Cao (chlorophyll a oxygenase) is a rieske non-heme iron-sulfur protein located within the plastid-envelope inner and thylakoid membranes, that catalyzes the conversion of chlorophyllide a to chlorophyllide b. CAO is found not only in plants but also in chlorophytes and prochlorophytes. This domain represents the N-terminal rieske domain of the oxygenase alpha subunit. ROs comprise a large class of aromatic ring-hydroxylating dioxygenases that enable microorganisms to tolerate and utilize aromatic compounds for growth. The oxygenase alpha subunit contains an N-terminal Rieske domain with an [2Fe-2S] cluster and a C-terminal catalytic domain with a mononuclear Fe(II) binding site. The Rieske [2Fe-2S] cluster accepts electrons from a reductase or ferredoxin component and transfers them to the mononuclear iron for catalysis. Cao is closely related to several other plant RO's including Tic 55, a 55 kDa protein associated with protein transport through the inner chloroplast membrane; Ptc 52, a novel 52 kDa protein isolated from chloroplasts; and LLS1/Pao (Lethal-leaf spot 1/pheophorbide a oxygenase).
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