Mechanisms of substrate binding with glutamine synthetase. Equilibrium isotope exchanges with the ovine brain, pea seed, and Escherichia coli enzymes.

Equilibrium isotope exchange kinetics were used to investigate the sequences of substrate binding with ovine brain, pea seed, and Escherichia coli glutamine synthetases. Without exception, the relative rates of exchange are (glutamate % glutamine) > (NH3 % glutamine) > (Pi % ATP) N (ADP % ATP). This suggests that the rate of net turnover at saturating substrate levels is limited more strongly by the rate of nucleotide release than by the rate of covalent interconversion. With the ovine brain enzyme, the kinetic patterns of equilibrium exchange are consistent with a partially ordered sequence of substrate binding, where ATP binds before NH,, glutamine leaves before ADP, but glutamate and phosphate are bound and released randomly on their respective sides of the reaction. This particular binding order does not permit observation of those partial exchange reactions expected if an enzyme-bound y-glutamyl phosphate intermediate were formed. Upon variation of the concentration of all substrates in constant ratio at equilibrium, the pea seed, the Mn2+-activated adenylylated and the Mg2+activated unadenylylated E. coli enzymes exhibit exchange kinetic patterns consistent with random substrate addition and release. The kinetic data also suggest synergism of substrate binding between glutamate and ATP. A brief section on theory and interpretation of kinetic patterns of, equilibrium exchanges as related to substrate-binding order is presented.