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this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.
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THI22 (systematic name YPR121W; UniProt Q06490) encodes a thiamine biosynthesis protein in Saccharomyces cerevisiae (strain ATCC 204508 / S288c). The protein is classified as a member of the thiaminase-2 family and contains several conserved domains including a haem oxygenase-like multi-helical domain (IPR016084), an HMP/HMP-P kinase domain (IPR004399, IPR013749), a ribokinase-like fold (IPR029056), and a thiaminase-2/PQQC domain (IPR004305). The protein is annotated as a precursor (with a signal peptide), suggesting it may be targeted to a secretory compartment.
THI22 is the third member of a three-gene family in S. cerevisiae that also includes THI20 and THI21 (kowalska2008thegenesand pages 4-8). The better-characterized family members, THI20 (YOL055C) and THI21 (YPL258C), encode trifunctional proteins with established roles in thiamine metabolism. The N-terminal domain of THI20 functions as both an HMP kinase (phosphorylating 4-amino-5-hydroxymethyl-2-methylpyrimidine, or HMP, to HMP-P) and an HMP-P kinase (phosphorylating HMP-P to HMP-PP) in ATP-dependent reactions (kowalska2008thegenesand pages 4-8, wronska2022engineeringbiotinsynthesis; pages 20-23, perli2020vitaminrequirementsand pages 7-8). The C-terminal domain of THI20 possesses thiaminase II activity, which degrades thiamine by hydrolyzing it to release HMP and a thiazole-derived product (kowalska2008thegenesand pages 4-8, wronska2022engineeringbiotinsynthesis; pages 20-23). This C-terminal domain exhibits high sequence similarity to the PET18 protein (kowalska2008thegenesand pages 4-8). THI20 is considered the major isoform, while THI21 is a paralog with likely similar activities (perli2020vitaminrequirementsand pages 7-8, wronska2022engineeringbiotinsynthesis; pages 20-23).
Critically, the specific enzymatic function of THI22 remains unresolved. Kowalska and Kozik (2008) explicitly state: "The function of the third member of the same gene family, THI22, is unknown at present" (kowalska2008thegenesand pages 4-8). Despite sharing family membership and domain annotations with THI20 and THI21, no direct biochemical characterization of THI22 enzymatic activity has been reported in the available literature.
The following table summarizes the key properties of the THI20/THI21/THI22 family:
| Gene Name | Systematic Name | Domain Architecture (N-terminal / C-terminal) | Known Enzymatic Activities | Key Role in Pathway | Evidence Level |
|---|---|---|---|---|---|
| THI20 | YOL055C | N-terminal HMP/HMP-P kinase domain; C-terminal thiaminase II domain; trifunctional family member (kowalska2008thegenesand pages 4-8, wronska2022engineeringbiotinsynthesis; pages 20-23, perli2020vitaminrequirementsand pages 7-8) | HMP kinase; HMP-P kinase; thiaminase II (thiamine-degrading) activity (kowalska2008thegenesand pages 4-8, wronska2022engineeringbiotinsynthesis; pages 20-23, perli2020vitaminrequirementsand pages 7-8) | Major isoform generating HMP-PP for the pyrimidine branch of thiamine biosynthesis; also participates in HMP salvage and thiamine degradation (perli2020vitaminrequirementsand pages 7-8, wronska2022engineeringbiotinsynthesis; pages 20-23, kowalska2008thegenesand pages 4-8, sabelleck2025thiamineisa pages 3-4) | Experimentally characterized for family activities; major isoform assignment and trifunctionality supported by reviews and cited primary work (kowalska2008thegenesand pages 8-11, wronska2022engineeringbiotinsynthesis; pages 20-23, perli2020vitaminrequirementsand pages 7-8) |
| THI21 | YPL258C | Inferred to share THI20-like architecture: N-terminal HMP/HMP-P kinase domain and C-terminal thiaminase II-related domain as a paralogous family member (kowalska2008thegenesand pages 4-8, perli2020vitaminrequirementsand pages 7-8) | HMP-P kinase; likely HMP kinase and thiaminase II-associated functions by homology/family assignment (kowalska2008thegenesand pages 4-8, perli2020vitaminrequirementsand pages 7-8, sabelleck2025thiamineisa pages 3-4) | Paralogue contributing to HMP-PP formation in the pyrimidine branch of thiamine biosynthesis; likely backup/secondary isoform relative to THI20 (perli2020vitaminrequirementsand pages 7-8, wronska2022engineeringbiotinsynthesis; pages 20-23, sabelleck2025thiamineisa pages 3-4) | Partly experimentally supported at family level, but less directly characterized than THI20; substantial inference from paralogy and pathway reviews (kowalska2008thegenesand pages 4-8, perli2020vitaminrequirementsand pages 7-8) |
| THI22 | YPR121W | Third member of the THI20/THI21/THI22 family; shares family membership and thiaminase-2/domain annotation, but detailed domain-function assignment is inferred rather than directly demonstrated (kowalska2008thegenesand pages 4-8) | Unknown specific enzymatic activity; possible relationship to HMP/HMP-P kinase/thiaminase family functions is inferred only from homology and annotation (kowalska2008thegenesand pages 4-8) | Putative participant in thiamine metabolism/biosynthesis context, but precise biochemical step remains unresolved in the literature reviewed (kowalska2008thegenesand pages 4-8, li2010thiaminebiosynthesisin pages 1-2, li2020transcriptomeanalysisreveals pages 5-7) | Inferred/uncharacterized: literature explicitly states THI22 function is unknown (kowalska2008thegenesand pages 4-8) |
Table: This table summarizes the Saccharomyces cerevisiae THI20/THI21/THI22 gene family, highlighting experimentally supported activities for THI20, likely paralogous functions for THI21, and the currently unresolved status of THI22. It is useful for distinguishing firm biochemical evidence from homology-based inference.
