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.
| 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. |
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Download this section (compressed HTML)Q: 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?
Q: 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?
Experiment: 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.
Experiment: 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.
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