Functional and structural role of the cytochrome b subunit of the membrane-bound hydrogenase complex of Alcaligenes eutrophus H16

This study shows that the product of the hoxZ gene of Alcaligenes eutrophus H16 is a b-type cytochrome (cytochrome b(z)), which is essential for anchoring the membrane-bound hydrogenase (MBH) complex to the periplasmic side of the membrane and for H-2-coupled respiration. The hoxZ product is not required for MBH translocation and H-2-dependent reduction of the redox dye, 2,3,5-triphenyl-2-tetrazolium chloride. The lack of cytochrome b(z) does not affect the electron-transport activities linked to oxidation of succinate and NADH, although it enhances the electron-flow rate through the cytochrome-e oxidase pathway in hoxZ Delta membranes. We show that the hoxZ product is a dihaem cytochrome b (haems with E-m7.0 of +10 mV and +166 mV) involved in H-2-dependent electron transfer. We conclude that cytochrome b, of the A. eutrophus MBH complex is the link necessary for transfer of electrons from H-2 to the ubiquinone pool and that it is required for attachment of MBH to the membrane.