Warfarin exerts its anticoagulant effect by inhibiting which vitamin K-dependent clotting factors?

Explanation

## Warfarin's Mechanism **Key Point:** Warfarin inhibits the vitamin K-dependent carboxylation of clotting factors II (prothrombin), VII, IX, and X. It blocks the enzyme vitamin K epoxide reductase (VKOR), preventing the regeneration of reduced vitamin K (the active cofactor). ## The Vitamin K Cycle 1. Vitamin K (phylloquinone) is reduced to vitamin K hydroquinone (active form) 2. Vitamin K hydroquinone acts as a cofactor for γ-carboxylation of glutamic acid residues on factors II, VII, IX, X 3. γ-carboxylated factors can bind calcium and phospholipids → functional clotting cascade 4. Warfarin blocks step 1 → reduced vitamin K cannot be regenerated → factors remain uncarboxylated and nonfunctional ## Mnemonic: **PIVKA** (Proteins Induced by Vitamin K Absence) **P**rothrombin (II), **I**ntermediaries (VII, IX), **V**itamin K-dependent factors, **K**ey ones, **A**ll anticoagulated Better mnemonic: **2, 7, 9, 10** — the four vitamin K-dependent factors inhibited by warfarin. ## Why NOT the Others? | Factor | Vitamin K-Dependent? | Warfarin-Affected? | | --- | --- | --- | | Factor II (Prothrombin) | Yes | **Yes** | | Factor V | No | No | | Factor VII | Yes | **Yes** | | Factor VIII | No | No | | Factor IX | Yes | **Yes** | | Factor X | Yes | **Yes** | | Factor XII (Hageman) | No | No | **High-Yield:** Factor V is NOT vitamin K-dependent; it is synthesized as an inactive precursor and activated by thrombin. This is a classic NEET PG trap. **Clinical Pearl:** The INR (International Normalized Ratio) reflects the PT, which is most sensitive to factors VII and X (short half-lives). Factor II (longest half-life, ~60 hours) takes longest to normalize after warfarin cessation.