In the intrinsic pathway of apoptosis, cytochrome c release from mitochondria is triggered by pro-apoptotic BAX/BAK proteins. Cytochrome c then binds APAF-1 and procaspase-9 to form which structure?

• A Necrosome
• B Apoptosome ✓
• C Inflammasome
• D Death-inducing signaling complex (DISC)

Explanation

The intrinsic (mitochondrial) apoptosis pathway proceeds as: mitochondrial outer membrane permeabilization (MOMP) → cytochrome c release → cytochrome c binds APAF-1 (apoptotic protease-activating factor 1) → conformational change → oligomerization into the apoptosome (a heptameric wheel-shaped complex) → procaspase-9 recruitment and activation → activated caspase-9 cleaves and activates effector caspases (caspase-3, -7). The necrosome (option A) mediates necroptosis (RIPK1/RIPK3/MLKL). DISC (option D) forms in the extrinsic pathway (death receptor activation). The inflammasome (option C) activates caspase-1 in innate immunity.

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

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