CYB5 (YNL111C, P40312) review notes

Identity

• Microsomal cytochrome b5, 120 aa, heme-binding cytochrome b5 domain, C-terminal tail anchor; ER membrane, cytoplasmic side (by similarity) [UniProt:P40312].
• Tail-anchored protein class inserted into the ER PMID:12514182.

Function

• Electron carrier for membrane oxygenases; role in fatty acid desaturation and sterol C-4 demethylation [UniProt:P40312].
• Yeast microsome studies: antibodies to cytochrome b5 inhibit C-4 demethylation but not 14alpha-demethylation PMID:6163470. The terminal cyanide-sensitive enzyme is Erg25 (C-4 methylsterol oxidase).
• Cyb5 with NADH-cytochrome b5 reductase can fully drive CYP51 (Erg11) PMID:10622712.
• Multicopy CYB5 suppresses ketoconazole hypersensitivity of the CPR (ncp1) mutant; cyb5 null viable PMID:8181746; CYB5 encodes the major microsomal b5 PMID:8181746.

Curation conclusions

• Core MF GO:0009055 electron transfer activity (heme b5 electron carrier); BP sterol/ergosterol biosynthesis (Erg25, Erg3, alternative supply to Erg11); CC ER membrane.
• Fatty acid desaturase Ole1 and the ceramide hydroxylase Scs7 carry their own fused b5 domains, so the role of free Cyb5 in fatty acid desaturation is less clear than for sterols.
• YeastCyc lists Cyb5 only as a generic reactant of ERG25/ERG3 reactions; module adds it.