Vitamin K is one of the fat soluble vitamins. It is mainly obtained from green leafy vegetables and is also supplied by the intestinal flora. It is famous for promoting the coagulation of the blood. Clotting of the blood is a defensive mechanism which has evolved to prevent the loss of blood when there is an injury. Subsequently, it has been shown that this action of vitamin K is actually brought about through the activation of some of the proteins involved in the process of blood coagulation. The most important of these is a protein called prothrombin.
Proteins are polymers built up from building blocks called amino acids. Normally, organisms use around 20 amino acids for this purpose. However, some proteins also contain some amino acids which do not fall under these 20 amino acids. These have been called as unusual amino acids. Examples of such unusual amino acids are hydroxyproline and hydroxylysine which are required for the formation of the structural protein collagen. Another example is gamma carboxy glutamic acid. Many of these unusual amino acids are formed post translationally. What this means is that normal amino acids are converted into unusual amino acids after the protein has been fully assembled.
Gamma carboxy glutamic acid has been abbreviated as gla and at least fourteen proteins in the body including the blood coagulation proteins contain gla. The conversion of some of the glutamic acid residues in these proteins to gla is achieved through the action of an enzyme called glutamyl gamma carboxylase. This enzyme shows an obligatory requirement for vitamin K. Thus, for example, prothrombin can participate in the process of blood coagulation only if its glutamic acid residues are converted into gla by the action of the vitamin K dependent enzyme.
Vitamin K is also concerned with bone formation. Supplementation with vitamin K has been shown to prevent hip fractures in post menopausal women. Vitamin K supplements also reduce the rate of osteoporosis and increase in bone density is seen. One of the proteins involved in bone formation is osteocalcin. This protein promotes the deposition of calcium in the bone formation. This is another protein which contains gla and therefore , formation of this protein also requires the help of vitamin K. Thus, vitamin K is also involved in bone formation.