Catalyzes the second reaction in the biosynthesis of heme A, a prosthetic group of mitochondrial cytochrome c oxidase (CcO) (PubMed:12474143). Heme A is synthesized from heme B by two sequential enzymatic reactions catalyzed by heme O synthase (HOS) and heme A synthase (HAS). HAS catalyzes the conversion of heme O to heme A by two successive hydroxylations of the methyl group at C8, in a reaction that involves matrix ferredoxin and ferredoxin reductase. The first hydroxylation forms heme I, the second hydroxylation results in an unstable dihydroxymethyl group, which spontaneously dehydrates, resulting in the formyl group of heme A (By similarity).
