Structural studies on bovine heart cytochrome c oxidase

Abstract

Among the X-ray structures of bovine heart cytochrome c oxidase (CcO), reported thus far, the highest resolution is 1.8Å. CcO includes 13 different protein subunits, 7 species of phospholipids, 7 species of triglycerides, 4 redox-active metal sites (CuA, heme a (Fea), CuB, heme a3 (Fea3)) and 3 redox-inactive metal sites (Mg2+, Zn2+ and Na+).The effects of various O2 analogs on the X-ray structure suggest that O2 molecules are transiently trapped at the CuB site before binding to Fea32+ to provide O2−. This provides three possible electron transfer pathways from CuB, Fea3 and Tyr244 via a water molecule. These pathways facilitate non-sequential 3 electron reduction of the bound O2− to break the OO bond without releasing active oxygen species.Bovine heart CcO has a proton conducting pathway that includes a hydrogen-bond network and a water-channel which, in tandem, connect the positive side phase with the negative side phase. The hydrogen-bond network forms two additional hydrogen-bonds with the formyl and propionate groups of heme a. Thus, upon oxidation of heme a, the positive charge created on Fea is readily delocalized to the heme peripheral groups to drive proton-transport through the hydrogen-bond network. A peptide bond in the hydrogen-bond network and a redox-coupled conformational change in the water channel are expected to effectively block reverse proton transfer through the H-pathway. These functions of the pathway have been confirmed by site-directed mutagenesis of bovine CcO expressed in HeLa cells. This article is part of a Special Issue entitled: Respiratory Oxidases.