Cytochrome c oxidase (COX) is the terminal enzyme of the mitochondrial respiratory chain.It is a multi-subunit enzyme complex that couples the transfer of electrons from cytochrome c to molecular oxygen and contributes to a proton electrochemical gradient across the inner mitochondrial membrane.The complex consists of 13 mitochondrial- and nuclear-encoded subunits.The mitochondrially-encoded subunits perform the electron transfer and proton pumping activities.The functions of the nuclear-encoded subunits are unknown but they may play a role in the regulation and assembly of the complex.This gene encodes the nuclear-encoded subunit IV isoform 1 of the human mitochondrial respiratory chain enzyme.It is located at the 3' of the NOC4 (neighbor of COX4) gene in a head-to-head orientation,and shares a promoter with it.[provided by RefSeq,Jul 2008]