Synthesis of hemes found in heme-copper oxidases of Bacillus subtilis

Sammanfattning: The last step of aerobic respiration, the reduction of dioxygen to water, is catalysed by terminal oxidases. These oxidases can be divided into two unrelated families, the heme-copper oxidases and the cytochrome bd-type oxidases. These enzymes contain two or more heme molecules as prosthetic groups. Heme is an iron-containing ring-closed tetrapyrrole. All eukaryotic and most bacterial oxidases belong to the heme-copper superfamily. The Gram-positive bacterium Bacillus subtilis can synthesise two heme-copper oxidases; cytochrome caa3 and cytochrome aa3. Both these oxidases contain two heme A molecules. The cytochrome caa3 also contain one heme C. Heme B (protoheme IX) is the last common precursor for the synthesis of both heme A and heme C. Heme C is formed by covalent attachment of protoheme IX to a cysteine-containing motif -CXXCH- in the apocytochrome c polypeptide. This attachment occurs on the outside of the cytoplasmic membrane in bacteria and requires transport of heme B across the membrane. The ccmABCDEFGH genes are necessary for heme C synthesis in Escherichia coli. CcmABC consitutes a putative ABC-type transport system proposed to transport heme out of the cell. Using E. coli mutant strains and a periplasmic heme-reporter system it was shown that CcmA and CcmC are not required for transport of heme B to the periplasm. CcdA is a membrane protein required for cytochrome c syntheis in Bacillus subtilis. It was demonstrated that CcdA-deficient cells are blocked in heme C synthesis at some step after heme and apocytochrome have been exported across the cytoplasmic membrane. Heme A is synthesised from heme B with heme O as an intermediate. Two polytopic membrane proteins, encoded by the ctaA and ctaB genes, are essential for heme A synthesis in B. subtilis. CtaB catalyses the conversion of heme B to heme O. A ctaB paralogue, ctaO, has been found in B. subtilis. The CtaO protein is not required for heme O synthesis, but seems to have heme O synthase activity. CtaA functions in the conversion of heme O to heme A and is known to bind heme B and heme A. A system for easy and rapid purification of B. subtilis CtaA has been worked out. The transmembrane topology of CtaA has been determined. Four histidine residues, which are invariant in CtaA from different organisms, have been mutagenised. Analysis of the isolated mutant proteins demonstrates that CtaA can bind heme O and provides information on properties of the enzyme.