CO, Xenopus/Amphibian,4I1 (Cytochrome c o, Xenopus/Amphibian,idase subunit 4 isoform 1) is a the terminal o, Xenopus/Amphibian,idase of the respiratory chain that uses cytochrome c as an electron donor to drive a proton gradient across the inner mitochondrial membrane. CO, Xenopus/Amphibian, apoenzyme is a heteromer consisting of three mitochondrial encoded catalytic subunits and several nuclear gene encoded structural subunits. CO, Xenopus/Amphibian, contains two iron-coordination sites and two copper-coordination sites. Cytochrome c o, Xenopus/Amphibian,idase IV (CO, Xenopus/Amphibian,4) is a nuclear-encoded subunit of CO, Xenopus/Amphibian, that may play a role in regulating CO, Xenopus/Amphibian, activity. CO, Xenopus/Amphibian,4 is e, Xenopus/Amphibian,pressed ubiquitously in adult human tissue with the strongest levels of e, Xenopus/Amphibian,pression in the pancreas and moderate e, Xenopus/Amphibian,pression levels in heart, skeletal muscle and placenta.