Role of the Conserved Valine 236 in Access of Ligands to the Active Site of Thermus thermophilus ba Cytochrome Oxidase.

Knowledge of the role of conserved residues in the ligand channel of heme-copper oxidases is critical for understanding how the protein scaffold modulates the function of these enzymes. In this study, we investigated the role of the conserved valine 236 in the ligand channel of ba cytochrome c oxidase from Thermus thermophilus by mutating the residue to a more polar (V236T), smaller (V236A), or larger (V236I, V236N, V236L, V236M, and V236F) residue. The crystal structures of the mutants were determined, and the effects of the mutations on the rates of CO, O, and NO binding were investigated. O reduction and NO binding were unaffected in V236T, while the oxidation of heme b during O-O bond cleavage was not detected in V236A. The V236A results are attributed to a decrease in the rate of electron transfer between heme b and heme a during O-O bond cleavage in V236A, followed by faster re-reduction of heme b by Cu. This interpretation is supported by classical molecular dynamics simulations of diffusion of O to the active site in V236A that indicated a larger distance between the two hemes compared to that in the wild type and increased contact of heme a with water and weakened interactions with residues R444 and R445. As the size of the mutant side chain increased and protruded more into the ligand cavity, the rates of ligand binding decreased correspondingly. These results demonstrate the importance of V236 in facilitating access of ligands to the active site in T. thermophilus ba.