Includes phytomenadione, phylloquinone, phytonadione, menaquinone.Activated by epoxide reductase to the reduced form, which is a cofactor for the γ-carboxylation of glutamic acid residues on proteins required for blood clotting (II, VII, IX, X, proteins C and S).- Synthesized by intestinal flora.

Deficiency:- Neonatal hemorrhage with ↑ PT and ↑ aPTT but normal bleeding time (neonates have sterile intestines and are unable to synthesize vitamin K).- Can also occur after prolonged use of broad-spectrum antibiotics.

- Not in breast milk; neonates are given vitamin K injection at birth to prevent hemorrhagic disease of the newborn.