EryCII (SACE_0725) is a cytochrome-P450-family homologue that has lost catalytic competence and instead acts as the activating partner of the desosaminyl transferase EryCIII in erythromycin biosynthesis in Saccharopolyspora erythraea. Although related to cytochrome P450s by sequence, EryCII lacks the heme-binding sites and therefore cannot perform P450 monooxygenase chemistry; it is a pseudoenzyme. Functionally, EryCII forms an elongated EryCII-EryCIII heterotetramer in which it stabilizes EryCIII and acts as an allosteric activator of the glycosyltransferase, enabling efficient transfer of TDP-D-desosamine to the macrolide scaffold (PMID:22056329, PMID:15303858). Mechanistically EryCII behaves like a conformational chaperone that templates the catalytically competent state of EryCIII, but - unlike a classical chaperone - it remains stably bound as a stoichiometric subunit and EryCIII is inactive without it, so it is best described as an activator/stabilizer subunit rather than a transient folding chaperone. EryCII is thus a clear example of domain-based over-annotation: its sequence-derived P450/heme/monooxygenase annotations do not reflect its true non-catalytic, GT-activating role.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004497 monooxygenase activity | IEA GO_REF:0000002 | REMOVE | Summary: InterPro-based (IEA) monooxygenase activity from the cytochrome P450 signature. EryCII is a P450 homologue that lacks the heme-binding sites and is catalytically dead; it has no monooxygenase activity. Its real role is as an allosteric activator of the EryCIII glycosyltransferase. Reason: Demonstrably incorrect. Beyond the UniProt CAUTION ("lacks the heme-binding sites"), the structure paper shows by structure-based alignment that EryCII lacks the conserved heme-ligating cysteine and is "not an active P450 enzyme", and the 2YJN structure is apo (no heme). EryCII functions as a GT activator, not a monooxygenase. Classic domain-propagation over-annotation of a pseudoenzyme. Supporting Evidence: file:SACEN/eryCII/eryCII-uniprot.txt Although related to the cytochrome P450 family, lacks the heme-binding sites. PMID:22056329 the conserved cysteine is absent (Supplementary Fig. S5); thus, they are not active P450 enzymes |
| GO:0005506 iron ion binding | IEA GO_REF:0000002 | REMOVE | Summary: IEA iron ion binding inherited from the P450 signature (P450s bind a heme iron). EryCII lacks the heme-binding sites and does not bind a catalytic iron. Reason: Incorrect; depends on heme/iron cofactor binding that EryCII has lost. The UniProt CAUTION (lacks the heme-binding sites) and the structure (no heme; absent conserved Cys) agree. Supporting Evidence: file:SACEN/eryCII/eryCII-uniprot.txt Although related to the cytochrome P450 family, lacks the heme-binding sites. PMID:22056329 Having lost the heme group in the central core, the protein may be more conformationally dynamic |
| GO:0016705 oxidoreductase activity, acting on paired donors, with incorporation or reduction of molecular oxygen | IEA GO_REF:0000002 | REMOVE | Summary: IEA oxidoreductase activity from the P450 signature. EryCII performs no oxygen-dependent oxidoreductase chemistry; it is a non-catalytic P450 homologue. Reason: Incorrect catalytic MF for a heme-less, non-catalytic P450 homologue (UniProt CAUTION plus the apo structure / absent conserved Cys). Supporting Evidence: file:SACEN/eryCII/eryCII-uniprot.txt Although related to the cytochrome P450 family, lacks the heme-binding sites. PMID:22056329 the conserved cysteine is absent (Supplementary Fig. S5); thus, they are not active P450 enzymes |
| GO:0020037 heme binding | IEA GO_REF:0000002 | REMOVE | Summary: IEA heme binding from the P450 signature. EryCII explicitly lacks the heme-binding sites, so it does not bind heme. Reason: Both the UniProt CAUTION ("lacks the heme-binding sites") and primary structural data agree: the conserved heme-ligating cysteine is absent and EryCII has "lost the heme group in the central core" (PMID:22056329); the 2YJN structure contains no heme ligand. The most clearly incorrect of the inherited P450 terms. Supporting Evidence: file:SACEN/eryCII/eryCII-uniprot.txt Although related to the cytochrome P450 family, lacks the heme-binding sites. PMID:22056329 Having lost the heme group in the central core, the protein may be more conformationally dynamic |
| GO:0008047 enzyme activator activity | IDA PMID:22056329 Structure of the glycosyltransferase EryCIII in complex with... | NEW | Summary: Proposed NEW annotation capturing EryCII's actual molecular function: it allosterically activates the glycosyltransferase EryCIII (EryCIII is inactive on its own; activity is restored by EryCII). Not present in GOA, which carries only the inherited (incorrect) P450 catalytic terms. Reason: Replaces the removed P450 catalytic terms with EryCII's true, experimentally supported activator role. Supporting Evidence: PMID:22056329 EryCII stabilizes EryCIII and also functions as an allosteric activator of the GT |
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Download this section (compressed HTML)Q: Would a dedicated "glycosyltransferase activator activity" term (child of GO:0008047) better capture the role of EryCII and analogous P450-homologue GT activators (e.g. in other deoxysugar pathways)?
Experiment: Confirm absence of heme binding and monooxygenase activity for purified EryCII, and quantify EryCIII GT stimulation as a function of EryCII, to formally retire the P450 catalytic annotations.
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