Some Spectroscopic Views of the CuA, Site in Cytochrome c Oxidase Preparations a

Cytochrome c oxidase couples the reduction of oxygen to water with the phosphorylation of ADP to ATP in all eukaryotic organisms as well as in some aerobic bacteria. Two heme groups of the a class and two copper ions are intimately involved in the electron transfer and oxygen-reducing activity of the enzyme. Cytochrome a and CU, are magnetically isolated implying a distance apart of > 12& whereas cytochrome a3 and the other copper ion, Cu,, are close to one another and act together to bind dioxygen and reduce it to water, stabilizing and retaining in bound form all the intermediates on the pathway. Cytochrome a and Cu, are thought to act as the electron entry pole of the enzyme and to provide electrons to the binuclear center. However, it is now apparent that this is an oversimplification because electron entry into cytochrome a and Cu, is accompanied by major conformational changes’ which seem to play a regulatory role on the ligand-binding properties of the binuclear center and electron flux through the enzyme. Recent EXAFS studies’ suggest that ligand exchange occurs at the Cu, site following reduction of the metal leading to the loss of a thiol group. This ligdnd switch cysteine tyrosinate forms the basis for a recent proposal for the mechanism of the proton pump. There is unequivocal evidence from a variety of spectroscopies that the heme of cytochrome a is liganded by two histidine ligands and remains as a low-spin complex throughout the enzyme cycle.’~~ Copper, has been the subject of investigation by electron paramagnetic resonance (EPR)5 and, more recently electron nuclear double resonance (ENDOR) spectroscopy!~~ However, because the optical absorption of the heme centers is so intense and extends over so wide a range of wavelength, the optical absorption, spectrum of Cui is largely unknown. Knowledge of the optical properties of this center would help to