Kinetic Mechanism of Escherichia c oli Asparagine Synthetase B

Escherichia coli asparagine synthetase B (AS-B) catalyzes the synthesis of asparagine from aspartate, glutamine, and ATP. A combination of kinetic, isotopic-labeling, and stoichiometry studies have been performed to define the nature of nitrogen transfer mediated by AS-B. The results of initial rate studies were consistent with initial binding and hydrolysis of glutamine to glutamate plus enzyme-bound ammonia. The initial velocity results were equally consistent with initial binding of ATP and aspartate prior to glutamine binding. However, product inhibition studies were only consistent with the latter pathway. Moreover, isotope-trapping studies confirmed that the enzyme−ATP−aspartate complex was kinetically competent. Studies using 18O-labeled aspartate were consistent with formation of a β-aspartyl-AMP intermediate, and stoichiometry studies revealed that 1 equiv of this intermediate formed on the enzyme in the absence of a nitrogen source. Taken together, our results are most consistent with initial formation of β -aspartyl-AMP intermediate prior to glutamine binding. This sequence leaves open many possibilities for the chemical mechanism of nitrogen transfer.