Thiamine (vitamin B1) biosynthesis in S. cerevisiae involves the separate synthesis of two precursor moieties — the pyrimidine moiety (HMP-PP, 4-amino-2-methyl-5-hydroxymethylpyrimidine diphosphate) and the thiazole moiety (HET-P, 5-(2-hydroxyethyl)-4-methylthiazole phosphate) — followed by their condensation into thiamine monophosphate (TMP), which is ultimately converted to the biologically active cofactor thiamine diphosphate (TDP) (wronska2022engineeringbiotinsynthesis; pages 20-23, kowalska2008thegenesand pages 4-8, kowalska2008thegenesand pages 1-4).
HMP-P is synthesized from pyridoxal-5-phosphate (PLP) and histidine by the HMP-P synthases encoded by THI5, THI11, THI12, and THI13, which function as suicide enzymes undergoing a single catalytic turnover (perli2020vitaminrequirementsand pages 7-8, perli2020vitaminrequirementsand pages 5-7). HMP-P is then phosphorylated to HMP-PP by HMP-P kinase in an ATP-dependent reaction catalyzed primarily by THI20 and THI21 (perli2020vitaminrequirementsand pages 7-8, wronska2022engineeringbiotinsynthesis; pages 20-23). In the salvage pathway, exogenous HMP can be taken up and phosphorylated to HMP-P by HMP kinase activity, also attributed to THI20/THI21 (kowalska2008thegenesand pages 4-8, kowalska2008thegenesand pages 1-4).
The thiazole precursor HET-P is synthesized by THI4 from NAD+, glycine, and a sulfur atom donated from a conserved cysteine residue in a suicide reaction (perli2020vitaminrequirementsand pages 7-8, perli2020vitaminrequirementsand pages 5-7). In the salvage pathway, free HET is phosphorylated by the HET kinase activity of the bifunctional THI6 protein (kowalska2008thegenesand pages 4-8).
The condensation of HMP-PP and HET-P into TMP is catalyzed by the TMP diphosphorylase activity of the bifunctional THI6 enzyme (kowalska2008thegenesand pages 8-11, kowalska2008thegenesand pages 4-8). Unlike bacteria, yeasts cannot directly phosphorylate TMP to TDP; instead, TMP is first dephosphorylated to free thiamine by non-specific phosphatases, and then thiamine is diphosphorylated to TDP by thiamine diphosphokinase (THI80) (kowalska2008thegenesand pages 4-8).
