glyA encodes serine hydroxymethyltransferase (SHMT, EC 2.1.2.1), a pyridoxal 5'-phosphate (PLP)-dependent enzyme that catalyzes the reversible interconversion of serine and glycine with tetrahydrofolate (THF). This reaction is crucial for the serine cycle, the central assimilatory pathway in methylotrophy, where it serves as the major source of one-carbon groups required for biosynthesis of purines, thymidylate, methionine, and other biomolecules. The enzyme functions as a homodimer in the cytoplasm. SHMT catalyzes: L-serine + THF β glycine + 5,10-methylenetetrahydrofolate + HβO. In methylotrophic metabolism, this enzyme is essential for converting C1 units from formaldehyde oxidation (via the H4MPT pathway) into the tetrahydrofolate pool, where they are used for serine biosynthesis from glycine, completing a key step in the serine cycle. The enzyme also exhibits THF-independent aldolase activity toward Ξ²-hydroxyamino acids. Genetic studies (insertional inactivation in M. extorquens AM1) have confirmed glyA is essential for methylotrophic growth: glyA null mutants lose all detectable SHMT activity and cannot grow on C1 compounds (including methanol) even when supplemented with glycine or serine, while still growing normally on succinate. SHMT activity is ~6-fold induced on methanol versus succinate and has been flagged as a potential rate-limiting step in the serine cycle.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004372 glycine hydroxymethyltransferase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Correct - SHMT catalyzes the reversible PLP-dependent interconversion of serine and glycine with THF as the one-carbon carrier; this is the core molecular function of glyA and is directly supported by organism-specific enzymology in M. extorquens AM1. Supporting Evidence: file:METEA/glyA/glyA-deep-research-falcon.md catalyzes the reversible folate-linked one-carbon transfer between serine and glycine: **L-serine + tetrahydrofolate (THF) β glycine + 5,10-methylenetetrahydrofolate (5,10-CH2-THF) + H2O** file:METEA/glyA/glyA-deep-research-falcon.md Insertional inactivation of **glyA** in AM1 eliminates measurable SHMT activity and causes a strong growth defect on **C1 substrates** |
| GO:0005737 cytoplasm | IEA GO_REF:0000120 | ACCEPT | Summary: Correct - SHMT is a soluble cytosolic enzyme participating in the serine cycle; no membrane/periplasmic/secretion signal is present and AM1 activity is measured in cell extracts. No direct AM1 localization experiment exists, so this remains an inference from enzymology and conserved bacterial SHMT biology. Supporting Evidence: file:METEA/glyA/glyA-deep-research-falcon.md all evidence places GlyA in intracellular folate/serine-cycle metabolism |
| GO:0005829 cytosol | IEA GO_REF:0000118 | ACCEPT | Summary: Correct - More specific than cytoplasm; SHMT operates as a soluble enzyme in the cytosolic compartment. As with cytoplasm, this is inferred from enzymology (activity in cell extracts) rather than a direct AM1 localization experiment. Supporting Evidence: file:METEA/glyA/glyA-deep-research-falcon.md soluble cytosolic enzyme |
| GO:0006730 one-carbon metabolic process | IEA GO_REF:0000043 | ACCEPT | Summary: Correct - SHMT is the central node linking folate-bound one-carbon units to amino acid metabolism in the serine cycle, integrating C1 units derived from methanol/formaldehyde oxidation into biosynthesis. Supporting Evidence: file:METEA/glyA/glyA-deep-research-falcon.md a key link between **folate-linked one-carbon metabolism** and assimilation of formaldehyde derived from methanol oxidation |
| GO:0008652 amino acid biosynthetic process | IEA GO_REF:0000043 | MODIFY | Summary: Correct but too general - SHMT catalyzes serine/glycine interconversion. A more specific child term, serine family amino acid biosynthetic process (GO:0009070), better captures the enzyme's role; the existing parent term should be kept as non-core. Reason: The generic amino acid biosynthetic process term is uninformative for an SHMT; the serine family is the relevant subgroup and a more specific term is available. Proposed replacements: serine family amino acid biosynthetic process Supporting Evidence: file:METEA/glyA/glyA-deep-research-falcon.md GlyA catalyzing the step that couples a folate-bound C1 unit with glycine to form serine |
| GO:0016740 transferase activity | IEA GO_REF:0000043 | KEEP AS NON CORE | Summary: Correct but too general - SHMT is a transferase, but the specific term "glycine hydroxymethyltransferase activity" (GO:0004372) is more informative and is already annotated. |
| GO:0019264 glycine biosynthetic process from L-serine | IEA GO_REF:0000120 | ACCEPT | Summary: Correct - SHMT catalyzes step 1/1 in glycine biosynthesis from L-serine. In the physiological methylotrophic direction in AM1 the reaction runs toward serine formation from glycine + an activated C1 unit, but the enzyme is fully reversible and this term is accurate. Supporting Evidence: file:METEA/glyA/glyA-deep-research-falcon.md GlyA catalyzing the step that couples a folate-bound C1 unit with glycine to form serine |
| GO:0030170 pyridoxal phosphate binding | IEA GO_REF:0000120 | ACCEPT | Summary: Correct - SHMT is a PLP-dependent enzyme; PLP forms an internal aldimine (Schiff base) with the active-site lysine (Lys242 in this protein), confirmed by conserved SHMT structural studies. Supporting Evidence: file:METEA/glyA/glyA-deep-research-falcon.md PLP forms an internal aldimine (Schiff base) with an active-site lysine |
| GO:0035999 tetrahydrofolate interconversion | IEA GO_REF:0000120 | ACCEPT | Summary: Correct - SHMT interconverts THF and 5,10-methylenetetrahydrofolate, the defining one-carbon transfer reaction of the serine cycle. In AM1 the physiologically relevant carrier is a polyglutamylated folate, which stimulates the SHMT reaction more strongly than monoglutamyl THF. Supporting Evidence: file:METEA/glyA/glyA-deep-research-falcon.md polyglutamylated THF species stimulate SHMT-catalyzed serine synthesis more strongly than monoglutamyl THF in vitro |
| GO:0046653 tetrahydrofolate metabolic process | IEA GO_REF:0000118 | KEEP AS NON CORE | Summary: Correct but redundant - More specific term "tetrahydrofolate interconversion" (GO:0035999) captures the same process more precisely. |
| GO:0006545 glycine biosynthetic process | IEA GO_REF:0000041 | KEEP AS NON CORE | Summary: Correct - SHMT catalyzes glycine biosynthesis from serine; parent term of GO:0019264. Kept as non-core because the more specific GO:0019264 (glycine biosynthetic process from L-serine) is the precise term for this enzyme. |
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