Structural characterization of the PCO/O2compound of cytochromecoxidase

Structural and functional mechanisms of cytochrome c oxidase

Cytochrome c oxidase (CcO) is the terminal enzyme in the electron transfer chain in mitochondria. It catalyzes the four-electron reduction of O2 to H2O and harnesses the redox energy to drive unidirectional proton translocation against a proton electrochemical gradient. A great deal of research has been conducted to comprehend the molecular properties of CcO. However, the mechanism by which the oxygen reduction reaction is coupled to proton translocation remains poorly understood. Here, we review the chemical properties of a variety of key oxygen intermediates of bovine CcO (bCcO) revealed by time-resolved resonance Raman spectroscopy and the structural features of the enzyme uncovered by serial femtosecond crystallography, an innovative technique that allows structural determination at room temperature without radiation damage. The implications of these data on the proton translocation mechanism are discussed.

The rate-limiting step in O(2) reduction by cytochrome ba(3) from Thermus thermophilus

Cytochrome ba(3) (ba(3)) of Thermus thermophilus (T. thermophilus) is a member of the heme-copper oxidase family, which has a binuclear catalytic center comprised of a heme (heme a(3)) and a copper (Cu(B)). The heme-copper oxidases generally catalyze the four electron reduction of molecular oxygen in a sequence involving several intermediates. We have investigated the reaction of the fully reduced ba(3) with O(2) using stopped-flow techniques. Transient visible absorption spectra indicated that a fraction of the enzyme decayed to the oxidized state within the dead time (~1ms) of the stopped-flow instrument, while the remaining amount was in a reduced state that decayed slowly (k=400s(-1)) to the oxidized state without accumulation of detectable intermediates. Furthermore, no accumulation of intermediate species at 1ms was detected in time resolved resonance Raman measurements of the reaction. These findings suggest that O(2) binds rapidly to heme a(3) in one fraction of the enzyme and progresses to the oxidized state. In the other fraction of the enzyme, O(2) binds transiently to a trap, likely Cu(B), prior to its migration to heme a(3) for the oxidative reaction, highlighting the critical role of Cu(B) in regulating the oxygen reaction kinetics in the oxidase superfamily.

Heme-heme communication during the alkaline-induced structural transition in cytochrome c oxidase

Alkaline-induced conformational changes at pH 12.0 in the oxidized as well as the reduced state of cytochrome c oxidase have been systematically studied with time-resolved optical absorption and resonance Raman spectroscopies. In the reduced state, the heme a(3) first converts from the native five-coordinate configuration to a six-coordinate bis-histidine intermediate as a result of the coordination of one of the Cu(B) ligands, H290 or H291, to the heme iron. The coordination state change in the heme a(3) causes the alteration in the microenvironment of the formyl group of the heme a(3) and the disruption of the H-bond between R38 and the formyl group of the heme a. This structural transition, which occurs within 1min following the initiation of the pH jump, is followed by a slower reaction, in which Schiff base linkages are formed between the formyl groups of the two hemes and their nearby amino acid residues, presumably R38 and R302 for the heme a and a(3), respectively. In the oxidized enzyme, a similar Schiff base modification on heme a and a(3) was observed but it is triggered by the coordination of the H290 or H291 to heme a(3) followed by the breakage of the native proximal H378-iron and H376-iron bonds in heme a and a(3), respectively. In both oxidation states, the synchronous formation of the Schiff base linkages in heme a and a(3) relies on the structural communication between the two hemes via the H-bonding network involving R438 and R439 and the propionate groups of the two hemes as well as the helix X housing the two proximal ligands, H378 and H376, of the hemes. The heme-heme communication mechanism revealed in this work may be important in controlling the coupling of the oxygen and redox chemistry in the heme sites to proton pumping during the enzymatic turnover of CcO.