mxaG (also known as moxG) encodes cytochrome c_L, a specialized c-type cytochrome that serves as the dedicated electron acceptor for the calcium-dependent methanol dehydrogenase MxaFI. The protein contains heme c covalently attached and exhibits a redox potential of +256 mV at pH 7.0, substantially higher than the analogous XoxG cytochrome (+172 mV) that serves the lanthanide-dependent XoxF system. Located in the periplasm, MxaG accepts electrons from reduced MxaFI following methanol oxidation and transfers them to downstream components of the respiratory electron transport chain. MxaG itself does not catalyze methanol oxidation; rather, it mediates electron transfer from the MDH redox cofactor PQQ to additional periplasmic cytochromes, coupling methanol oxidation to respiration and energy conservation. The gene is part of the mxa operon (mxaFJGIRSACKLDEHB) and is subject to the same complex regulatory control as mxaFI, being activated by MxbM in the absence of lanthanides and repressed when lanthanides are present. Crystal structure has been solved at 1.60 Å resolution (PDB: 2C8S), revealing the molecular architecture of this electron transfer protein.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005506 iron ion binding | IEA GO_REF:0000002 | ACCEPT | Summary: Correct - MxaG binds iron in the heme prosthetic group [file:METEA/mxaG/mxaG-uniprot.txt, "Binds 1 heme c group covalently per subunit" and "Iron" in keywords]. The UniProt feature table specifies the axial iron-coordinating histidine (residue 94, ligand Fe), consistent with iron ion binding in a c-type cytochrome heme. Reason: The heme c iron is the redox-active center of cytochrome c_L; iron ion binding is a correct molecular function supported by the UniProt covalent heme c binding sites and the Fe axial-binding residue. |
| GO:0009055 electron transfer activity | IEA GO_REF:0000002 | ACCEPT | Summary: Correct and core - MxaG (cytochrome c_L) functions as the dedicated electron acceptor for periplasmic PQQ-dependent methanol dehydrogenase, accepting electrons from reduced MxaFI and transferring them onward to additional cytochromes in the respiratory electron transport chain [file:METEA/mxaG/mxaG-uniprot.txt, "Electron acceptor for MDH. Acts in methanol oxidation"]. Reason: This is the primary molecular function of the gene product. Multiple authoritative sources name MxaG as the cytochrome c_L electron-transfer partner of methanol dehydrogenase, transferring electrons from PQQ to downstream cytochromes. Supporting Evidence: file:METEA/mxaG/mxaG-deep-research-falcon.md All methylotrophic PQQ-ADHs are periplasmic enzymes associated with a cytochrome cL (MxaG, XoxG, and ExaG, respectively) that transfers electrons from PQQ to additional cytochromes in the electron transport chain file:METEA/mxaG/mxaG-deep-research-falcon.md the specific cytochrome c that accepts electrons from methanol dehydrogenase file:METEA/mxaG/mxaG-deep-research-falcon.md reduced PQQ is reoxidized by transferring electrons to the heme of cytochrome cL, which is then oxidized by downstream cytochromes |
| GO:0015945 methanol metabolic process | IEA GO_REF:0000043 | ACCEPT | Summary: Correct and core - MxaG participates in methanol metabolism as the electron acceptor in the calcium-dependent methanol oxidation pathway, feeding electrons from periplasmic methanol oxidation into respiration [file:METEA/mxaG/mxaG-uniprot.txt, "Acts in methanol oxidation" and "Methanol utilization" in keywords]. Reason: MxaG is genetically part of the mxa methanol dehydrogenase cluster and functionally required for methanol oxidation by accepting electrons from the MDH PQQ cofactor; the deep research confirms its role in the periplasmic methanol-oxidation electron-transfer module. Supporting Evidence: file:METEA/mxaG/mxaG-deep-research-falcon.md methanol oxidation occurs in the periplasm via PQQ-dependent ADHs file:METEA/mxaG/mxaG-deep-research-falcon.md Electrons are transferred from PQQ to cytochrome cL and then to additional cytochromes/respiratory chain components, coupling methanol oxidation to energy generation |
| GO:0020037 heme binding | IEA GO_REF:0000002 | ACCEPT | Summary: Correct - MxaG contains one heme c group covalently bound per subunit [file:METEA/mxaG/mxaG-uniprot.txt, "Binds 1 heme c group covalently per subunit"]. The covalent heme c is the redox cofactor through which MxaG accepts and donates electrons. Reason: Covalent heme c binding is documented in UniProt (CXXCH-type c-type cytochrome with covalent thioether attachment at residues 90 and 93 and Fe axial coordination at residue 94); the heme is essential to its electron-transfer function. |
| GO:0042597 periplasmic space | IEA GO_REF:0000120 | ACCEPT | Summary: Correct and core - MxaG is localized to the periplasm where it accepts electrons from periplasmic MxaFI [file:METEA/mxaG/mxaG-uniprot.txt, "SUBCELLULAR LOCATION: Periplasm"]. It bears an N-terminal signal peptide (residues 1-25) consistent with periplasmic export. Reason: UniProt assigns periplasmic localization and a cleaved signal peptide; the deep research independently places cytochrome c_L in the periplasmic methanol-oxidation electron-transfer chain alongside the periplasmic PQQ-dependent dehydrogenases. Supporting Evidence: file:METEA/mxaG/mxaG-deep-research-falcon.md All methylotrophic PQQ-ADHs are periplasmic enzymes associated with a cytochrome cL (MxaG, XoxG, and ExaG, respectively) that transfers electrons from PQQ to additional cytochromes in the electron transport chain |
| GO:0046872 metal ion binding | IEA GO_REF:0000043 | KEEP AS NON CORE | Summary: Correct but general - MxaG binds iron in the heme group. The more specific terms GO:0005506 (iron ion binding) and GO:0020037 (heme binding) better capture this function. Reason: This is a redundant generic parent of the iron ion binding annotation already present; it is retained as non-core because the specific iron ion binding and heme binding terms convey the actual molecular context. |
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