In the intrinsic apoptotic pathway, cytochrome c release from mitochondria leads to apoptosome formation. The PRECISE molecular event triggered by cytochrome c in the cytosol is:

• A Cytochrome c directly cleaves and activates procaspase-3 without need for an adaptor protein
• B Cytochrome c activates FLIP, which then recruits procaspase-8 to form the apoptosome
• C Cytochrome c binds FADD, initiating the death-inducing signalling complex (DISC) formation
• D Cytochrome c binds Apaf-1, promoting its oligomerization and recruitment of procaspase-9 via CARD-CARD interaction ✓

Explanation

Released cytochrome c binds Apaf-1 (apoptotic protease activating factor 1) along with dATP/ATP, causing Apaf-1 to undergo conformational change and oligomerize into a heptameric wheel structure — the apoptosome. Exposed CARDs (caspase recruitment domains) of Apaf-1 recruit procaspase-9 via its CARD, causing autoproteolytic activation of caspase-9, which then cleaves and activates effector caspases 3, 6, and 7. FADD and DISC are components of the extrinsic (death receptor) pathway; FLIP inhibits caspase-8.

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

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