Mutational scanning of a hairpin loop in the tryptophan synthase beta-subunit implicated in allostery and substrate channeling.

The tryptophan synthases from Escherichia coli and Salmonella typhimurium are tetrameric enzymes, with an elongated TrpA.TrpB.TrpB.TrpA structure. Structural studies have identified residues 277-283 of TrpB as a potentially important region for the allosteric communication between the TrpA and TrpB subunits and for the transport of indole between their active sites through a hydrophobic tunnel. To explore the functional role of this region, we analyzed the effects of 19 single and double mutations in TrpB on the tryptophan synthase (TSase) and serine deaminase (SDase) activities of the TrpB2 dimer, either in the presence or in the absence of the TrpA subunit. The mutations of residues 273-283 could be divided into 4 classes. Mutations 1278A, F280G and M282A decreased the SDase and TSase activities of TrpB2 to similar extents. F280A decreased the SDase activity of TrpB2 more than its TSase activity, whereas the reverse was true for Y279L. F280A decreased the activation factor of TrpB2 by TrpA, whereas F280G increased it. The reaction steps and intramolecular contacts that could be affected by the mutations are described. The sequence 278-IYFGM-282, which is present in E. coli and S. typhimurium, is only found in 5 out of 42 organisms, whereas the sequence VLHGX is found in 21 organisms. Our results identified several mutations that could be used as structural probes to analyze precisely the roles of residues 278-282 and their evolution.