Vitamin K (phylloquinone, Fig. 3), the only form of vitamin K found in plants, functions as an electron carrier in the photosynthetic membranes of the chloroplasts. There it serves to carry electrons from the photosystem I receptor in an electron transport chain related to that of mitochondria. The latter utilizes ubiquinone rather than phylloquinone. Some photosynthetic bacteria utilize menaquinone, in which the number of isoprenoid units in the polyprenyl side chain is greater. In the human body vitamin K has a quite different and specialized function in the modification of the side chains of glutamic acid units in a small group of proteins. Among these are prothrombin and other blood clotting proteins. Selected glutamic acid side chains (at 10 positions near the N-terminal end of the prothrombin chain) are modified by addition of an extra carboxyl group at the gamma (or C4) position of the side chain to give γ -carboxyglutamate (Gla) units. The side chain now contains two negatively charged —COO− groups and is able to better bind to calcium ions (Ca2+), which help bind the clotting factors to the phospholipid membrane in the blood clotting complex. Formation of Gla requires both vitamin K and O2, as indicated in Fig. 21. Here the quinone form of vitamin K has been reduced to the dihydro form. It has been shown experimentally to be converted to the epoxide derivative as the glutamyl (Glu) side chain is converted to that of Gla. One possible mechanism is depicted in Fig. 22. The O2 molecule has added to the dihydro-vitamin K to form a peroxide which is used to generate an HO− ion in the active site where it is in a position to remove the hydrogen in the γ position to form H2O. The resulting anion adds to CO2 to form the Gla.
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