mtdA encodes a bifunctional NADP-dependent dehydrogenase that catalyzes the oxidation of both methylene-H4MPT (EC 1.5.1.-) and methylene-H4F (EC 1.5.1.5) to their respective methenyl forms. The enzyme functions in step 2/5 of formaldehyde degradation via the H4MPT route, but with a 20-fold preference for H4MPT over H4F as substrate (Vmax 600 vs 30 ΞΌmol/min/mg). MtdA plays a critical regulatory role in controlling the segregation of C1 carbon flux between assimilation and oxidation pathways. While mtdB serves as the main methylene-H4MPT dehydrogenase in vivo, mtdA's dual substrate specificity allows it to bridge the H4MPT and tetrahydrofolate metabolic pools. The enzyme functions as a homotrimer in the cytoplasm with optimal activity at pH 6.0 and 45Β°C. Multiple crystal structures have been solved (PDB: 1LU9, 1LUA, 6TGE, 6TLK, 6TM3), including complexes with NADP showing re-face stereospecificity. Direct protein sequencing has confirmed the N-terminal sequence.
Definition: Catalysis of the reaction 5,10-methylenetetrahydromethanopterin + NADP+ = 5,10-methenyl-5,6,7,8-tetrahydromethanopterin + NADPH. This is the primary physiological reaction of M. extorquens MtdA (EC 1.5.1.-, RHEA:24682) and is distinct from the F420-dependent activity captured by GO:0030268.
Justification: MtdA's primary, highest-efficiency activity is NADP-dependent oxidation of methylene-H4MPT, but the only existing methylenetetrahydromethanopterin dehydrogenase MF term (GO:0030268) is defined with coenzyme F420 as the electron acceptor, and GO:0018532 is obsolete. UniProt's GO mapping therefore only assigns the secondary H4F activity (GO:0004488), leaving the enzyme's principal molecular function without an exact GO term.
Parent term: oxidoreductase activity
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004488 methylenetetrahydrofolate dehydrogenase (NADP+) activity | IEA GO_REF:0000120 | ACCEPT | Summary: Correct - MtdA catalyzes methylene-H4F dehydrogenation with NADP+ as cofactor, though with ~20-fold lower catalytic efficiency than its primary substrate methylene-H4MPT [file:METEA/mtdA/mtdA-uniprot.txt, "catalyze the reversible dehydrogenation of methylene-H(4)F with 20-fold"; file:METEA/mtdA/mtdA-uniprot.txt, "EC=1.5.1.5"]. This is the only molecular_function term assigned by UniProt's GO mapping (UniProtKB-EC); however it captures only the secondary H4F activity, not the enzyme's primary NADP-dependent methylene-H4MPT dehydrogenase reaction (EC 1.5.1.-, RHEA:24682), for which no exact GO MF term currently exists (see proposed_new_terms). Falcon deep research corroborates the dual specificity. Supporting Evidence: file:METEA/mtdA/mtdA-deep-research-falcon.md NADP+-dependent methylene-pterin dehydrogenase file:METEA/mtdA/mtdA-deep-research-falcon.md catalytic efficiency for methylene-H4F is reported to be PMID:9765566 dehydrogenation of methylene tetrahydrofolate (methylene H4F) with NADP+ |
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | ACCEPT | Summary: Correct - MtdA is localized to the cytoplasm where it participates in C1 metabolism [file:METEA/mtdA/mtdA-uniprot.txt, "SUBCELLULAR LOCATION: Cytoplasm"]. Direct cell-fractionation evidence (Hagemeier et al. 2000) supports a soluble/cytosolic enzyme: methylene-H4MPT dehydrogenase activity partitioned into the ultracentrifugation supernatant while the membrane fraction was inactive. Supporting Evidence: file:METEA/mtdA/mtdA-deep-research-falcon.md membrane fraction lacked NAD(P)-dependent methylene-H4MPT dehydrogenase activity file:METEA/mtdA/mtdA-deep-research-falcon.md consistent with MtdA not being membrane-associated |
| GO:0006730 one-carbon metabolic process | IEA GO_REF:0000043 | ACCEPT | Summary: Correct - MtdA is central to C1 metabolism, bridging the H4MPT and H4F pathways [file:METEA/mtdA/mtdA-uniprot.txt, "formaldehyde degradation; formate from"; file:METEA/mtdA/mtdA-uniprot.txt, "formaldehyde (H(4)MPT route): step 2/5"]. Falcon deep research describes MtdA as providing a mechanistic link between H4MPT-linked formaldehyde processing and the H4F C1 pool used for assimilation and biosynthesis. Supporting Evidence: file:METEA/mtdA/mtdA-deep-research-falcon.md a mechanistic link between formaldehyde processing and the H4F C1 pool used for assimilation file:METEA/mtdA/mtdA-deep-research-falcon.md generating methenyl-/formyl-H4F for biosynthetic C1 metabolism |
| GO:0016491 oxidoreductase activity | IEA GO_REF:0000120 | KEEP AS NON CORE | Summary: Correct but too general - MtdA is a bifunctional NADP-dependent dehydrogenase; more specific MF terms are available (GO:0004488 for the H4F activity, and the proposed new term for the primary NADP-dependent methylene-H4MPT dehydrogenase activity). Reason: The parent oxidoreductase term is not wrong but is uninformative relative to the specific dehydrogenase activities MtdA catalyzes; retained as non-core. |
| GO:0046294 formaldehyde catabolic process | IEA GO_REF:0000041 | ACCEPT | Summary: Correct - MtdA catalyzes step 2/5 in formaldehyde degradation via the H4MPT route [file:METEA/mtdA/mtdA-uniprot.txt, "formaldehyde degradation; formate from"; file:METEA/mtdA/mtdA-uniprot.txt, "formaldehyde (H(4)MPT route): step 2/5"]. Falcon deep research identifies the H4MPT-linked pathway as the major formaldehyde oxidation/detoxification route in M. extorquens AM1, with MtdA oxidizing methylene-H4MPT to methenyl-H4MPT en route to formate/CO2. Supporting Evidence: file:METEA/mtdA/mtdA-deep-research-falcon.md major formaldehyde oxidation/detoxification route file:METEA/mtdA/mtdA-deep-research-falcon.md oxidizing methylene-H4MPT to methenyl-H4MPT |
Loading supporting contentβ¦
Download this section (compressed HTML)Q: Does MtdA contribute non-redundantly to the H4F (serine cycle / biosynthetic) C1 pool in vivo, or is its physiological role essentially confined to the H4MPT formaldehyde-oxidation branch?
Experiment: Construct conditional/depletion alleles of mtdA (since clean nulls appear non-viable) and perform 13C-formaldehyde flux tracing to quantify the relative contribution of MtdA to methenyl-H4MPT versus methenyl-H4F pools under methylotrophic and heterotrophic growth.
Hypothesis: MtdA's essentiality reflects an indispensable role in supplying methenyl-/formyl-H4F for biosynthesis rather than (or in addition to) formaldehyde oxidation.
Loading supporting contentβ¦
Download this section (compressed HTML)Loading supporting contentβ¦
Download this section (compressed HTML)