Effective Pumping Proton Collection Facilitated by a Copper Site (CUB) of Bovine Heart Cytochrome c Oxidase, Revealed by a Newly Developed Time-resolved Infrared System

Background: Cytochrome c oxidase reduces O-2 coupled with proton pumping. Results: A newly developed time-resolved infrared system reveals transient conformational changes in the proton-pumping pathway upon CO binding to Cu-B in the O-2 reduction site. Conclusion: Cu-B promotes proton collection and effective blockage of back-leak of pumping protons. Significance: These critical findings in bioenergetics stimulate the new infrared approach for mechanistic investigation of any other protein function. X-ray structural and mutational analyses have shown that bovine heart cytochrome c oxidase (CcO) pumps protons electrostatically through a hydrogen bond network using net positive charges created upon oxidation of a heme iron (located near the hydrogen bond network) for O-2 reduction. Pumping protons are transferred by mobile water molecules from the negative side of the mitochondrial inner membrane through a water channel into the hydrogen bond network. For blockage of spontaneous proton back-leak, the water channel is closed upon O-2 binding to the second heme (heme a(3)) after complete collection of the pumping protons in the hydrogen bond network. For elucidation of the structural bases for the mechanism of the proton collection and timely closure of the water channel, conformational dynamics after photolysis of CO (an O-2 analog)-bound CcO was examined using a newly developed time-resolved infrared system feasible for accurate detection of a single C=O stretch band of -helices of CcO in H2O medium. The present results indicate that migration of CO from heme a(3) to Cu-B in the O-2 reduction site induces an intermediate state in which a bulge conformation at Ser-382 in a transmembrane helix is eliminated to open the water channel. The structural changes suggest that, using a conformational relay system, including Cu-B, O-2, heme a(3), and two helix turns extending to Ser-382, Cu-B induces the conformational changes of the water channel that stimulate the proton collection, and senses complete proton loading into the hydrogen bond network to trigger the timely channel closure by O-2 transfer from Cu-B to heme a(3).