heme A biosynthesis PMID:10675592 UPa:UPA00269 pathway PMID:16156667 Biosynthesis of heme A, a prosthetic group in many respiratory oxidases. It is synthesised from heme B (protoheme IX) with heme O as an intermediate. In Bacillus subtilis two genes required for heme A synthesis, ctaA and ctaB, have been identified. CtaB is the heme O synthase and CtaA is involved in the conversion of heme O to heme A. The model described by Brown et al. [PMID:10675592] is that Heme O Synthase (HOS) and Heme A Synthase (HAS) form a complex and that heme O is transferred directly from HOS to HAS. The biochemical reactions are not yet clearly defined. PMID:15491161 GO:0006784 KEGG:map00860 PMID:16321940 KEGG:map01110 cofactor biosynthesis porphyrin-containing compound metabolism porphyrin-containing compound biosynthesis