title: A novel reaction catalyzed by unadenylylated glutamine synthetase from Escherichia coli. AMP-dependent synthesis of pyrophosphate and L-Glutamate from orthophosphate and L-glutamine.
abstract: The unadenylylated, manganese form of glutamine synthetase (L-glutamate: ammonia ligase (ADP forming), EC 6.3.1.2 from Escherichia coli catalyzes a novel, AMP-dependent (reversible) synthesis of pyrophosphate and L-glutamate from orthophosphate and L-glutamine: Formula (See Text). The hydrolysis of the L-glutamine amide bond is coupled to the stoichiometric synthesis of pyrophosphate, although as PPi accumulates, additional hydrolysis of L-glutamine occurs in a secondary reaction catalyzed by the [manganese x enzyme x AMP x PPi] complex. The synthesis of PPi probably occurs at the subunit catalytic site in the positions normally occupied by the beta, gamma-phosphates of ATP. To promote PPi synthesis, AMP apparently binds to the subunit catalytic site rather than to the allosteric inhibitor site; equilibrium binding results suggest that Pi directs the binding of AMP to the active site. In this reaction, Mg2+ will not substitute for Mn2+, and adenylylated glutamine synthetase is inactive. Pyrophosphate is synthesized by the unadenylylated, manganese enzyme at approximately 2% of the rate of that of ATP in the reverse biosynthetic reaction. If P1 is replaced by arsenate, the enzymatic rate of the AMP-supported hydrolysis of L-glutamine is 100-fold faster than is PPi synthesis and is one-half the rate of the ADP-supported, irreversible arsenolysis of L-glutamine. This latter activity also is supported by GMP and IMP, suggesting that the catalytic site of glutamine synthetase has a rather broad specificity for the nucleotide base. The reactions supported by AMP directly relate to the mechanism of glutamine synthetase catalysis.
mesh_terms: D000249:Adenosine Monophosphate / Q000494:pharmacology*; D011756:Diphosphates / Q000378:metabolism*; D004926:Escherichia coli / Q000378:metabolism*; D005974:Glutamate-Ammonia Ligase / Q000378:metabolism*; D005971:Glutamates / Q000096:biosynthesis*; D005973:Glutamine / Q000378:metabolism; D007700:Kinetics; D008345:Manganese / Q000494:pharmacology; D010710:Phosphates / Q000378:metabolism
chemical_list: D011756:Diphosphates; D005971:Glutamates; D010710:Phosphates; D005973:Glutamine; D000249:Adenosine Monophosphate; D008345:Manganese; D005974:Glutamate-Ammonia Ligase