Structure of the cytochrome <i>aa</i>_<i>3</i> -600 heme-copper menaquinol oxidase bound to inhibitor HQNO shows TM0 is part of the quinol binding site.

Abstract

Virtually all proton-pumping terminal respiratory oxygen reductases are members of the heme-copper oxidoreductase superfamily. Most of these enzymes use reduced cytochrome <i>c</i> as a source of electrons, but a group of enzymes have evolved to directly oxidize membrane-bound quinols, usually menaquinol or ubiquinol. All of the quinol oxidases have an additional transmembrane helix (TM0) in subunit I that is not present in the related cytochrome <i>c</i> oxidases. The current work reports the 3.6-Å-resolution X-ray structure of the cytochrome <i>aa</i>_<i>3</i> -600 menaquinol oxidase from <i>Bacillus subtilis</i> containing 1 equivalent of menaquinone. The structure shows that TM0 forms part of a cleft to accommodate the menaquinol-7 substrate. Crystals which have been soaked with the quinol-analog inhibitor HQNO (<i>N</i>-oxo-2-heptyl-4-hydroxyquinoline) or 3-iodo-HQNO reveal a single binding site where the inhibitor forms hydrogen bonds to amino acid residues shown previously by spectroscopic methods to interact with the semiquinone state of menaquinone, a catalytic intermediate.