SUR2 (SYR2; YDR297W, P38992) notes

Identity and activity

• Sterol desaturase / fatty acid hydroxylase (His-box diiron) family; sphingolipid C4-hydroxylase converting sphinganine and dihydroceramide to phytosphingosine/phytoceramide, cyt b5-dependent [UniProt:P38992].
• Genetic and biochemical evidence [PMID:9556590 "yeast strains carrying a disrupted SYR2 allele produced sphingoid long chain bases lacking the 4-hydroxyl group present in wild type strains"; "4-hydroxylase activity was increased in microsomes prepared from a SYR2 overexpression strain"].
• Not essential PMID:9556590.
• ER localization PMID:8868422.

Curation decisions

• IBA sphingolipid delta-4 desaturase activity (PTN000933245, donor S. pombe SPBC887.15c) REMOVED: Sur2 hydroxylates C4; delta-4 desaturation is the DEGS family, absent from S. cerevisiae. PAINT node worth checking.
• YeastPathways RCA cytosol -> MODIFY to ER membrane.
• Core MF GO:0102772 sphingolipid C4-monooxygenase activity (EC 1.14.18.5). Note the GO definition is written for dihydroceramide; free sphinganine (RHEA:33519) is not among its Rhea xrefs, although YeastPathways uses the free-base reaction.