Knowledge Bank

Cytochrome c Oxidase contains two heme irons, a and $a3$, and two coppers, A and B. The protein catalyzes the reduction of dioxygen to water within the $a3$ heme pocket. Carbon monoxide, like $O2$, binds to the $a3$ iron, and it is photolizable with visible light. After photolysis the CO relaxes back to the $a3$Fe following a power law time dependence: (l + t/to). This relationship is expected from a distribution in activation enthalpy caused by a distribution in protein conformation (1), The peak of this enthalpy distribution, Hp, can be obtained by fitting the CO kinetics, simultaneously at all temperatures, to the function $(1+Ane-Hp/KTt)-n$ and is found to be 40kJ/mole. This value, much larger than for simpler heme proteins, has been explained by FTIR which has been used to identify the photolyzed CO associating with the B Cu (2). Since the CO cannot escape the heme pocket at these low temperatures ($T\leq$210K), we infer that the Fe and the Cu are separated by a clear pathway, 4 to 6 {\AA} long, within this pocket. That the two metals can both bind CO and that they are in close proximity should be helpful in elucidating the mechanism of $O2$ reduction. A passable intermediate would be Fe-0-0-Cu. The transmission spectrum of the Fe bound CO vibration has been fit very well to a Gaussian bandshape convoluted with a sin x/x instrument function at all temperatures between 10 and 275K. The Gaussian shape also reflects the distribution in conformational states frozen into the protein at low temperature. This work was supported in part by grants from NIH(HL-17839), AHA(78-1089) and the Ohio State University Instruction and Research Computer Center.