Cytochrome c is released from mitochondria during apoptosis. Once released into the cytosol, cytochrome c binds to a protein called APAF-1 (apoptosis-activating factor-1), forming a multimeric structure called the apoptosome. This complex binds to caspase-9, the critical initiator caspase of the mitochondrial pathway, and promotes its autocatalytic cleavage, generating catalytically active forms of the enzyme.
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