cytochrome c oxidase

Cytochrome c oxidase is a large, multi-subunit transmembrane protein complex located in the inner mitochondrial membrane, comprising Complex IV of the electron transport chain (ETC). It contains heme groups (cytochrome a and a3) and copper centers (CuA and CuB) that facilitate the four-electron reduction of O2 to two molecules of H2O, the final step of oxidative phosphorylation. This reaction is coupled to proton pumping across the inner mitochondrial membrane, contributing to the electrochemical gradient that drives ATP synthase (Complex V).

Clinical significance: Cyanide (CN-) and azide bind tightly to the ferric iron (Fe3+) in the a3-CuB binuclear center, inhibiting electron transfer to oxygen. This blocks oxidative phosphorylation despite adequate oxygen delivery, causing 'histotoxic hypoxia'—cells cannot utilize oxygen even though it is present, leading to a shift to anaerobic metabolism, lactic acidosis, and cellular energy failure, particularly affecting high-ATP-demand tissues like the brain and heart. Treatment for cyanide toxicity includes hydroxocobalamin (binds CN- to form cyanocobalamin), sodium thiosulfate (converts CN- to thiocyanate via rhodanese), and historically amyl nitrite/sodium nitrite (induces methemoglobinemia, which has higher affinity for cyanide, pulling it off cytochrome c oxidase).

Carbon monoxide also inhibits cytochrome c oxidase (in addition to binding hemoglobin), contributing to its toxicity beyond hypoxemia alone.

Genetic defects in cytochrome c oxidase subunits are associated with mitochondrial diseases such as Leigh syndrome, presenting with lactic acidosis, developmental regression, and CNS involvement.

High-yield board points: Know that cyanide/azide inhibit Complex IV specifically; understand the concept of histotoxic hypoxia (normal PaO2, normal oxygen-carrying capacity, but impaired utilization); differentiate from anemic hypoxia (low O2 content) and ischemic hypoxia (reduced blood flow).