xoxG encodes a periplasmic monoheme class I c-type cytochrome that serves as the dedicated physiological electron acceptor for XoxF-type lanthanide-dependent methanol dehydrogenases. It is the functional analog of MxaG, the cytochrome that serves the calcium-dependent MxaFI system. The protein carries a covalently bound heme c attached via the canonical CXXCH motif (Cys95/Cys98), with the heme iron axially ligated by His99 and Met143. XoxG has an unusually low midpoint reduction potential (+172 mV at pH 7.0), substantially lower than MxaG (~+256 mV), a property attributed to a heme environment with increased solvent exposure and proposed to tune it to the redox chemistry of the lanthanide-PQQ cofactor in XoxF. XoxG is purified from the periplasmic fraction with its signal peptide cleaved (mature N-terminus Gln27) and accepts electrons from reduced XoxF following methanol oxidation, transferring them onward into the respiratory electron transport chain. The gene is organized within the xoxF1-xoxG-xoxJ module and is genetically essential for XoxF-dependent methanol growth under lanthanide conditions, with loss of xoxG phenocopying loss of xoxF1/xoxF2.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0009055 electron transfer activity | IEA GO_REF:0000002 | ACCEPT | Summary: Correct - XoxG is the physiological electron acceptor that accepts electrons from reduced XoxF (lanthanide-dependent methanol dehydrogenase) following methanol oxidation and transfers them onward into the respiratory electron transport chain. This is the core molecular function of XoxG and is strongly supported by direct biochemistry. Supporting Evidence: file:METEA/xoxG/xoxG-deep-research-falcon.md XoxG is a **periplasmic, heme c–binding cytochrome** purified from the periplasmic fraction with a **cleaved signal peptide**, and it functions as the **physiological electron acceptor** for lanthanide-dependent XoxF methanol dehydrogenase activity file:METEA/xoxG/xoxG-deep-research-falcon.md the functional model is that XoxG transfers electrons onward into the respiratory chain |
| GO:0020037 heme binding | IEA GO_REF:0000002 | ACCEPT | Summary: Correct - XoxG is a c-type cytochrome carrying a covalently bound heme c attached via the canonical CXXCH motif (Cys95/Cys98). Heme binding is essential for its electron-transfer function. Supporting Evidence: file:METEA/xoxG/xoxG-deep-research-falcon.md The heme is attached via the canonical **CXXCH** motif (Cys95/Cys98) |
| GO:0046872 metal ion binding | IEA GO_REF:0000043 | KEEP AS NON CORE | Summary: Correct but general - XoxG binds iron within the heme prosthetic group, with the heme iron axially ligated by His99 and Met143. The more specific term GO:0020037 (heme binding) is also present and captures this function more precisely, so this general term is retained as non-core. Supporting Evidence: file:METEA/xoxG/xoxG-deep-research-falcon.md The heme iron is axially ligated by **His99 and Met143** |
| GO:0042597 periplasmic space | IDA PMID:31017712 Biochemical and Structural Characterization of XoxG and XoxJ... | NEW | Summary: XoxG is purified from the periplasmic fraction with its signal peptide cleaved (mature N-terminus Gln27). It localizes to the periplasm where it functions as the electron acceptor for periplasmic XoxF methanol dehydrogenase. Reason: XoxG is a periplasmic cytochrome c that must reside in the same compartment as XoxF methanol dehydrogenase to facilitate electron transfer. Direct evidence (purification from the periplasmic fraction and signal peptide cleavage to mature Gln27) supports periplasmic localization. Supporting Evidence: PMID:31017712 2019 Aug 7. Biochemical and Structural Characterization of XoxG and XoxJ and Their Roles in Lanthanide-Dependent Methanol Dehydrogenase Activity. |
| GO:0046170 methanol catabolic process | IDA PMID:31017712 Biochemical and Structural Characterization of XoxG and XoxJ... | NEW | Summary: XoxG is the dedicated electron acceptor for the lanthanide-dependent oxidation of methanol to formaldehyde by XoxF, i.e. the first committed step of periplasmic methanol catabolism. Reason: XoxG accepts electrons from reduced XoxF following methanol oxidation and is required for XoxF turnover, placing it directly in the methanol catabolic pathway. The previously proposed GO:0015946 (methanol oxidation) is the archaeal methanol-to-methyl-CoM pathway and is biologically inappropriate for this aerobic methylotrophic system, so the correct catabolic term GO:0046170 is used instead. Supporting Evidence: PMID:31017712 2019 Aug 7. Biochemical and Structural Characterization of XoxG and XoxJ and Their Roles in Lanthanide-Dependent Methanol Dehydrogenase Activity. |
| GO:0015945 methanol metabolic process | IMP PMID:31017712 Biochemical and Structural Characterization of XoxG and XoxJ... | NEW | Summary: XoxG is genetically essential for methanol metabolism via the lanthanide-dependent pathway; loss of xoxG produces a growth defect equivalent to loss of the XoxF dehydrogenases when lanthanides are present. Reason: Genetic studies demonstrate that loss of xoxG results in growth phenotypes equivalent to loss of xoxF1/xoxF2 when lanthanides are present, supporting an essential role in methanol metabolic process via the lanthanide-dependent pathway. Supporting Evidence: PMID:31017712 2019 Aug 7. Biochemical and Structural Characterization of XoxG and XoxJ and Their Roles in Lanthanide-Dependent Methanol Dehydrogenase Activity. |
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