COFACTORS IN BLOOD COAGULATION (clotting factors): Role of vitamin K & calcium.

Co-factors in Blood clotting

Various substances are required for the proper functioning of the coagulation cascade:

Calcium and phospholipid (a platelet membrane constituent) are required for the tenase and prothrombinase complexes to function. Calcium mediates the binding of the complexes via the terminal gamma-carboxy residues on FXa and FIXa to the phospholipid surfaces expressed by platelets, as well as procoagulant microparticles or microvesicles shed from them. Calcium is also required at other points in the coagulation cascade.
Vitamin K is an essential factor to a hepatic gamma-glutamyl carboxylase that adds a carboxyl group to glutamic acid residues on factors II, VII, IX and X, as well as Protein S, Protein C and Protein Z. In adding the gamma-carboxyl group to glutamate residues on the immature clotting factors Vitamin K is itself oxidized. Another enzyme, Vitamin K epoxide reductase, (VKORC) reduces vitamin K back to its active form. Vitamin K epoxide reductase is pharmacologically important as a target for anticoagulant drugs warfarin and related coumarins such as acenocoumarol, phenprocoumon, and dicumarol. These drugs create a deficiency of reduced vitamin K by blocking VKORC, thereby inhibiting maturation of clotting factors. Other deficiencies of vitamin K (e.g., in malabsorption), or disease (hepatocellular carcinoma) impairs the function of the enzyme and leads to the formation of PIVKAs (proteins formed in vitamin K absence); this causes partial or non-gamma carboxylation, and affects the coagulation factors' ability to bind to expressed phospholipid.

 Vitamin K

Vitamin K is a fat-soluble vitamin. The "K" is derived from the German word "koagulation." Coagulation refers to the process of blood clot formation. Vitamin K is essential for the functioning of several proteins involved in blood clotting. There are two naturally occurring forms of vitamin K. Plants synthesize phylloquinone, which is also known as vitamin K₁. Bacteria synthesize a range of vitamin K forms using repeating 5-carbon units in the side chain of the molecule. These forms of vitamin K are designated menaquinone-n (MK-n), where n stands for the number of 5-carbon units. MK-n are collectively referred to as vitamin K₂. MK-4 is not produced in significant amounts by bacteria; instead, it appears to be synthesized by animals (including humans) from phylloquinone. MK-4 is also formed from menadione, a synthetic form of vitamin K present in animal feed. MK-4 is found in a number of organs other than the liver at higher concentrations than phylloquinone. This fact, along with the existence of a unique pathway for its synthesis, suggests that MK-4 has a unique biological function that has not yet been identified.

VITAMIN K

Function
The only known biological role of vitamin K is as a cofactor for an enzyme that catalyzes the carboxylation of the amino acid, glutamic acid, resulting in its conversion to gamma-carboxyglutamic acid (Gla). Although vitamin K-dependent gamma-carboxylation occurs only on specific glutamic acid residues in a small number of vitamin K-dependent proteins, it is critical to the calcium-binding function of those proteins.

Coagulation (clotting)
The ability to bind calcium ions (Ca²⁺) is required for the activation of the seven vitamin K-dependent clotting factors, or proteins, in the coagulation cascade. The term, coagulation cascade, refers to a series of events, each dependent on the other, that stop bleeding through clot formation. Vitamin K-dependent gamma-carboxylation of specific glutamic acid residues in those proteins makes it possible for them to bind calcium. Factors II (prothrombin), VII, IX, and X make up the core of the coagulation cascade. Protein Z appears to enhance the action of thrombin (the activated form of prothrombin) by promoting its association with phospholipids in cell membranes. Protein C and protein S are anticoagulant proteins that provide control and balance in the coagulation cascade; protein Z also has an anticoagulatory function. Control mechanisms for the coagulation cascade exist, because uncontrolled clotting may be as life threatening as uncontrolled bleeding. Vitamin K-dependent coagulation factors are synthesized in the liver. Consequently, severe liver disease results in lower blood levels of vitamin K-dependent clotting factors and an increased risk of uncontrolled bleeding (hemorrhage).
Some people are at risk of forming clots, which could block the flow of blood in arteries of the heart, brain, or lungs, resulting in heart attack, stroke, or pulmonary embolism, respectively. Some oral anticoagulants, such as warfarin (Coumadin), inhibit coagulation through antagonism of the action of vitamin K. Although vitamin K is a fat-soluble vitamin, the body stores very little of it, and its stores are rapidly depleted without regular dietary intake. Perhaps, because of its limited ability to store vitamin K, the body recycles it through a process called the vitamin K cycle. The vitamin K cycle allows a small amount of vitamin K to function in the gamma-carboxylation of proteins many times, decreasing the dietary requirement. Warfarin prevents the recycling of vitamin K by inhibiting two important reactions and creating a functional vitamin K deficiency. Inadequate gamma-carboxylation of vitamin K-dependent coagulation proteins interferes with the coagulation cascade, which inhibits blood clot formation. Large quantities of dietary or supplemental vitamin K can overcome the anticoagulant effect of vitamin K antagonists, so patients taking these drugs are cautioned against consuming very large or highly variable quantities of vitamin K in their diets. Experts now advise a reasonably constant dietary intake of vitamin K that meets current dietary recommendations (90-120 mcg/day) for patients on vitamin K antagonists like warfarin.
NOTE: 
intrinsic pathway and extrinsic pathway are analyzed to study the reaction sequence of blood clotting mechanism. since blood clotting is a cascade, importance of each co-factors can be studies in detail. in the absence of a particular co-factor the reaction can not proceed further; thus by using fluorescent markers the cascade can be studied along with respective co-factors.

PATHWAY OF BLOOD CLOTTING