A 38-year-old woman from Delhi presents with progressive muscle weakness, fatigue, and recurrent infections over 3 months. She has a history of rheumatoid arthritis treated with methotrexate for 2 years. Laboratory investigations show: Hb 9.2 g/dL, WBC 3.2 × 10⁹/L, platelets 120 × 10⁹/L. Serum folate is low at 2.1 ng/mL (normal 5.4 ng/mL). Her bone marrow shows megaloblastic changes. Which type of enzyme inhibition mechanism best explains methotrexate's toxicity in this patient?

Question

A 38-year-old woman from Delhi presents with progressive muscle weakness, fatigue, and recurrent infections over 3 months. She has a history of rheumatoid arthritis treated with methotrexate for 2 years. Laboratory investigations show: Hb 9.2 g/dL, WBC 3.2 × 10⁹/L, platelets 120 × 10⁹/L. Serum folate is low at 2.1 ng/mL (normal >5.4 ng/mL). Her bone marrow shows megaloblastic changes. Which type of enzyme inhibition mechanism best explains methotrexate's toxicity in this patient?

Accepted Answer

B. Competitive inhibition of dihydrofolate reductase with reversible binding to the active site. ## Mechanism of Methotrexate Toxicity

**Key Point:** Methotrexate is a structural analogue of dihydrofolate (DHF) and acts as a **competitive inhibitor** of dihydrofolate reductase (DHFR), the enzyme that converts DHF to tetrahydrofolate (THF).

## Why Competitive Inhibition?

Methotrexate competes with the natural substrate (DHF) for binding to the active site of DHFR. This competition is:
- **Reversible** — the drug binds non-covalently through hydrogen bonds and van der Waals forces
- **Substrate-dependent** — excess folate can partially overcome the inhibition
- **Kinetically characterized** — increases apparent Km while Vmax remains unchanged

## Clinical Consequence: Folate Depletion

1. DHFR inhibition → ↓ THF production
2. ↓ THF → impaired one-carbon transfer reactions
3. ↓ dTMP and purine synthesis → impaired DNA replication
4. Rapidly dividing cells (bone marrow, GI epithelium) are most affected
5. Result: **Megaloblastic anemia, pancytopenia, mucositis**

**High-Yield:** This patient's low serum folate and megaloblastic bone marrow changes are classic signs of competitive DHFR inhibition leading to folate deficiency. The reversible nature is why **folinic acid (leucovorin) rescue** works — it bypasses the blocked DHFR step.

## Comparison with Other Inhibition Types

| Inhibition Type | Reversibility | Substrate Effect | Km/Vmax Change | MTX Fit? |
|---|---|---|---|---|
| **Competitive** | Reversible | ↑ Km, ↓ Vmax | Km ↑, Vmax unchanged | **YES** |
| Non-competitive | Reversible | No substrate rescue | Both ↓ | No |
| Irreversible/Suicide | Irreversible | Permanent inactivation | N/A | No |
| Uncompetitive | Reversible | Binds ES complex only | Both ↓ proportionally | No |

**Clinical Pearl:** Methotrexate's competitive inhibition is **therapeutically exploitable** — high-dose MTX followed by leucovorin rescue is a standard chemotherapy protocol. Leucovorin (5-formyl-THF) enters cells via a different transporter and replenishes THF pools, selectively sparing normal cells while cancer cells (with higher DHFR activity) are still affected.

[cite:Lehninger Principles of Biochemistry Ch 8]

Answer

A. Non-competitive inhibition causing allosteric changes in dihydrofolate reductase structure

Answer

C. Uncompetitive inhibition affecting the enzyme-substrate complex formation

Answer

D. Irreversible covalent inhibition of dihydrofolate reductase via suicide substrate mechanism

Explanation

Practice similar questions