In complex IV (cytochrome "c" oxidase; ), sometimes called cytochrome AA3, four electrons are removed from four molecules of cytochrome "c" and transferred to molecular oxygen (O2), producing two molecules of water. The complex contains coordinated copper ions and several heme groups. At the same time, eight protons are removed from the mitochondrial matrix (although only four are translocated across the membrane), contributing to the proton gradient. The exact details of proton pumping in complex IV are still under study. Cyanide is an inhibitor of complex 4.