Which enzyme family is primarily responsible for executing the apoptotic cascade and is absent or non-functional in necrosis?

Explanation

## Caspases: The Executioners of Apoptosis **Key Point:** Caspases (cysteine-aspartic proteases) are the central executioners of apoptosis. Their activation and function are the defining biochemical feature that distinguishes apoptosis from necrosis. ### Caspase Classification and Function **Initiator Caspases** (extrinsic and intrinsic pathways) - Caspase-8 (death receptor pathway) - Caspase-9 (mitochondrial pathway) - Activated by adaptor proteins and oligomerization **Executioner Caspases** - Caspase-3 (primary executioner) - Caspase-6, Caspase-7 - Cleave structural and regulatory proteins ### Caspase Substrates and Outcomes 1. **Poly(ADP-ribose) polymerase (PARP)** — DNA repair inhibition 2. **Lamin A/C** — nuclear lamina breakdown → chromatin condensation 3. **Inhibitor of caspase-activated DNase (ICAD)** — release of CAD/DFF45 → internucleosomal DNA cleavage 4. **Cytoskeletal proteins** — cell rounding and blebbing **Mnemonic:** **CASPASE** = **C**ysteine-**A**spartic protease, **S**erine protease family, **A**ctivates **P**roteolysis, **A**poptosis **S**ignal **E**xecution ### Why Caspases Are Absent in Necrosis ```mermaid flowchart TD A[Cell Injury]:::outcome --> B{Energy Available?}:::decision B -->|Yes| C[Apoptotic Pathway]:::action C --> D[Caspase Activation]:::action D --> E[Controlled Cell Death]:::outcome B -->|No| F[Necrotic Pathway]:::action F --> G[No Caspase Activation]:::action G --> H[Uncontrolled Cell Lysis]:::outcome ``` **High-Yield:** Necrosis is an ATP-independent process and does NOT activate caspases. Apoptosis is ATP-dependent and REQUIRES caspase cascade activation. This is a fundamental distinction tested frequently in NEET PG. **Clinical Pearl:** Caspase inhibitors (e.g., Z-VAD-FMK) can block apoptosis in experimental settings, demonstrating the essential role of caspases. In necrosis, such inhibitors have no effect.