Crystallographic studies of phosphonate-based alpha-reaction transition-state analogues complexed to tryptophan synthase

In an effort to use a structure-based approach for the design of new herbic ides, the crystal structures of complexes of tryptophan synthase with a ser ies of phosphonate enzyme inhibitors were determined at 2.3 Angstrom or hig her resolution. These inhibitors were designed to mimic the transition stat e formed during the ct-reaction of the enzyme and, as expected, have affini ties much greater than that of the natural substrate indole-3-glycerol phos phate or its nonhydrolyzable analogue indole propanol phosphate (IPP), Thes e inhibitors are ortho-substituted arylthioalkylphosphonate derivatives tha t have an sp(3)-hybridized sulfur atom, designed to mimic the putative tetr ahedral transition state at the C3 atom of the indole, and lack the C2 atom to allow for higher conformational flexibility. Overall, the inhibitors bi nd in a fashion similar to that of IPP. Glu-49 and Phe-212 are the two acti ve site residues whose conformation changes upon inhibitor binding, A very short hydrogen bond between a phosphonate oxygen and the Ser-235 hydroxyl o xygen may be responsible for stabilization of the enzyme-inhibitor complexe s. Implications for the mechanism of catalysis as well as directions for mo re potent inhibitors are discussed.