The active site of the copper-containing protein, cytochrome c oxidase CcO , consists of a binuclear center formed by a high-spin heme a3 and a copper ion CuB . This binuclear center is the location where the reduction of molecular oxygen to water takes place during aerobic respiration.The crucial role of the active site in the enzyme's catalytic mechanism can be explained through the following steps:1. Electron transfer: Cytochrome c oxidase receives electrons from cytochrome c, a small electron carrier protein. These electrons are transferred to the heme a3-CuB binuclear center via another heme group, heme a, and a copper center CuA .2. Oxygen binding: Molecular oxygen O2 binds to the heme a3 iron atom at the active site. The presence of the CuB ion helps to stabilize the binding of oxygen and facilitates its reduction.3. Proton transfer: Protons are transferred from the mitochondrial matrix to the active site through a series of proton-conducting channels. These protons are essential for the reduction of oxygen to water.4. Oxygen reduction: The electrons and protons at the active site are used to reduce the bound oxygen molecule. This process occurs in a stepwise manner, with the formation of intermediate species such as peroxide and oxoferryl species. The final product of this reduction is water H2O .5. Proton pumping: During the catalytic cycle, cytochrome c oxidase also pumps protons from the mitochondrial matrix to the intermembrane space, contributing to the generation of a proton gradient across the inner mitochondrial membrane. This proton gradient is essential for the production of ATP through oxidative phosphorylation.In summary, the active site of cytochrome c oxidase, consisting of the heme a3-CuB binuclear center, plays a crucial role in the enzyme's catalytic mechanism by facilitating the reduction of molecular oxygen to water and contributing to the generation of a proton gradient for ATP production in aerobic respiration.