Step by Step
GLA
γ-Carboxylation — the key reaction
Vitamin K is a cofactor for γ-glutamyl carboxylase — the enzyme that adds a carboxyl group (-COOH) to glutamate residues (creating γ-carboxyglutamate, Gla) on vitamin K-dependent proteins. Gla residues bind Ca²⁺ → allows clotting factors to bind phospholipid surfaces → activation of the coagulation cascade. Without Gla, clotting factors cannot function.
Memory trick: Vitamin K = 'Koagulation' (German). γ-carboxylation = the stamp that activates clotting factors.
Fac
Vitamin K-dependent proteins
Coagulation factors: II (prothrombin), VII, IX, X — all require Gla for activity. Anticoagulant proteins: Protein C and Protein S — also require Gla. Note: Protein C and S are anticoagulant — vitamin K deficiency impairs these too, but net effect is bleeding (procoagulant factors more important). Bone proteins: osteocalcin and matrix Gla protein (important in bone mineralization).
War
Warfarin — vitamin K antagonist
Warfarin (Coumadin) inhibits vitamin K epoxide reductase (VKOR) — preventing recycling of vitamin K back to its active (reduced) form. Effect: depletes functional vitamin K → impairs γ-carboxylation of factors II, VII, IX, X and proteins C and S → anticoagulation. Factor VII has the shortest half-life → PT/INR rises first. Monitored by INR. Reversed by vitamin K (takes 12-24h) or FFP (fresh frozen plasma, immediate).
Memory trick: Warfarin = 'War on vitamin K.' Inhibits VKOR → depletes active vitamin K.
Def2
Vitamin K deficiency — who's at risk
Newborns: gut sterile (no bacteria to make menaquinone), low liver stores, low breast milk content → hemorrhagic disease of the newborn (HDN). Prophylactic vitamin K injection at birth is standard. Fat malabsorption: Crohn's, celiac, cystic fibrosis, cholestasis → all four fat-soluble vitamins depleted. Prolonged antibiotics: kill gut bacteria that synthesize menaquinone (K2). Broad-spectrum antibiotics + poor oral intake = risk.
Applied Walkthrough
1
A newborn is brought to the ER on day 3 of life with bleeding from the umbilical stump and prolonged PT — hemorrhagic disease of the newborn (vitamin K deficiency).
2
The parents declined the routine vitamin K injection at birth (not uncommon in home births). IV vitamin K is given — bleeding resolves within hours.
3
A patient on long-term warfarin for atrial fibrillation presents with INR of 9.2 (supratherapeutic) and bleeding. Management: hold warfarin + give vitamin K (if non-urgent) or FFP + 4-factor PCC (if urgent bleeding).
4
A patient with cholestatic liver disease develops prolonged PT — both vitamin K malabsorption (fat-soluble vitamin) AND impaired liver synthesis of clotting factors contribute to coagulopathy.
Exam Application
Exams test the mechanism of vitamin K (γ-carboxylation of glutamate residues), which factors are vitamin K-dependent (II, VII, IX, X + proteins C and S), warfarin's mechanism (inhibits VKOR), why PT rises before aPTT with warfarin (factor VII shortest half-life), and vitamin K deficiency in newborns. The reversal of warfarin anticoagulation is also high yield.
⚠ Common Trap
Students say vitamin K deficiency only affects PT — it also affects aPTT (factors IX and X are in the intrinsic pathway too). However PT rises FIRST because factor VII (extrinsic pathway) has the shortest half-life (~6 hours). Also: proteins C and S are anticoagulant, but net effect of vitamin K deficiency is still bleeding because the procoagulant factors matter more acutely.
✓ Quick Self-Check
1. What is the biochemical function of vitamin K?
Cofactor for γ-glutamyl carboxylase — adds carboxyl groups to glutamate residues (Gla) on clotting factors, allowing them to bind Ca²⁺ and phospholipid surfaces to activate the coagulation cascade.
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2. Which clotting factors require vitamin K?
Factors II, VII, IX, and X — plus anticoagulant proteins C and S.
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3. How does warfarin work?
Inhibits vitamin K epoxide reductase (VKOR) → prevents recycling of vitamin K to its active form → depletes Gla-containing clotting factors over days.
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4. Why does PT rise before aPTT with warfarin?
Factor VII (extrinsic pathway, measured by PT) has the shortest half-life (~6 hours) — it is depleted first by warfarin.
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5. Why do newborns receive vitamin K at birth?
Newborns have sterile guts (no bacteria to make menaquinone), minimal liver stores, and low breast milk vitamin K → hemorrhagic disease of the newborn risk.
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