Escherichia coli glutamine synthetase adenylyltransferase (ATase, EC 2.7.7.49): kinetic characterization of regulation by PII, PII-UMP, glutamine, and alpha-ketoglutarate

Glutamine synthetase adenylyltranferase (ATase, EC 2.7.7.49) catalyzes the adenylylation and deadenylylation of glutamine synthetase (GS), regulating GS activity. The adenylyltransferase (AT) reaction is activated by glutamine and by the unmodified form of the PII signal transduction protein and is inhibited by the uridylylated form of PII, PII-UMP. Conversely, the adenylyl-removing (AR) reaction is activated by PII-UMP and is inhibited by glutamine and by PII. Both AT and AR reactions are regulated by alpha-ketoglutarate, which binds to PII and PII-UMP. Here, we present a kinetic analysis of the AT and AR activities and their regulation. Both AT and AR reactions used a sequential mechanism of rapid equilibrium random binding of substrates and products. Activators and inhibitors had little effect on the binding of substrates, instead exerting their effects on catalysis. Our results were consistent with PII, PII-UMP, and glutamine shifting the enzyme among at least six different enzyme forms, two of which were inactive, one of which exhibited AR activity, and three of which exhibited AT activity. In addition to a site for glutamine, the enzyme appeared to contain two distinct sites for PII and PII-UMP. The PII, PII-UMP, and glutamine sites were in communication so that the apparent activation and inhibition constants for regulators depended upon each other. The binding of PII was favored by glutamine and its level reduced by PII-UMP, whereas glutamine and PII-UMP competed for the enzyme. alpha-Ketoglutarate, which acts exclusively through its binding to PII and PII-UMP, did not alter the binding of PII or PII-UMP to the enzyme. Rather, alpha-ketoglutarate dramatically affected the extent of activation or inhibition of the enzyme by PII or PII-UMP. A working hypothesis for the regulation of the AT and AR activities, consistent with all data, is presented.