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Vitamin K

Vitamin K is a necessary participant in synthesis of several proteins that mediate both coagulation and anticoagulation. Vitamin K deficiency is manifest as a tendency to bleed excessively. Indeed, many commercially-available rodent poisons are compounds that interfere with vitamin K and kill by inducing lethal hemorrhage.

Structure

At least two naturally-occuring forms of vitamin K have been identified, and these are designated vitamins K1 and K2. Both are quinone derivatives. The structure of vitamin K1 is depicted below.

Physiologic Effects of Vitamin K

Vitamin K serves as an essential cofactor for a carboxylase that catalyzes carboxylation of glutamic acid residues on vitamin K-dependent proteins. The key vitamin K-dependent proteins include:

These proteins have in common the requirement to be post-translationally modified by carboxylation of glutamic acid residues (forming gamma-carboxyglutamic acid) in order to become biologically active. Prothrombin, for example, has 10 glutamic acids in the amino-terminal region of the protein which are carboxylated. Without vitamin K, the carboxylation does not occur and the proteins that are synthesized are biologically inactive.

What essential function do gamma-carboxyglutamic acid residues endow upon a protein? There appear to be two major effects:

Sources of Vitamin K

Vitamin K is found in a number of foods, including leafy greens, cauliflower and, if you consider it a food, liver. However, the chief source of vitamin K is synthesis by bacteria in the large intestine, and in most cases, absence of dietary vitamin K is not at all deleterious. Vitamin K is a fat-soluble vitamin and both dietary and microbial vitamin K are absorbed into intestinal lymph along with other lipids. The fetus obtains vitamin K from its mother by transplacental transfer.

The Vitamin K Cycle

As a cofactor to the carboxylase that generates gamma-carboxyglutamic acid, Vitamin K undergoes a cycle of oxidation and reduction that allows its reuse. The essential details of this cycle are:

Vitamin K Deficiencies

Deficiency in vitamin K and resulting hemorrhagic disease can result from several situations:

Two other vitamin K deficiency states have received considerable recent attention:

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