Probing the haem d-binding site in cytochrome bd quinol oxidase by site-directed mutagenesis

Cytochrome bd is a cyanide-resistant terminal quinol oxidase under micro-aerophilic growth conditions and generates a proton motive force via scalar protolytic reactions. Protons used for dioxygen reduction are taken up from the cytoplasm and delivered to haem d through a proton channel. Electrons are transferred from quinols to haem d through haem b(558) and haem b(595). All three haems are bound to subunit I but only the axial ligand of haem d remains to be determined. Haems b(595) and d form a haemhaem binuclear centre and substitutions of either His19 in helix I (haem b(595) ligand) and Glu99 in helix III eliminated or severely reduced both haems. To probe the location of the haem d ligand, we introduced mutations around His19 and Glu99 and examined the cyanide-resistance of the oxidase activity and spectroscopic properties. In contrast to mutations around His19, I98F and L101T reduced the IC(50) for cyanide to 0.18 and 0.41 mM, respectively, from 1.4 mM of the wild-type. Blue shifts in the peak of I98F suggest that Ile98 is in the vicinity of the haem d-binding site. Our data are consistent with the proposal that Glu99 serves as a haem d ligand of cytochrome bd.