Figure 3.

Vitamin K exists as a family of vitamers. A, Phylloquinone (or vitamin K-1) consists of a quinone ring that confers vitamin K oxygenation activity and a long hydrophobic tail consisting of 4 isoprenoid units with a single double bond. B, Menaquinone (or vitamin K-2) retains the same quinone ring functionality but varies in the composition of the hydrophobic tail. Several menaquinone forms are known with different numbers of isoprenoid units, and each unit contains a double bond. The isoprenoid tails are not involved in oxygenation chemistry but could serve to bind the vitamers to the endoplasmic reticulum membrane where the carboxylase and VKOR are localized.