Vitamin K - Chemical Structure

Chemical Structure

All members of the vitamin K group of vitamins share a methylated naphthoquinone ring structure (menadione), and vary in the aliphatic side chain attached at the 3-position (see figure 1). Phylloquinone (also known as vitamin K₁) invariably contains in its side chain four isoprenoid residues, one of which is unsaturated.

Menaquinones have side chains composed of a variable number of unsaturated isoprenoid residues; generally they are designated as MK-n, where n specifies the number of isoprenoids.

It is generally accepted that the naphthoquinone is the functional group, so that the mechanism of action is similar for all K-vitamins. Substantial differences may be expected, however, with respect to intestinal absorption, transport, tissue distribution, and bio-availability. These differences are caused by the different lipophilicity of the various side chains, and by the different food matrices in which they occur.

There are three synthetic forms of vitamin K, vitamins K₃, K₄, and K₅, which are used in many areas including the pet food industry (vitamin K₃) and to inhibit fungal growth (vitamin K₅).