Heme A synthase from Bacillus subtilis and Aeropyrum pernix

Respiration in animals, plants and many bacteria is dependent on heme A, which functions as a prosthetic group in a-type cytochromes (terminal energy-transducing oxidases in the electron transport chain that reduce molecular oxygen to water). Heme A synthase catalyses a chemically very demanding reaction, the conversion of one specific methyl side-group of heme O to a formyl group of heme A. This thesis addresses the structure, evolution and enzyme reaction mechanism of heme A synthase. In the experimental work, the heme A synthase from the gram-positive, soil-bacterium, Bacillus subtilis and the hyper-thermophilic aerobic archaeon Aeropyrum pernix were exploited as model enzymes. In addition to increased knowledge about heme A synthesis, the research results contribute to our understanding of some mitochondria-linked diseases, bacterial physiology, membrane protein evolution, and enzyme catalysis.

Major results of the thesis: (i) A shortened, fully active, mutant of B. subtilis heme A synthase polypeptide (CtaA), obtained using promoted evolution, was shown to have essentially wild-type properties. (ii) A novel (compact) variant of heme A synthase polypeptide (cCtaA), of about half the size of CtaA, was identified in A. pernix. Some properties of the isolated enzyme, produced in Escherichia coli, were determined. (iii) Using a site-specific mutation approach, roles of invariant histidine and cysteine residues in the heme A synthases were analyzed. Some histidine residues probably function as axial heme ligands and cysteine residues can form disulfide bonds in the enzyme. (iv) A procedure for in vitro synthesis of the A. pernix cCtaA membrane protein in the presence of detergent was established. And (v) A system, LALA, for glycerol- or glycerol-3-phosphate dependent gene expression in gram-positive bacteria was constructed and validated in B. subtilis.