The Mechanism of Tryptophan Binding to Tryptophan Synthase from Escherichia coli

The kinetics of the binding of l‐tryptophan to the αholoβ₂ complex of tryptophan synthase from Escherichia coli have been measured by rapid‐mixing techniques under conditions where tryptophan release is mainly reate‐determining in tryptophan synthesis. The dependence of the three observable rate processes on the concentration of l‐tryptophan suggests a mechanism in which a rapid binding step is followed by two isomerizations. The effect of the substrate analogue indolepropanol phosphate on the kinetics of binding and synthesis from l‐serine and indole supports a branched mechanism with an unproductive enzyme‐ligand complex being the major species. The productive enzyme‐ligand complex absorbs light at 473 nm but not at 500 nm. These l‐tryptophan exist on the enzyme. The effects of protons, indole and indolepropanol phosphate on the three rate processes explain the dependence of k_caton the three non‐competitive ligands.