Role of His residues in Bacillus subtilis cytochrome b558 for haem binding and assembly of succinate:quinone oxidoreductase (complex II)

• Cytochrome 5558 in the cytoplasmic membrane ofBacilius subtiiis constitutes the anchor and electronacceptor to the flavoprotein (Fp) and iron-sulphurprotein (Ip) in succinate:quinone oxidoreductase, andseemingly contains two haem groups. EPR and MCDspectroscopic data indicate bis-imidazole ligation ofthe haem. Apo-cytochrome was found in the mem-brane fraction of haem-deficient B. subtilis, suggest-ing that during biogenesis of the oxidoreductase thecytochrome b558 polypeptide is embedded into themembrane prior to the incorporation of haem andsubsequent binding of Fp and Ip. The six His residuesin cytochrome b558 were individually changed to Tyrto attempt identification of residues serving as haemaxial ligands and to analyse the role of His residues forassembly and function of the oxidoreductase. Fromthe properties of the mutants, His-47 can be excludedas a haem ligand. The remaining His residues (atpositions 13,28,70,113 and 155) are located in or closeto four predicted transmembrane segments. TheTyr-28 and Tyr-70 mutant proteins appeared to lackone of the two haems. Only the Tyr-13 and Tyr-47mutant cytochromes were found to function asanchors for Fp and Ip, but the Tyr-13 mutant cyto-chrome assembles into an enzymatically defectivesuccinate:quinone oxidoreductase. It is concludedfrom a combination of the experimental findings,sequence comparisons and membrane topology datathat His-28, His-70 and His-155 are probably haemaxial ligands in a dihaem cytochrome 6558. His-70 andHis-155 may be tigands to the same haem.

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NATURVETENSKAP  -- Biologi -- Biokemi och molekylärbiologi (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences -- Biochemistry and Molecular Biology (hsv//eng)

NATURVETENSKAP  -- Biologi -- Mikrobiologi (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences -- Microbiology (hsv//eng)

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