant genomes encode several plant-type PEPCs (PTPCs, ~110 kDa, homotetrameric
      Class-1 PEPCs with conserved serine-phosphorylation sites) plus a distantly related
      bacterial-type PEPC (BTPC); PPC16/Q02909 is a PTPC.
- id: PMID:36309625
  title: Role of C4 photosynthetic enzyme isoforms in C3 plants and their potential
    applications in improving agronomic traits in crops.
  findings:
  - statement: >
      All genes of the C4 photosynthetic pathway are present in C3 plants but the C3
      isoforms perform diverse non-photosynthetic functions.
  - statement: >
      Non-photosynthetic PEPC isoforms support major metabolic pathways including the TCA
      cycle, cellular pH maintenance, and nutrient uptake/assimilation.
- id: PMID:29240945
  title: Transcript profiling indicates a widespread role for bacterial-type
    phosphoenolpyruvate carboxylase in malate-accumulating sink tissues.
  findings:
  - statement: >
      Plant genomes encode several plant-type PEPC (PTPC) isozymes plus a distantly related
      bacterial-type PEPC (BTPC); the two are separate genes.
  - statement: >
      PEPC in biosynthetically active sink tissues maintains rapid anaplerotic PEP
      carboxylation and respiratory CO2 refixation rather than autotrophic carbon fixation.
- id: PMID:10938812
  title: Weather and nodule mediated variations in delta 13C and delta 15N values in
    field-grown soybean (Glycine max L.) with special interest in the analyses of xylem
    fluids.
  findings:
  - statement: >
      In field-grown soybean, the contribution of PEPC-mediated CO2 fixation in root
      nodules to whole-plant carbon incorporation was not significant, consistent with an
      anaplerotic rather than net autotrophic carbon-fixing role for PEPC.
- id: file:SOYBN/PPC16/PPC16-deep-research-falcon.md
  title: Deep research report (falcon / Edison Scientific Literature) on soybean PPC16
    (Q02909) phosphoenolpyruvate carboxylase.
  findings:
  - statement: >
      Soybean primary literature (Sugimoto et al. 1992) cloned gmppc16 as a full-length
      PEPC cDNA from a developing-seed library and concluded the encoded protein resembles
      C3-type/housekeeping PEPCs more than C4/CAM photosynthetic forms; the report confirms
      PPC16 is the broadly expressed housekeeping isozyme of a C3 plant with no
      photosynthetic role.
  - statement: >
      gmppc16 mRNA was detected by Northern blot at similar levels in leaf, stem, root and
      developing seed, supporting a constitutive housekeeping rather than tissue-specialized
      expression pattern.
  - statement: >
      The report places the housekeeping/C3 PEPC reaction (PEP + HCO3- -> oxaloacetate + Pi,
      EC 4.1.1.31, irreversible, Mg2+-dependent) in cytosolic anaplerotic carbon metabolism,
      replenishing TCA-cycle C4 intermediates and supplying carbon skeletons for nitrogen
      assimilation and amino-acid/storage-protein synthesis during seed filling.
  - statement: >
      Genome-wide analysis (Wang et al. 2016) shows soybean has 10 PEPC genes (7 plant-type,
      3 bacterial-type) predicted predominantly cytosolic; a later seed-development study
      (Yamamoto et al. 2020) identifies Gmppc16 as Glyma.12g161300, a plant-type PEPC
      isogene, supporting its classification as a PTPC paralog within the family.
  - statement: >
      The nodule-enhanced GmPEPC7 is 92.5% identical at the amino-acid level to gmppc16
      (Hata et al. 1998), confirming gmppc16 is a close paralog representing the
      broadly-expressed housekeeping lineage and consistent with cytosolic localization
      stated for soybean nodule PEPCs.

PPC16

Gene Description

Existing Annotations Review

Core Functions

PPC16 catalyses the irreversible beta-carboxylation of phosphoenolpyruvate with bicarbonate to form oxaloacetate plus phosphate (EC 4.1.1.31). This is the defining molecular activity of the protein and matches the curated UniProt catalytic activity and Rhea reaction RHEA:28370.