On the Role of αThr183 in the Allosteric Regulation and Catalytic Mechanism of Tryptophan Synthase

pH Dependence of Tryptophan Synthase Catalytic Mechanism

“…2 In order to identify the ionizable residues responsible for the control of the chemical transformation from the external aldimine to ␣-aminoacrylate, there are two sources of information, the three-dimensional structures of the native enzyme and catalytic intermediates and the kinetic data obtained for the wild type enzyme and mutants. The more relevant available structures are as follows: (i) the internal aldimine in the presence of either sodium (12,15,35,40), potassium, or cesium ions (40), also with bound ␣-subunit ligands (12,55); (ii) the external aldimine of the wild type enzyme in the presence of sodium ions (35); and (iii) the ␣-aminoacrylate Schiff base in the presence of sodium ions and 5-fluoroindole propanol phosphate, an ␣-subunit ligand (12). Other structures of the internal and external aldimine, determined on mutant enzymes (35,36,41,59,60), should be considered with caution, because mutation might have small but critical consequences on the location and local environment of ionizable residues.…”

“…On the basis of the three-dimensional structures of the internal aldimine (12,15,35), the external aldimine (35), and the aminoacrylate (12), there are several ionizable residues within the ␤-active site that, to some extent, could assist the reaction with an acid-base catalysis: ␤His-86, ␤Lys-87, ␤Glu-109, ␤His-115, ␤Lys-167, and ␤Asp-305 (Fig. 7).…”

“…Stereoview of the active site of the internal aldimine (a), external aldimine (b), and ␣-aminoacrylate (c). The internal aldimine structure was generated from Protein Data Bank code 1kfk (35), the external aldimine from code 1kfj (35), and the aminoacrylate from code 1a5s (12). PLP is displayed in yellow with the phosphate group colored by atom type.…”

“…(vi) The conversion of the external aldimine to the aminoacrylate is accompanied by the release of a proton originating from the Schiff base nitrogen (24). Surprisingly, no studies have addressed the pH dependence of the catalytic rates on the S. typhimurium enzyme, the form for which the three-dimensional structure of the internal aldimine (12,15,35), the external aldimine (35,36), and the ␣-aminoacrylate (12) has been determined. Recently, it was also found that monovalent cations and ␣-subunit ligands affect the distribution of intermediates as well as the catalytic properties of TS, triggering regulatory signals and stabilizing alternative open and closed conformations of the ␣-and ␤-subunits (23,27,(37)(38)(39)(40)(41).…”

Sensor Domain of Histidine Kinase KinB of Pseudomonas

“…For example, a similar guanidinium-guanidinium ion pair (Arg-3/Arg-89) is present in tryptophan synthase from Salmonella typhimurium. This interaction is stabilized by an aspartic acid carboxylate (PDB entry 1KFC) (49). However, this pair as well as many others reported previously are exposed toward the solvent region instead of being buried in the core of the protein (48).…”