Vitamin-K-dependent carboxylation: Synthesis and biological properties of diastereoisomeric γ-substituted glutamic acid containing peptidic substrates

Pentapeptides Phe-Leu-X-Glu-Val where X is successively L-threo-γ-fluoro-glutamyl, L-erythro-γ-fluoro-glutamyl, L-threo-γ-methyl-glutamyl or L-erythro-γ-methyl-glutamyl have been synthesized and tested as substrates for the vitamin K-dependent carboxylation. L-threo- or L-erythro-γ-methyl-glutamyl are not carboxylated but both corresponding peptides are inhibitors of the reaction. The L-threo-γ-methyl-glutamyl containing peptide has the highest affinity described so far for the active site of the carboxylase (80 M). In the γ-fluoro-glutamyl series, only the L-erythro-γ-fluoro-glutamyl residue is carboxylated, showing that the enzymatic hydrogen abstraction is stereospecific and corresponds to the elimination of the pro S hydrogen of glutamic acid. The lack of in vitro dicarboxylation of model peptides in contrast with the in vivo polycarboxylation of endogenous precursors is discussed along with the regiospecificity of the reaction with the different substrates.