VITAMIN K
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Carboxcylase
In mammals the only known function of vitamin K is that it serves as a cofactor for glutamyl carboxylase, an endoplasmic enzyme (see figure 1) involved in the posttranslational carboxylation of glutamate (Glu) into carboxy glutamate (Gla) residues. Hence, the vitamin K-dependent step is a carboxylation reaction taking place during the later stages of protein biosynthesis (see figure 2).
Figure 1: Hypothetical transmembrane structure of human glutamyl carboxylase The resulting Gla-residues are found in a limited number of proteins (Table 1), and only at certain well-defined positions. In case of vitamin K-deficiency the carboxylation reaction cannot proceed, 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. Vitamin K is the cofactor in this reaction, which is catalyzed by the enzyme glutamyl carboxylase. Recommended literature:
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