Prothrombin, calcium binding

Compounds with vitamin K activity (Table 6.2) are required in our diets for y-carboxyglutamate biosynthesis (Table 4.1). This amino acid is produced from certain protein glutamyl residues by carboxylation. Proteins that contain y-carboxyglutamate are blood prothrombin and coagulation factors VII, IX, and X (see Chapter 7). Other proteins of this type are osteocalcin from bone and several kidney and muscle calcium-binding proteins. 

The Gla-containing proteins of the blood coagulation system are all modular with the Gla-domain N terminal. Factors VII, IX, and X and protein C form a group with the same domain structure. The Gla domain is followed by two EGF-like domains, of which the first one also binds calcium (see Section IV.C), and a serine protease domain, also with a calcium-binding site (see Section IV.D). Prothrombin and protein S have somewhat different domain structures (145, 146). In prothrombin the Gla domain is followed by a hexapeptide with a disulfide loop, two kringle domains, and the C-terminal serine protease domain. In protein S the Gla domain is followed by the thrombin-sensitive loop, four EGF-like domains, and the C-terminal domain that is homologous to plasma steroid hormone-binding proteins. 

FIGURE 10.43 The calcium-binding region of prothrombin. Prothrombin binds calcium ions with the modified amino acid 7-carboxyglutamate (red). 

Prothrombin and several other proteins of the blood clotting system (Factors VII, IX and X, and proteins C and S) each contain between four and six y-carboxygluta-mate residues which chelate calcium ions and so permit the binding of the blood clotting proteins to membranes. In vitamin K deficiency or in the presence of warfarin, an abnormal precursor of prothrombin (preprothrombin) containing little or no y-carboxyglutamate, and incapable of chelating calcium, is released into the circulation. 

Nelsestuen G. L. Role of gamma carboxy glutamic acid. An unusual transition required for calcium-dependent binding of prothrombin to phospholipid. J Biol Chem 1976 251,5648. 

S. W. Tendian, N. L. Thompson, and B. R. Lentz, Calcium-independent binding of prothrombin to negatively charged membranes, Biophys. J. 57, 72a (1990). 

Platelet membrane phosphatidylserine is critical to the formation of the tenase complex since on its surface activated factor VIII (Villa) generates a high-afflnity binding site for activated factor IX (IXa) in the presence of calcium. Subsequently, this complex activates factor X (2, 13). Platelet membrane phosphatidylserine similarly anchors activated factor V (Va), favoring the calcium-dependent binding of activated factor X (Xa). The prothrombinase complex is generated on the surface of the anionic platelet membrane phosphatidylserine when factor Va binds prothrombin. The prothrombinase complex cleaves prothrombin to produce thrombin, which has a multifunctional role (14). 

The most obvious effect of a deficiency in vitamin K in animals is delayed blood clotting, which has been traced to a decrease in the activity of prothrombin and of clotting factors VII, IX, and X (Chapter 12, Fig. 12-17). Prothrombin formed by the liver in the absence of vitamin K lacks the ability to chelate calcium ions essential for the binding of prothrombin to phospholipids and to its activation to thrombin. The structural differences between this abnormal protein and the normal prothrombin have been pinpointed at the N terminus of the 560 residue glycoprotein.e f Tryptic peptides from the N termini differed in electrophoretic mobility. As detailed in Chapter 12, ten residues within the first 33, which were identified as glutamate residues by the sequence analysis on normal prothrombin, are actually y-carboxyglutamate (Gla). The same amino acid is present near the N termini of clotting factors VII, IX, and X. 

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