- Cytochrome c oxidase subunit I (COI or COX1) is a protein encoded by the mitochondrial genome that plays a vital role in cellular respiration, specifically within the electron transport chain.
- It is one of the key subunits of cytochrome c oxidase (Complex IV)—the terminal enzyme of the mitochondrial respiratory chain—which catalyzes the final step in oxidative phosphorylation: the transfer of electrons from cytochrome c to molecular oxygen, reducing it to water. This reaction is crucial for maintaining the proton gradient across the inner mitochondrial membrane, which drives the synthesis of ATP, the cell’s main energy currency.
- The COI protein is a transmembrane component of Complex IV and is highly conserved across a wide range of eukaryotic organisms, reflecting its essential function in energy metabolism. It contains multiple metal-binding sites, including heme and copper centers (heme a, heme a3, and CuB), which facilitate electron transfer and the reduction of oxygen. These redox-active centers are embedded within the COI subunit and are critical for the enzymatic activity of cytochrome c oxidase. The integrity of COI is therefore fundamental to efficient mitochondrial function and overall cellular energy production.
- Beyond its biochemical role, COI is widely used as a genetic marker in molecular biology, particularly in DNA barcoding for species identification. Due to its moderate rate of evolutionary change, the COI gene sequence provides a reliable means of distinguishing between species while remaining conserved enough to design universal primers for a broad range of animals. A 648-base pair fragment of the COI gene, typically located at the 5′ end, is the standard region used in animal DNA barcoding. This segment offers enough variability to differentiate most species but is stable enough within a species to ensure accurate identification.
- The utility of COI in DNA barcoding was first proposed and standardized in the early 2000s and has since been applied to tens of thousands of animal species. It has become a cornerstone of biodiversity assessment, ecological research, and forensic biology. For instance, COI barcoding is used to verify the identity of fish and meat products, detect invasive or endangered species, and catalog organisms in environmental samples through metabarcoding techniques.
- Despite its effectiveness in animals, COI is less suitable for plants and fungi, which generally require different genomic regions for accurate barcoding due to lower variability in mitochondrial DNA. In plants, chloroplast genes such as rbcL and matK are preferred, while fungi are often identified using ITS (internal transcribed spacer) sequences from nuclear ribosomal DNA.
