The term vitamin K covers a number of compounds, with a double ring -
2-methyl-1,4-naphthoquinone - and a side chain.
Vitamin K1 - phylloquinone - is produced by plants and is
found in the leaves. It has a
phytyl side chain .
Animals
convert it into
Vitamin K2, of which there are several types called menaquinones, differing in
isoprenoid chain length. This is the main storage form in animals. Bacteria in the colon can also convert K1 into vitamin K2, and may lengthen the isoprenoid side chain to produce a range of vitamin K2 forms.
Synthetic forms of vitamin K (K3, K4, and K5) are of uncertain efficacy.
Vitamin K functions as a cofactor in the formation of several important proteins:
coagulation factors II (prothrombin), VII, IX, and X, produced in the liver are essential for normal blood clotting and osteocalcin in bone is invoved in in bone mineralization and calcium ion homeostasis.
This involves post translational modification (carboxylation) of glutamate residues in precusors to these Gla proteins, which means that they react with calcium ions.