Synthesis of D-@-Glutamine from P-Glutamic Acid by Glutamine Synthetase*

In contrast to fi-glutamine and @glutamylhydroxamic acid, fl-glutamic acid does not possess an asymmetric carbon atom, but has, according to the nomenclature of Schwartz and Carter (2), a meso carbon atom. The present experiments were designed to determine whether glutamine synthetase distinguishes between the carboxyl groups of P-glutamic acid. It is noteworthy in this connection that the enzyme exhibits significant activity toward u-glutamic acid as well a~ Gglutamic acid (3, 4), and it might therefore be expected to catalyze synthesis of both isomers of /3-glutamine from &glutamic acid. In the present studies, the enzymatically synthesized P-glutamine was found to exhibit levorotatory optical activity. Efforts to obtain the pure optically active isomers of Land n-P-glutamine for comparative purposes by resolution of ox,-@-glutamine with alkaloids were not successful. Therefore, another approach to the problem was employed which was based on earlier work indicating that chymotrypsin acts stereospecifitally on dimethyl&hydroxyglutarate and diethyl-N-acetyl$ glutamate (5, 6). Chymotrypsin cleaves a specific ester bond of the latter compound to yield a monoethyl ester of N-acetyl-/3glutamic acid. This isomer is identical with the dextrorotatory isomer obtained by resolution of the racemic monoethyl ester of N-acetyl-P-glutamic acid with cinchonidine. The configuration of this isomer was shown to be the same as that of the monomethyl ester of fl-hydroxyglutaric acid produced by the action of chymotrypsin on dimethyl-@-hydroxyglutarate. The acetoxy derivative of the enzymatically produced isomer (3-acetoxy-4carbomethoxylbutanoic acid), by its relationship with methyl-3n-acetoxypentanoate (7), can be assigned the L configuration. The corresponding isomer of the monoethyl ester of N-acetyl-Pglutamic acid (3-cacetylamino-4-carboethoxylbutanoic acid) is