Cytochrome c release from mitochondria during intrinsic apoptosis leads to formation of the apoptosome. Which protein does cytochrome c bind to form this complex?

• A FADD (Fas-associated death domain protein)
• B Apaf-1 (apoptotic protease activating factor-1) ✓
• C Procaspase-8 in the death-inducing signaling complex (DISC)
• D Bcl-2, which then activates Bax to release further cytochrome c

Explanation

Released cytochrome c binds Apaf-1 in the cytosol, triggering Apaf-1 oligomerisation into a heptameric wheel structure — the apoptosome. This recruits and activates procaspase-9, which then activates executioner caspases-3, -6, and -7. FADD and DISC are components of the extrinsic pathway (death receptor-mediated). Procaspase-8 is activated by DISC. Bcl-2 is anti-apoptotic and inhibits cytochrome c release by neutralising pro-apoptotic Bax/Bak.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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