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

Question

Accepted Answer

C. Caspases. ## 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.

Answer

A. Kinases

Answer

B. Proteases

Answer

D. Phosphatases

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

Caspase Classification and Function

Caspase Substrates and Outcomes

Why Caspases Are Absent in Necrosis

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