Structural studies on bovine heart cytochrome c oxidase

Among the X-ray structures of bovine heart cytochrome c oxidase (CcO), reported thus far, the highest resolution is 1.8 angstrom. CcO includes 13 different protein subunits, 7 species of phospholipids, 7 species of triglycerides, 4 redox-active metal sites (Cu-A, heme a (Fe-a), Cu-B, heme a(3) (Fe-a3)) and 3 redox-inactive metal sites (Mg2+, Zn2+ and Na+). The effects of various O-2 analogs on the X-ray structure suggest that O-2 molecules are transiently trapped at the CUB site before binding to Fe-a3(2+) to provide O-2(-). This provides three possible electron transfer pathways from Cu-B, Fe-a3 and Tyr244 via a water molecule. These pathways facilitate non-sequential 3 electron reduction of the bound O-2(-) to break the O-O bond without releasing active oxygen species. Bovine heart CcO has a proton conducting pathway that includes a hydrogen-bond network and a waterchannel 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 Fe-a 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. (C) 2011 Published by Elsevier B.V.