The Metabolic Functions Of Vitamin K

The main metabolic function of vitamin K is as the coenzyme in the carboxyla-tion ofprotein-incorporated glutamate residues to yield y -carboxyglutamate -a unique type of carboxylation reaction, clearly distinct from the biotin-dependent carboxylation reactions (Section 11.2.1).

Four vitamin K-dependent proteins involved in blood coagulation, prothrombin and Factors VII, IX, and X, were discovered early in the investigations of the vitamin, as a result of the hemorrhagic disease caused by deficiency. The function of y-carboxyglutamate in these proteins is to chelate calcium and induce a conformational change that permits binding of the proteins to membrane phospholipids. Unlike the high-affinity calcium binding proteins, such as calmodulin and calbindin (Section 3.3.3.1), the vitamin K-dependent calcium binding proteins have a relatively low affinity for calcium, with values of Kdiss in the millimolar range.

In addition to blood clotting, y-carboxyglutamate-containing proteins are found in

1. bone (osteocalcin and bone matrix Gla protein);

2. the kidney cortex (nephrocalcin); hydroxyapatite and calcium oxalate containing urinary stones;

3. atherosclerotic plaque - this protein is sometimes called atherocalcin, but is probably the same as the bone matrix Gla protein (Section 5.5.3.3);

4. the intermembrane space of mitochondria, where they may have a role in the mitochondrial accumulation of calcium; and

5. the central nervous system and probably other tissues (Gas6).

A number of proteins involved in cell signaling and as cell surface receptors also contain y-carboxyglutamate (Nelsestuen et al., 2000).

A specific vitamin K binding protein has been identified in the nucleus in osteoblasts, suggesting that the vitamin may also have direct nuclear actions (Hoshi et al., 1999). Phylloquinone, but not menaquinones, down-regulates osteoclastic bone resorption by inducing apoptosis in osteoclasts (Kameda etal., 1996).

The vitamin also activates serine palmitoyltransferase, the first enzyme of phosphosphingolipid synthesis, and in bacteria it can, together with inorganic phosphate, replace part of the ATP requirement of galactocerebroside sulfo-transferase (Tsaioun, 1999). In animals, lipid sulfatides are decreased in vitamin K deficiency and increased with higher intakes (Sundaram et al., 1996).

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