3.D.4.5.2
Cytochrome aa3 quinol oxidase subunits I- IV (E - H). The 3-D structure has been determined (PDB# 6KOE). The quinol oxidases have an additional transmembrane helix (TMS0) in subunit I that is not present in the related cytochrome c oxidases. Xu et al. 2020 reported the 3.6 Å-resolution X-ray structure of the cytochrome aa3-600 menaquinol oxidase from Bacillus subtilis containing 1 equivalent of menaquinone. The structure shows that TM0 forms part of a cleft to accommodate the menaquinol-7 substrate. Crystals which had been soaked with the quinol-analog inhibitor HQNO (N-oxo-2-heptyl-4-hydroxyquinoline) or 3-iodo-HQNO revealed 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 (Xu et al. 2020).