The following table summarizes the complete pathway:
| Pathway step | Reaction | Gene(s) / enzyme | Substrates | Product(s) | Enzyme class / note |
|---|---|---|---|---|---|
| 1. HMP-P synthesis | Pyridoxal-5-phosphate + histidine → HMP-P | THI5, THI11, THI12, THI13 | Pyridoxal-5-phosphate, histidine | HMP-P | HMP-P synthase; suicide enzyme family in yeast (perli2020vitaminrequirementsand pages 7-8, perli2020vitaminrequirementsand pages 5-7) |
| 2. HMP-P phosphorylation | HMP-P + ATP → HMP-PP | THI20, THI21; THI22 possible by homology but unproven | HMP-P, ATP | HMP-PP | HMP-P kinase; THI20 is the major isoform, THI21 is paralogous; THI22 remains uncharacterized (kowalska2008thegenesand pages 4-8, perli2020vitaminrequirementsand pages 7-8, wronska2022engineeringbiotinsynthesis; pages 20-23, sabelleck2025thiamineisa pages 3-4) |
| 3. HMP salvage phosphorylation | HMP + ATP → HMP-P | THI20, THI21 | HMP, ATP | HMP-P | HMP kinase activity in the salvage branch; assigned to the trifunctional THI20/THI21 family (kowalska2008thegenesand pages 4-8, perli2020vitaminrequirementsand pages 7-8) |
| 4. HET-P synthesis | NAD+ + glycine + cysteine-derived sulfur → HET-P | THI4 | NAD+, glycine, sulfur from conserved Cys in Thi4 | HET-P | Thiazole synthase; suicide enzyme with single-turnover sulfur donation (perli2020vitaminrequirementsand pages 7-8, perli2020vitaminrequirementsand pages 5-7) |
| 5. HET salvage phosphorylation | HET + ATP → HET-P | THI6 | HET, ATP | HET-P | Hydroxyethylthiazole kinase; bifunctional THI6 protein (kowalska2008thegenesand pages 8-11, kowalska2008thegenesand pages 4-8, hohmann1998thiaminmetabolismand pages 1-3) |
| 6. TMP formation | HMP-PP + HET-P → TMP | THI6 | HMP-PP, HET-P | TMP | TMP diphosphorylase / thiamine-phosphate pyrophosphorylase; bifunctional THI6 enzyme (kowalska2008thegenesand pages 8-11, perli2020vitaminrequirementsand pages 7-8, wronska2022engineeringbiotinsynthesis; pages 20-23, kowalska2008thegenesand pages 4-8) |
| 7. Thiamine formation | TMP → thiamine | Unspecified phosphatase(s) | TMP | Thiamine | Dephosphorylation step; likely carried out by nonspecific phosphatases in yeast (kowalska2008thegenesand pages 4-8, hohmann1998thiaminmetabolismand pages 3-5) |
| 8. TDP formation | Thiamine + ATP → TDP | THI80 | Thiamine, ATP | TDP (ThDP) | Thiamine diphosphokinase / thiamine pyrophosphokinase (kowalska2008thegenesand pages 8-11, perli2020vitaminrequirementsand pages 7-8, kowalska2008thegenesand pages 4-8) |
| 9. Thiamine degradation | Thiamine → HMP + thiazole-derived product | THI20 | Thiamine | HMP + thiazole-related product(s) | Thiaminase II activity in the C-terminal domain of Thi20; family-level evidence also implicates THI21-like proteins (kowalska2008thegenesand pages 4-8, wronska2022engineeringbiotinsynthesis; pages 20-23, perli2020vitaminrequirementsand pages 7-8) |
| 10. Thiamine uptake | External thiamine → internal thiamine | THI7 / THI10 | Extracellular thiamine | Intracellular thiamine | High-affinity plasma membrane thiamine transporter; uptake supports salvage and cofactor production (kowalska2008thegenesand pages 4-8, hohmann1998thiaminmetabolismand pages 6-8, perli2020vitaminrequirementsand pages 7-8) |
Table: This table summarizes the core thiamine biosynthesis, salvage, degradation, and uptake steps in Saccharomyces cerevisiae. It is useful for locating where THI22 fits relative to the better-characterized THI20/THI21 family members and the rest of the pathway.
Although no direct biochemical evidence exists for THI22's enzymatic activity, several lines of evidence support its involvement in thiamine metabolism:
Domain architecture: THI22 contains the same InterPro domains as THI20/THI21, including the HMP/HMP-P kinase domain (IPR004399, IPR013749) and the thiaminase-2/PQQC domain (IPR004305). The presence of a ribokinase-like fold (IPR029056) is consistent with kinase activity on small-molecule substrates. This domain composition strongly suggests that THI22 may possess HMP/HMP-P kinase activity and/or thiaminase II activity, by analogy to its family members.
Thiaminase-2 family membership: UniProt classifies THI22 in the thiaminase-2 family. Thiaminase II enzymes catalyze the hydrolysis of thiamine to release the pyrimidine moiety (HMP) and a thiazole product. In THI20, this degradation activity is localized to the C-terminal domain (kowalska2008thegenesand pages 4-8, wronska2022engineeringbiotinsynthesis; pages 20-23).
Signal peptide / precursor status: The UniProt annotation identifies THI22 as a "Precursor" with a potential signal peptide. This is distinct from THI20, which is a cytoplasmic protein. The presence of a signal peptide suggests that THI22 may be targeted to the secretory pathway and could function in the periplasm or cell wall, potentially in a salvage or degradation role analogous to the periplasmic acid phosphatase PHO3, which dephosphorylates extracellular thiamine phosphates (kowalska2008thegenesand pages 4-8, hohmann1998thiaminmetabolismand pages 6-8).
