Catalysis uncoupling in a glutamine amidotransferase bienzyme by unblocking the glutaminase active site

Nitrogen is incorporated into various metabolites by multifunctional glutamine amidotransferases via reactive ammonia generated by glutaminase hydrolysis of glutamine. Although this process is generally tightly regulated by subsequent synthase activity, little is known about how the glutaminase is inhibited in the absence of an activating signal. Here, we use imidazoleglycerolphosphate synthase as a model to investigate the mechanism of glutaminase regulation. A structure of the bienzyme-glutamine complex reveals that the glutaminase active site is in a catalysis-competent conformation but the ammonia pathway toward the synthase active site is blocked. Mutation of two residues blocking the pathway leads to a complete uncoupling of the two reactions and to a 2800-fold amplification of glutaminase activity. Our data advance the understanding of coupling enzymatic activities in glutamine amidotransferases and raise hypotheses of the underlying molecular mechanism.