Cytochrome c oxidase subunit 5B, also known as COX5B, 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. Two isoforms of COX5 exist, COX5a and COX5b. When oxygen levels within the cell are high, transcription of COX5A (the aerobic isoform) is up-regulated as the rate of cellular respiration increases. Conversely, when oxygen levels are low, COX5B (the hypoxic isoform) transcription increases and functions to maximize the turnover rate of the COX apoenzyme. Recombinant human COX5B protein, fused to His-tag at N-terminus, was expressed in E.coli and purified by using conventional chromatography techniques.