Cytochrome c release from the mitochondria into the cytoplasm is a pivotal step in intrinsic apoptosis. It forms the apoptosome by binding which protein to activate caspase-9?

• A Bcl-2
• B FADD (Fas-associated death domain protein)
• C FLIP (FLICE inhibitory protein)
• D Apaf-1 (Apoptotic protease activating factor 1) ✓

Explanation

Released cytochrome c binds Apaf-1 in the presence of dATP, causing conformational changes that enable Apaf-1 oligomerisation into a heptameric wheel structure, the apoptosome. The apoptosome recruits and activates procaspase-9, which then cleaves and activates executioner caspases-3, -6, and -7. Bcl-2 is an anti-apoptotic protein at the outer mitochondrial membrane that prevents cytochrome c release. FADD adapts death receptors (FasL, TRAIL) to caspase-8 in extrinsic apoptosis. FLIP inhibits caspase-8 activation at the death-inducing signalling complex.

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

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