Catalyzes the formation of N(5)-methyl-L-glutamine from glutamate and methylamine. In vitro, can also use ethylamine, hydroxylamine and ammonia, with 75%, 40% and 1% activity compared to methylamine, respectively. ATP + L-glutamate + methylamine = ADP + H(+) + N(5)-methyl-L-glutamine + phosphate ATP + ethylamine + L-glutamate = ADP + H(+) + N(5)-ethyl-L-glutamine + phosphate Formation of theanine is repressed by a high concentration of glutamic acid. By methylamine. Could be used for theanine (N(5)-ethyl-L-glutamine) production, by coupling the reaction with the ATP-regeneration system of yeast sugar fermentation. Theanine occurs naturally in the tea plant (Camellia sinensis) and provides flavor to green tea. It has also several favorable physiological effects on mammals. Belongs to the glutamine synthetase family. Type 3 subfamily.