This entry includes fatty acid and carotene hydroxylases and sterol desaturases. Beta-carotene hydroxylase is involved in zeaxanthin synthesis by hydroxylating beta-carotene, but the enzyme may be involved in other pathways [1]. This family includes C-5 sterol desaturase and C-4 sterol methyl oxidase [2,3,6]. Members of this family are involved in cholesterol biosynthesis and biosynthesis a plant cuticular wax [4]. These enzymes contain two copies of a HXHH motif which coordinate two irons at the catalytic centre. Members of this family are ER integral membrane proteins that share a novel mushroom-shaped fold consisting of four transmembrane (TM1-TM4) helices that anchor them to the membrane capped by a cytosolic domain containing the unique histidine- coordinating di metal centre [5].

1: Fah1p, a Saccharomyces cerevisiae cytochrome b5 fusion protein, and its Arabidopsis thaliana homolog that lacks the cytochrome b5 domain both function in the alpha-hydroxylation of sphingolipid-associated very long chain fatty acids. Mitchell AG, Martin CE; J Biol Chem 1997;272:28281-28288. PMID:9353282

2: The zeaxanthin biosynthesis enzyme beta-carotene hydroxylase is involved in myxoxanthophyll synthesis in Synechocystis sp. PCC 6803. Lagarde D, Vermaas W; FEBS Lett 1999;454:247-251. PMID:10431816

3: Cloning, disruption and sequence of the gene encoding yeast C-5 sterol desaturase. Arthington BA, Bennett LG, Skatrud PL, Guynn CJ, Barbuch RJ, Ulbright CE, Bard M; Gene 1991;102:39-44. PMID:1864507

4: Cloning and characterization of ERG25, the Saccharomyces cerevisiae gene encoding C-4 sterol methyl oxidase. Bard M, Bruner DA, Pierson CA, Lees ND, Biermann B, Frye L, Koegel C, Barbuch R; Proc Natl Acad Sci U S A 1996;93:186-190. PMID:8552601

5: Molecular characterization of the CER1 gene of arabidopsis involved in epicuticular wax biosynthesis and pollen fertility. Aarts MG, Keijzer CJ, Stiekema WJ, Pereira A; Plant Cell 1995;7:2115-2127. PMID:8718622

6: The Crystal Structure of an Integral Membrane Fatty Acid alpha-Hydroxylase. Zhu G, Koszelak-Rosenblum M, Connelly SM, Dumont ME, Malkowski MG; J Biol Chem. 2015;290:29820-29833. PMID:26515067

7: Delta7-sterol-C5-desaturase: molecular characterization and functional expression of wild-type and mutant alleles. Husselstein T, Schaller H, Gachotte D, Benveniste P; Plant Mol Biol. 1999;39:1006113919172. PMID:10344195