Vitamin K-dependent carboxylation. Study of the hydrogen abstraction stereochemistry with gamma-fluoroglutamic acid-containing peptides

Two pentapeptides Phe-Leu-X-Glu-Val where X is either the L-threo-gamma-fluoroglutamic acid or the L-erythro-isomer have been synthesized and tested as substrates in the vitamin K-dependent carboxylation. Both peptides are carboxylated, but the reaction occurs exclusively on the glutamic acid of the L-threo-gamma-fluoroglutamate-containing peptide, whereas both glutamic and fluoroglutamic residues of the L-erythro-gamma-fluoroglutamate-containing peptide are carboxylated. These results reveal that the enzymatic hydrogen abstraction step is stereospecific and corresponds, in the gamma-fluoroglutamate case, to the elimination of the hydrogen equivalent to the pro-S hydrogen of glutamic acid.