Beta-carotene ketolase/oxygenase (CrtW, also known as CrtO), the carotenoid astaxanthin biosynthetic enzyme, initially catalyzes the addition of two keto groups to carbons C4 and C4' of beta-carotene. Carotenoids are important natural pigments produced by many microorganisms and plants. Astaxanthin is reported to be an antioxidant, an anti-cancer agent, and an immune system stimulant. A number of bacteria and green algae can convert beta-carotene into astaxanthin by using several ketocarotenoids as intermediates and CrtW and a beta-carotene hydroxylase (CrtZ). CrtW initially converts beta-carotene to canthaxanthin via echinenone, and CrtZ initially mediates the conversion of beta-carotene to zeaxanthin via beta-cryptoxanthin. After a few more intermediates are formed, CrtW and CrtZ act in combination to produce astaxanthin. Sequences of this domain family appear to be structurally related to membrane fatty acid desaturases and alkane hydroxylases. They all share in common extensive hydrophobic regions that are capable of spanning the membrane bilayer at least twice. Comparison of these sequences also reveals three regions of conserved histidine cluster motifs that contain eight histidine residues: HXXXH, HXXHH, and HXXHH. These histidine residues are reported to be catalytically essential and proposed to be the ligands for the iron atoms contained within homologs, stearoyl CoA desaturase and alkane hydroxylase.
Jiyao W et al.(2023). "The conserved domain database in 2023.",