Carboxcylase
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In mammals the only known function of vitamin K
is that it serves as a cofactor for gamma-glutamyl carboxylase,
an endoplasmic enzyme (see figure 1) involved in the posttranslational
carboxylation of glutamate residues (Glu) into gammacarboxy glutamate
(Gla). Hence, the vitamin K-dependent step is a carboxylation reaction
taking place during the later stages of protein biosynthesis (see
figure 2).
The resulting Gla-residues are found in a limited number of proteins (Table 1), and in these proteins only at certain well-defined positions. In case of vitamin K-deficiency the carboxylation reaction cannot proceed, hence the Gla-proteins are released in the circulation as 'descarboxy proteins', which are biologically inactive. Gla-residues form calcium-binding groups in proteins, so the main physico-chemical difference between normal and descarboxy proteins is their large difference in both binding of calcium from solution and the adsorption of these proteins onto insoluble calcium salts.
Figure 2. The vitamin K-dependent carboxylation reaction. Protein-bound glutamate residues are carboxylated into gammacarboxy glutamate and vitamin K is the cofactor in this reaction, which is catalyzed by the enzyme gammaglutamyl carboxylase. Recommended literature:
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