Cytochrome C is an electron transporting protein that resides within the intermembrane space of the mitochondria, where it plays a critical role in the process of oxidative phosphorylation and production of cellular ATP. An increasing amount of interest has been directed toward the role which cytocrome C has been demonstrated to play in apoptotic processes. Following exposure to apoptotic stimuli, cytochrome C is rapidly released from the mitochondria into the cytosol, an event which may be required for the completion of apoptosis in some systems. Cytosolic cytochrome C functions in the activation of caspase 3, an ICE family molecule that is a key effector of apoptosis.
Function: Electron carrier protein. The oxidized form of the cytochrome c heme group can accept an electron from the heme group of the cytochrome c1 subunit of cytochrome reductase. Cytochrome c then transfers this electron to the cytochrome oxidase complex, the final protein carrier in the mitochondrial electron-transport chain. Plays a role in apoptosis. Suppression of the anti-apoptotic members or activation of the pro-apoptotic members of the Bcl-2 family leads to altered mitochondrial membrane permeability resulting in release of cytochrome c into the cytosol. Binding of cytochrome c to Apaf-1 triggers the activation of caspase-9, which then accelerates apoptosis by activating other caspases.
Subcellular Location: Mitochondrion intermembrane space. Note=Loosely associated with the inner membrane.
Post-translational modifications: Binds 1 heme group per subunit. Phosphorylation at Tyr-49 and Tyr-98 both reduce by half the turnover in the reaction with cytochrome c oxidase, down-regulating mitochondrial respiration.
DISEASE: Defects in CYCS are the cause of thrombocytopenia type 4 (THC4) [MIM:612004]; also known as autosomal dominant thrombocytopenia type 4. Thrombocytopenia is the presence of relatively few platelets in blood. THC4 is a non-syndromic form of thrombocytopenia. Clinical manifestations of thrombocytopenia are absent or mild. THC4 may be caused by dysregulated platelet formation.