THI22 is part of the THI regulon, a group of genes whose expression is tightly regulated in response to intracellular thiamine diphosphate (TDP) levels. Under thiamine-replete conditions, the expression of all known THI structural genes (except THI80, which is constitutively expressed) is repressed (kowalska2008thegenesand pages 4-8, hohmann1998thiaminmetabolismand pages 5-6). Under thiamine-depleted conditions, a ternary complex of the positive transcriptional regulators Thi2p (a zinc finger transcription factor), Thi3p (a TDP-binding sensor protein), and Pdc2p activates transcription of THI genes (kowalska2008thegenesand pages 4-8, hohmann1998thiaminmetabolismand pages 8-9, brion2014decipheringregulatoryvariation pages 1-2). TDP itself serves as the intracellular negative signal: when TDP levels are sufficient, TDP binds Thi3p, preventing formation of the activator complex and silencing THI gene transcription (kowalska2008thegenesand pages 4-8, hohmann1998thiaminmetabolismand pages 8-9).
An additional layer of regulation was demonstrated by Li et al. (2010), who showed that multiple THI genes are regulated by the NAD+-dependent histone deacetylase Hst1. Conditions that reduce intracellular NAD+ or increase nicotinamide lead to derepression of THI genes, establishing a regulatory link between thiamine and NAD+ homeostasis (li2010thiaminebiosynthesisin pages 1-2).
Transcriptomic studies have confirmed that THI22 is co-expressed with other THI genes. For example, Li et al. (2020) reported that THI22 was upregulated in proanthocyanidin-treated yeast cells alongside other thiamine pathway genes including THI6, THI21, and THI80, consistent with its membership in the THI regulon.
No direct experimental data on the subcellular localization of THI22 has been reported in the available literature. However, two important considerations inform predictions:
The UniProt annotation as a precursor with a signal peptide suggests targeting to the secretory pathway, potentially to the periplasm, cell wall, or extracellular space. This would be unusual for a thiamine biosynthetic enzyme, as de novo thiamine biosynthesis in yeast is generally considered to occur in the cytoplasm (kowalska2008thegenesand pages 4-8).
By contrast, the characterized family member THI20 is a cytoplasmic enzyme. The high-throughput yeast GFP localization study by Huh et al. (2003) did not report a clear localization for THI22.
If THI22 is indeed targeted to the secretory pathway, this may suggest a specialized role distinct from the cytoplasmic HMP/HMP-P kinase function of THI20/THI21 — perhaps in extracellular thiamine salvage or degradation.
The presence of three paralogous genes (THI20, THI21, THI22) in this family represents a form of genetic redundancy that is a recurrent theme in the yeast thiamine biosynthesis pathway. The four THI5/THI11/THI12/THI13 genes encoding HMP-P synthase show a similar pattern of gene amplification, likely reflecting a selective advantage in thiamine-poor environments (hohmann1998thiaminmetabolismand pages 5-6, perli2020vitaminrequirementsand pages 7-8). In the case of the THI20/THI21/THI22 family, THI20 is clearly the major functional isoform, while THI21 and THI22 may serve as backup copies or specialized variants that contribute under particular environmental conditions (perli2020vitaminrequirementsand pages 7-8, wronska2022engineeringbiotinsynthesis; pages 20-23).
The involvement of two suicide enzymes (THI4 for thiazole and THI5-family for pyrimidine synthesis) makes de novo thiamine biosynthesis energetically expensive in yeast — for each molecule of thiamine produced, two complete proteins must be synthesized and degraded (perli2020vitaminrequirementsand pages 7-8). This energetic cost underscores the importance of tight transcriptional regulation and the evolutionary rationale for maintaining redundant genes and efficient salvage pathways.
THI22 (YPR121W) encodes a thiamine biosynthesis protein in S. cerevisiae that belongs to the thiaminase-2 family and the THI20/THI21/THI22 gene family. While its paralogs THI20 and THI21 have been characterized as trifunctional enzymes with HMP kinase, HMP-P kinase, and thiaminase II activities, the specific biochemical function of THI22 remains experimentally unresolved (kowalska2008thegenesand pages 4-8). Its domain architecture (HMP/HMP-P kinase domain plus thiaminase-2/PQQC domain) strongly suggests it participates in the pyrimidine branch of thiamine biosynthesis and/or thiamine salvage/degradation. The presence of a signal peptide distinguishes THI22 from the cytoplasmic THI20 and may indicate a role in extracellular or periplasmic thiamine metabolism. THI22 is a member of the THI regulon and is transcriptionally regulated by thiamine availability through the Thi2p/Thi3p/Pdc2p activator complex and by NAD+ levels through the histone deacetylase Hst1 (kowalska2008thegenesand pages 4-8, hohmann1998thiaminmetabolismand pages 8-9, li2010thiaminebiosynthesisin pages 1-2). Future biochemical characterization of the purified THI22 protein will be necessary to determine whether it possesses HMP/HMP-P kinase and/or thiaminase II activity, and to define its precise role in thiamine metabolism.
References
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