Cytochrome oxidase is the terminal oxidase of most of aerobic organisms and reduces molecular oxygen (O2) to water (1). The electrons and protons required for the formation of water molecules are transferred from both sides of the mitochondrial inner membranes in eukaryotic cells and of the cell membrane in prokaryotic cells (1). The migration of positive and negative charges from the different sides of the membrane produces electric potential across the membrane. In addition to the O2 reduction, this enzyme pumps protons from the inside to the outside of the membrane to produce a proton gradient across the membrane in addition to the membrane potential produced by the net migration of the positive charges (1,2). This enzyme contains heme iron and copper ions in the catalytic center (1). Because of the intriguing reaction of this copper-containing enzyme in addition to its physiological importance, many articles have been published on its structure and function, isolated from various organisms and tissues, in the last 75 yr or so (1,3,4) since its discovery by Warburg (5). Progress in understanding of the reaction mechanism of cytochrome-c oxidase has been accelerated by X-ray structures of the enzyme isolated from mammalian tissue and bacterial cells, which began to appear as late as in 1995 (6,7).