| Claim/role | Evidence type (genetics/biochemistry/structure/bioinformatics) | Key quantitative values | Notes (organism specificity) | Primary source with publication date and URL |
|---|---|---|---|---|
| xoxG (MexAM1_META1p1741; UniProt C5B121) is the cytochrome c gene in the xoxF-xoxG-xoxJ module of *Methylorubrum extorquens* AM1 | Bioinformatics/genome context | Adjacent to xoxF1 (META1_1740) and xoxJ (META1_1742) in an operon-like arrangement analogous to the Ca-dependent mxaFJG system | Correct target organism and locus; supports assignment of XoxG as the cognate cytochrome partner of XoxF, not an unrelated xoxG homolog from another taxon (pqac-00000016, pqac-00000017) | Schmidt et al., 2010-08, *Microbiology*. https://doi.org/10.1099/mic.0.038570-0 ; Roszczenko-Jasińska et al., 2020-07, *Scientific Reports*. https://doi.org/10.1038/s41598-020-69401-4 |
| XoxG is a periplasmic c-type cytochrome | Biochemistry | Purified from the periplasmic fraction; mature N-terminus starts at Gln27 after signal peptide cleavage; UV-vis spectrum showed characteristic c-type cytochrome features including a weak 695 nm band consistent with Met ligation | Directly measured in *M. extorquens* AM1 XoxG; aligns with UniProt/domain annotation for a cytochrome c-like protein (pqac-00000009) | Featherston et al., 2019-09, *ChemBioChem*. https://doi.org/10.1002/cbic.201900184 |
| XoxG is the physiological electron acceptor for lanthanide-dependent XoxF methanol dehydrogenase | Biochemistry | XoxF activity was assayed with purified XoxG as the electron acceptor; La-, Ce-, and Nd-XoxFs showed similar Vmax values, but Km for XoxG increased about 3-fold from La to Nd | This is the central experimentally supported function in AM1; places XoxG in periplasmic electron transfer during methanol oxidation (pqac-00000000, pqac-00000001, pqac-00000009, pqac-00000011) | Featherston et al., 2019-09, *ChemBioChem*. https://doi.org/10.1002/cbic.201900184 |
| XoxG is essential for XoxF-dependent methanol growth in the presence of lanthanum | Genetics | In methanol + La3+, loss of xoxG phenocopied loss of xoxF1 and xoxF2; reporter data showed xoxG mutant had mxa promoter activity 365 ± 24 RFU/OD600 in methanol without La, but no xox1 promoter signal was reported under the tested conditions | Evidence is from reconstructed mutants in *M. extorquens* AM1; supports a direct functional requirement rather than mere operon association (pqac-00000004, pqac-00000015, pqac-00000016) | Roszczenko-Jasińska et al., 2020-07, *Scientific Reports*. https://doi.org/10.1038/s41598-020-69401-4 |
| XoxG is a monoheme class I c-type cytochrome with covalently attached heme c | Structure/biochemistry | X-ray structure solved to 2.71 Å; heme attached via Cys95 and Cys98 in the CXXCH motif; axial ligands His99 and Met143 | Structural work performed on AM1 XoxG directly; supports heme-c mediated electron-transfer role (pqac-00000003, pqac-00000009) | Featherston et al., 2019-09, *ChemBioChem*. https://doi.org/10.1002/cbic.201900184 |
| XoxG has an unusually low reduction potential, likely tuned for lanthanide-dependent XoxF catalysis | Biochemistry/structure | Em = +172 ± 1 mV at pH 7.0; lower than the corresponding MxaG value cited as about +256 mV; typical His/Met monoheme cytochromes are usually about +200 to +370 mV | Measured directly for AM1 XoxG; interpreted as a specialization for electron transfer from Ln-PQQ XoxF, especially with lighter lanthanides (pqac-00000000, pqac-00000003, pqac-00000009, pqac-00000010, pqac-00000012) | Featherston et al., 2019-09, *ChemBioChem*. https://doi.org/10.1002/cbic.201900184 |
| The low Em is explained by a distinctive heme environment with increased solvent exposure | Structure | XoxG lacks helix IV and lacks the Ca2+ site found near the heme in MxaG; heme propionates hydrogen-bond to Arg118 and Lys132; solvent exposure of the HP6/HP7 face is proposed to depress Em | Mechanistic structural inference is based on the solved AM1 XoxG crystal structure and comparison with MxaG/other cytochromes (pqac-00000003, pqac-00000012, pqac-00000018, pqac-00000019, pqac-00000020) | Featherston et al., 2019-09, *ChemBioChem*. https://doi.org/10.1002/cbic.201900184 |
| XoxG likely interacts directly with XoxF through defined surface loops | Structure/modeling | Homology-guided model using a fused PQQ-dehydrogenase/cytochrome template suggested loops between helices I-II and III-V contact XoxF | Interaction interface is inferred, not yet captured in a co-crystal for AM1; still useful for functional annotation of partner specificity (pqac-00000003, pqac-00000012) | Featherston et al., 2019-09, *ChemBioChem*. https://doi.org/10.1002/cbic.201900184 |
| XoxG appears tuned to favor lighter lanthanides in the cognate XoxF enzyme | Biochemistry/physiological interpretation | Apparent Km for XoxG rises from La-XoxF to Nd-XoxF; authors extrapolated a possible Km near 10 μM for Sm-XoxF, potentially too high for physiological activity at native periplasmic XoxG concentrations | This inference is specific to the AM1 XoxF5/XoxG pair and helps explain why AM1 performs best with lighter lanthanides (pqac-00000001, pqac-00000011) | Featherston et al., 2019-09, *ChemBioChem*. https://doi.org/10.1002/cbic.201900184 |
| Functional dependence on cytochrome c maturation/heme export systems supports XoxG’s role as a heme protein in methanol oxidation | Genetics/pathway support | Deletion of cytochrome c biogenesis and heme export genes (e.g., cycK, ccmB, ccmC) eliminated methanol growth with and without La3+ | Indirect but organism-specific support that active heme-containing cytochromes such as XoxG are required for the pathway in AM1 (pqac-00000004, pqac-00000016) | Roszczenko-Jasińska et al., 2020-07, *Scientific Reports*. https://doi.org/10.1038/s41598-020-69401-4 |


*Table: This table summarizes the experimentally supported functional annotation of xoxG (C5B121; MexAM1_META1p1741) in *Methylorubrum extorquens* AM1, integrating genetics, biochemistry, structure, and operon context. It is useful as a traceable evidence map for assigning XoxG as the periplasmic cytochrome c electron acceptor in lanthanide-dependent methanol oxidation.*