Cytochrome A

Cytochrome a is a protein found in Complex III of the Electron Transport Chain. Cytochromes are a family of proteins that contain an iron-containing heme group. They are found in mitochondria (cristae) and chloroplasts as part of the electron transport chain (in thylakoids).

Cytochrome a is involved in electron transfer within the complex. Complex IV contains a cytochrome a/a3-domain that transfers electrons and catalyzes the reaction of oxygen to water.

Cytochrome a is found in:

• Complex III: Cytochrome b and cytochrome c1 are found in complex III
• Complex IV: Cytochrome a and cytochrome a3 are found in complex IV
• Cytochrome A–cytochrome A3 subcomplex: Hydrogen cyanide blocks oxidative phosphorylation and mitochondrial oxygen utilization by binding to this subcomplex

Cytochrome aa3 is the terminal respiratory enzyme of all eukaryotes and many bacteria and archaea. It reduces O2 to water and harnesses the free energy from the reaction to generate the transmembrane electrochemical potential.