COX5A, also known as COX, COX-VA and VA, belongs to the cytochrome c oxidase subunit 5A family. Cytochrome c oxidase (COX), the terminal component of the mitochondrial respiratory chain, catalyzes the electron transfer from reduced cytochrome c to oxygen. This component is a heteromeric complex consisting of 3 catalytic subunits encoded by mitochondrial genes and multiple structural subunits encoded by nuclear genes. The mitochondrially-encoded subunits function in electron transfer, and the nuclear-encoded subunits may function in the regulation and assembly of the complex. COX5A is the heme A-containing chain of cytochrome c oxidase, the terminal oxidase in mitochondrial electron transport. Recombinant human COX5A protein, fused to His-tag at N-terminus, was expressed in E.coli and purified by using conventional chromatography techniques.