SoxG (PP_0326) is the gamma subunit of the cytosolic heterotetrameric sarcosine oxidase SoxABDG. It belongs to the SoxG family, which shares a folate-binding-like fold with the GcvT/TrmE superfamily; in the characterised orthologous enzyme the gamma subunit contacts two domains of the folate-binding alpha subunit. It is a component of the complex that oxidises sarcosine and transfers its methyl group to tetrahydrofolate, forming glycine, 5,10-methylenetetrahydrofolate and hydrogen peroxide. In KT2440 the complex acts in sarcosine catabolism downstream of creatine and glycine betaine degradation.
Definition: Catalysis of the reaction: sarcosine + (6S)-5,6,7,8-tetrahydrofolate + O2 = (6R)-5,10-methylene-5,6,7,8-tetrahydrofolate + glycine + H2O2.
Justification: This is the physiological reaction of heterotetrameric SoxABDG-type sarcosine oxidases (EC 1.5.3.24, RHEA:70455). GO:0008115 sarcosine oxidase activity is defined by the formaldehyde-forming reaction (EC 1.5.3.1, RHEA:13313), which these enzymes perform only when folate is absent, so no current GO term captures transfer of the oxidised methyl group to tetrahydrofolate.
Parent term: oxidoreductase activity, acting on the CH-NH group of donors, oxygen as acceptor
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0008115 sarcosine oxidase activity | IEA GO_REF:0000120 | KEEP AS NON CORE | Summary: GO:0008115 describes the folate-free, formaldehyde-forming reaction, a real but conditional activity of the SoxABDG complex; the physiological reaction is folate-coupled. Reason: UniProt combined-IEA row (GO_REF:0000120) from the SoxG signature IPR006280 and the submitter-supplied EC 1.5.3.1. GO:0008115 is defined as "H2O + O2 + sarcosine = formaldehyde + glycine + H2O2". Heterotetrameric sarcosine oxidases perform this reaction when tetrahydrofolate is absent, so the term is true of the complex, but with folate they transfer the oxidised methyl group to tetrahydrofolate instead (EC 1.5.3.24, RHEA:70455), which has no GO term (see proposed_new_terms). The formaldehyde branch is therefore a genuine conditional secondary activity, kept as non-core, and the physiological activity is recorded in core_functions. The gamma subunit carries none of the flavin sites, so it contributes to rather than enables the activity; the qualifier defect is recorded for UniProt. Supporting Evidence: PMID:15946648 The gamma subunit is in contact with two domains of alpha subunit and has possibly a folate-binding structure. PMID:16934831 In a second reaction, TSOX catalyzes the transfer of the oxidized 1-carbon fragment from the sarcosine imine PMID:16934831 In the absence of H 4 folate, turnover occurs at a somewhat slower rate and the sarcosine imine is hydrolyzed to glycine and formaldehyde. |
| GO:0032921 sarcosine oxidase complex | NAS PMID:35266867 Creatine utilization as a sole nitrogen source in Pseudomona... | NEW | Summary: SoxG is the gamma subunit of the tetrameric SoxBDAG sarcosine oxidase. Reason: No complex annotation exists. A same-species study names tetrameric SoxBDAG as the KT2440 enzyme converting sarcosine to glycine, and the orthologous enzyme is a four-subunit heterotetramer; GO:0032921 is defined as a heterotetramer with sarcosine oxidase activity. Supporting Evidence: PMID:35266867 sarcosine is then converted to glycine and formaldehyde by the tetrameric sarcosine oxidase (SoxBDAG) PMID:15946648 Sarcosine oxidase from Corynebacterium sp. U-96 is a heterotetrameric enzyme. |
| GO:0005737 cytoplasm | ISS PMID:15914915 Corynebacterium sp. U-96 contains a cluster of genes of enzy... | NEW | Summary: SoxG is a subunit of the soluble cytosolic SoxABDG sarcosine oxidase. Reason: No location annotation exists for this subunit. The protein has no predicted transmembrane segment or signal peptide, the orthologous heterotetramer is functionally expressed as a soluble enzyme, and GO:0032921 sarcosine oxidase complex is part_of cytoplasm; SoxB of the same complex carries the cytoplasm annotation. Supporting Evidence: PMID:15914915 Functional expression of sarcosine oxidase in Escherichia coli was accomplished, but the beta subunit and the betadelta complex were expressed at a low level as a soluble protein. |
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Download this section (compressed HTML)Q: Does SoxG bind folate or participate in channelling the iminium intermediate to the alpha-subunit folate site, as its folate-binding-like fold suggests?
Experiment: Reconstitute KT2440 SoxABD with and without SoxG and compare complex assembly, folate binding and the ratio of 5,10-methylenetetrahydrofolate to formaldehyde produced.
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