Studies of the mechanism of glutamine synthetase utilizing pH-dependent behavior in catalysis and binding

The pKa values of enzyme groups of Escherichia coli glutamine synthetase which affect catalysis and/or substrate binding were determined by measuring the pH dependence of Vmax and V/K. Analysis of these data revealed that two enzyme groups are required for catalysis with apparent pKa values of approximately 7.1 and 8.2. The binding of ATP is essentially independent of pH in the range studied while the substrate ammonia must be deprotonated for the catalytic reaction. Using methylamine and hydroxylamine in place of ammonia, the pKa value of the deprotonated amine substrate as expressed in the V/K profiles was shifted to a lower pKa value for hydroxylamine and a higher pKa value for methylamine. These data indicate that the amine substrate must be deprotonated for binding. Hydroxylamine is at least as good a substrate as ammonia judged by the kinetic parameters whereas methylamine is a poor substrate as expressed in both the V and V/K values. Glutamate binding was determined by monitoring fluorescence changes of the enzyme and the data indicate that a protonated residue (pKa = 8.3 +/- 0.2) is required for glutamate binding. Chemical modification by reductive methylation with HCHO indicated that the group involved in glutamate binding most likely is a lysine residue. In addition, the Ki value for the transition state analog, L-3-amino-3-carboxy-propanesulfonamide was measured as a function of pH and the results indicate that an enzyme residue must be protonated (pKa = 8.2 +/- 0.1) to assist in binding. A mechanism for the reaction catalyzed by glutamine synthetase is proposed from the kinetic data acquired herein. A salt bridge is formed between the gamma-phosphate group of ATP and an enzyme group prior to attack by the gamma-carboxyl of glutamate on ATP to form gamma-glutamyl phosphate. The amine substrate subsequently attacks gamma-glutamyl phosphate resulting in formation of the tetrahedral adduct before phosphate release. A base on the enzyme assists in the deprotonation of ammonia during its attack on gamma-glutamyl phosphate or after the protonated carbinol amine is formed. Based on the kinetic data with the three amine substrates, catalysis is not rate-limiting through the pH range 6-9.