Some properties of glutamine synthetase from Anabaena cylindrica

Some properties of the biosynthetic and γ-glutamyltransferase activities of glutamine synthetase (EC 6.3.1.2) from Anabaena cylindrica are described, including requirement for divalent cations, pH optimum and Km for substrates. The γ-glutamyltransferase reaction was inhibited by L-glutamate, ammonia and ATP. The inhibition by L-glutamate and ammonia was competitive for L-glutamine and non-competitive for hydroxylamine. Both the biosynthetic and the γ-glutamyltransferase activities of the desalted enzyme were much more sensitive to inactivarion by treatments such as urea, hydroxylamine and incubation at 50° C than the preparation which contained a divalent cation. The effects of some substrates of these reactions on protection against thermal denaturation and hydroxylamine were examined. An interpretation of these results in terms of the sequence of binding of substrates both in the biosynthetic and the γ-glutamyltransferase reactions are discussed.