High stability of the radical at the catalytic center of cytochrome c oxidase

In aerobic organisms, cellular respiration is associated with electron transfer through a respiratory system of membrane-bound complexes. This electron flow is terminated by the reduction of dioxygen to water by respiratory oxidases. Cytochrome c oxidase (CcO) is a widely distributed heme-copper-oxygen reductase (HCO) found in all mitochondria and some bacteria. However, the sequential reduction of O₂ to water in CcO generates a protein-based radical at the catalytic heme a₃-Cu_B site. To avoid the potential damage from the radical, CcO has apparently developed protective mechanisms. Protection by transfer of the highly oxidizing equivalent over considerable distances away from the catalytic site by redox-active Tyr/Trp chains has been previously demonstrated in bovine CcO. However, the rate of the radical migration from the catalytic center has not yet been determined for any HCO. In this work, we show that the radical escapes from the catalytic center of the ferryl P_M intermediate of bovine CcO within minutes, which is much longer than the time of its functional reduction during cellular respiration. Apparently, this high stability has evolved to avoid the dissipation of energy released during the oxygen reduction with substrate electrons.