COQ3 notes (P27680, YOL096C)

• UbiG/COQ3-family SAM-dependent O-methyltransferase; cloned by complementation of a coq3 mutant PMID:1885593.
• One enzyme performs both O-methylations (DHHB -> HMHB, EC 2.1.1.114; demethyl-QH2 -> QH2, EC 2.1.1.64) [PMID:10419476 "one O-methyltransferase catalyzes both steps in Q biosynthesis in eukaryotes and prokaryotes"; "Both yeast and rat Coq3p recognize the demethyl-Q(3) precursor as a substrate."].
• Location: peripheral, matrix side of the mitochondrial inner membrane PMID:10419476.
• CoQ synthome member; stability depends on the other Coq proteins [PMID:10760477 "multi-subunit complex"; PMID:24406904 "Coq4 may be a crucial component through which Coq3, Coq5, Coq6, Coq7, and Coq9 associate to form the CoQ-synthome"].
• Redox state of the final-step substrate (quinol vs quinone) unresolved; UniProt lists both RHEA:44400 and RHEA:81215 [UniProt:P27680].
• YeastPathways RCA cytosol rows (PWY3O-19, PWY3O-862) are pathway-default compartment errors -> REMOVE.
• Review: 26 ACCEPT, 2 REMOVE (cytosol RCA), 1 NEW (GO:0110142 ubiquinone biosynthesis complex, part_of).