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[Biochemistry](https://durgical.com/en/biochemistry/) · Molecule

# Vitamin K (phylloquinone)

Vitamin K lets the liver activate clotting factors; without it blood takes too long to clot.

Updated 2026-11-01

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## Key facts

- Coenzyme of γ-glutamyl carboxylase: it adds a carboxyl to glutamate (Gla) residues of factors II, VII, IX and X and of proteins C and S.
- Gla residues trap calcium, which anchors the factors to platelet membranes.
- Each carboxylation leaves the vitamin as an epoxide; VKORC1 recycles it, and that is warfarin's target.
- Newborns have little vitamin K, so it is given at birth to prevent haemorrhagic disease.

## In detail

- Sources: K1 (phylloquinone) in green leaves (spinach, broccoli, cabbage); K2 (menaquinones) is made by colonic bacteria and found in fermented foods.
- Reaction: vitamin K hydroquinone, O₂ and CO₂ let γ-glutamyl carboxylase (in the hepatocyte endoplasmic reticulum) turn glutamate into γ-carboxyglutamate.
- Recycling: vitamin K epoxide returns to quinone and hydroquinone via vitamin K epoxide reductase (VKORC1); warfarin blocks it.
- Other Gla proteins: osteocalcin (bone) and matrix Gla protein (prevents arterial calcification).
- Deficiency: newborns without prophylaxis, fat malabsorption, long courses of broad-spectrum antibiotics, malnutrition, liver disease. Mucosal bleeding, bruising, intracranial haemorrhage in infants.
- Excess: natural K1 is barely toxic; synthetic menadione can cause haemolysis and jaundice in newborns.
- Lab: prothrombin time (PT/INR) lengthens first, because factor VII has the shortest half-life; aPTT lengthens later. PIVKA-II (uncarboxylated prothrombin) confirms deficiency.

**Also called**
Phylloquinone (K1), Menaquinones (K2), Menadione (K3)

**Formula**
C₃₁H₄₆O₂

**PubChem**
[CID 5284607](https://pubchem.ncbi.nlm.nih.gov/compound/5284607)

## Structure

- A naphthoquinone ring (2-methyl-1,4-naphthoquinone) with a side chain: phytyl in K1, 4 to 13 isoprene units in the K2 forms.
- The ring cycles through quinone, hydroquinone (the active form) and epoxide on each turn of the cycle.

## Clinical notes

- Warfarin inhibits VKORC1 and lowers active factors; its effect is reversed with vitamin K and, when urgent, with prothrombin complex concentrate or plasma.
- Dietary vitamin K shifts the INR of people on warfarin; a steady intake is advised, not avoidance.
- VKORC1 and CYP2C9 variants explain much of the difference in warfarin dose between people.

## Drugs that act here

- [Phytomenadione (vitamin K1)](https://durgical.com/en/pharmacology/phytomenadione/)

## Related conditions

- [Vitamin K deficiency bleeding of the newborn](https://durgical.com/en/conditions/vitamin-k-deficiency-bleeding-of-the-newborn/)
- [Cirrhosis](https://durgical.com/en/conditions/cirrhosis/)
- [Deep vein thrombosis](https://durgical.com/en/conditions/deep-vein-thrombosis/)

## Related

- [Vitamin K epoxide reductase (VKORC1)](https://durgical.com/en/biochemistry/enzymes/vitamin-k-epoxide-reductase/)
- [Vitamin E (α-tocopherol)](https://durgical.com/en/biochemistry/molecules/vitamin-e/)
- [Calcium and phosphate](https://durgical.com/en/biochemistry/topics/calcium-and-phosphate/)

## Test yourself

Think of the answer, then tap the question to see it.

## Which reaction does the vitamin K-dependent enzyme catalyse?

γ-carboxylation of glutamate residues to calcium-binding γ-carboxyglutamate (Gla) in factors II, VII, IX and X and proteins C and S.

## Why does PT/INR lengthen before aPTT in vitamin K deficiency?

Factor VII, in the extrinsic pathway that PT measures, has the shortest half-life (about 6 hours) and falls first.

## Where does warfarin act?

It inhibits vitamin K epoxide reductase (VKORC1), blocking vitamin K recycling, so the liver releases uncarboxylated factors.

## Why do newborns receive vitamin K at birth?

Little crosses the placenta, breast milk contains little, and the gut has no bacteria yet to make it.

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Molecules are shown in their neutral form, as the databases deposit them; in the cell the acids and phosphates are ionised.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Phylloquinone (CID 5284607)](https://pubchem.ncbi.nlm.nih.gov/compound/5284607) (PubChem, NCBI, 2026) · Public domain; [Vitamin K: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002407.htm) (MedlinePlus, US National Library of Medicine, 2026); [Prothrombin Time Test and INR (PT/INR): MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/prothrombin-time-test-and-inr-ptinr/) (MedlinePlus, US National Library of Medicine, 2026); [VKORC1 gene: MedlinePlus Genetics](https://medlineplus.gov/genetics/gene/vkorc1/) (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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