complex IV

Complex IV, also known as cytochrome c oxidase, is the fourth and final complex in the mitochondrial electron transport chain (ETC) located in the inner mitochondrial membrane. It accepts electrons one at a time from reduced cytochrome c (which received them from Complex III via the Q cycle) and uses them to reduce molecular oxygen (O2) to two molecules of water. This reaction requires four electrons and four protons per O2 molecule. Complex IV contains two heme groups (cytochrome a and cytochrome a3) and two copper centers (CuA and CuB), which are essential for electron transfer and oxygen binding/reduction.

As electrons pass through Complex IV, protons are pumped from the mitochondrial matrix into the intermembrane space, contributing to the proton gradient (electrochemical gradient) that drives ATP synthase (Complex V) to produce ATP via oxidative phosphorylation. Complex IV pumps protons directly (unlike Complex II, which does not pump protons).

Clinical significance: Complex IV is the site of action for several classic toxins/poisons that cause cellular (histotoxic) hypoxia by blocking oxidative phosphorylation: - Cyanide (CN-) binds to the ferric (Fe3+) form of cytochrome a3, halting electron transport and ATP production, leading to cellular asphyxiation despite adequate blood oxygenation. Treatment includes hydroxocobalamin, sodium thiosulfate, and amyl nitrite. - Carbon monoxide (CO) also binds to Complex IV in addition to hemoglobin, though hemoglobin binding is more clinically prominent. - Sodium azide and hydrogen sulfide are also inhibitors used experimentally and encountered in toxicology.

Mitochondrial diseases such as Leigh syndrome and MELAS can involve mutations affecting Complex IV subunits, leading to lactic acidosis, neurodegeneration, and muscle weakness due to impaired ATP production. Complex IV deficiency is one of the most common causes of mitochondrial respiratory chain disorders in pediatric patients.

Understanding Complex IV is essential for grasping oxidative phosphorylation, toxicology of cyanide/CO poisoning, and the pathophysiology of mitochondrial myopathies—all high-yield topics for USMLE Step 1.