LOW-SPIN HEME-A IN THE HEME-A BIOSYNTHETIC PROTEIN CTAA FROM BACILLUS-SUBTILIS

Synthesis of heme A from heme B (protoheme IX) most likely occurs in t wo steps with heme O as an intermediate. Bacillus subtilis CtaB, an in tegral membrane protein, functions in farnesylation of heme B to form heme O. CtaA, also a membrane protein, is required for heme A synthesi s from heme O and appears to be a monooxygenase and/or a dehydrogenase . Wild-type ctaA and ctaB expressed together from plasmids in B. subti lis resulted in CtaA containing equimolar amounts of low-spin heme B a nd heme A; this form of CtaA was named cyt ba-CTA. A mutant ctaB gene was identified and characterised. It encodes a truncated CtaB polypept ide. Wild-type ctaA and the mutant ctaB gene on plasmids resulted in C taA containing mainly low-spin heme B, this variant was named cyt b-CT A. The heme B component in cyt ba-CTA and cyt b-CTA showed identical p roperties; a mid-point redox potential of +85 mV, an EPR g(max) signal at 3.7, and a split alpha-band light absorption peak. The heme A comp onent in cyt ba-CTA showed a mid-point potential of +242 mV, an EPR g( max) signal at 3.5, and the alpha-band light absorption peak at 585 nm . It is suggested that the CtaA protein contains two heme binding site s, one for heme B and one for substrate heme. The heme B would play a role in electron transfer, i.e. function as a cytochrome, in the monoo xygenase and/or dehydrogenase reaction catalysed by CtaA whereas heme O/heme A would be substrate/product.