Affinity labeling of the active site of Escherichia coli glutamine synthetase by 5'-p-fluorosulfonylbenzoyladenosine.

The interaction of Escherichia coli glutamine synthetase with the adenosine 5'-triphosphate analogue, 5'-p-fluorosulfonylbenzoyladenosine (5'-FSO2BzAdo), has been studied. This interaction results in the covalent attachment of the 5'-FSO2BzAdo to the enzyme with concomitant loss of catalytic activity. Although adenine nucleotides interact with glutamine synthetase at three distinct sites--a noncovalent AMP effector site, a regulatory site of covalent adenylylation, and the catalytic ATP/ADP binding site--our studies suggest that reaction with 5'-FSO2BzAdo occurs only at the active center. When glutamine synthetase was incubated with 5'-FSO2BzAdo, the decrease in catalytic activity obeyed pseudo-first order kinetics. The plot of the observed rate constant of inactivation versus the concentration of 5'-FSO2BzAdo was hyperbolic, consistent with reversible binding of the analogue to the enzyme prior to covalent attachment. Protection against inactivation was afforded by ATP and ADP; L-glutamate did not protect the enzyme against inactivation, but rather enhanced the rate of inactivation, consistent with the observations of others (Timmons, R. B., Rhee, S. G., Luterman, D. L., and Chock, P. B. (1974) Biochemistry 13, 4479-4485) that there is synergism in the binding of the two substrates to the enzyme. The incorporation of approximately 1.09 mol of the 5'-FSO2BzAdo/mol of glutamine synthetase subunit resulted in the total loss of enzymatic activity. The results suggest that 5'-FSO2BzAdo occupies the ATP binding site at the active center of glutamine synthetase and binds covalently to an amino acid residue nearby