Hydrogen exchange kinetics and the mechanism of reaction B of yeast tryptophan synthase

Catalytic mechanism of the tryptophan synthase alpha(2)beta(2) complex. Effects of pH, isotopic substitution, and allosteric ligands

The mechanism of the tryptophan synthase alpha(2)beta(2) complex from Salmonella typhimurium is explored by determining the effects of pH, of temperature, and of isotopic substitution on the pyridoxal phosphate-dependent reaction of L-serine with indole to form L-tryptophan. The pH dependence of the kinetic parameters indicates that three ionizing groups are involved in substrate binding and catalysis with pK(a)1 = 6.5, pK(a)2 = 7.3, and pK(a)3 = 8.2-9. A significant primary isotope effect (approximately 3.5) on V and V/K is observed at low pH (pH 7), but not at high pH (pH 9), indicating that the base that accepts the alpha-proton (betaLys-87) is protonated at low pH, slowing the abstraction of the alpha-proton and making this step at least partially rate-limiting. pK(a)2 is assigned to betaLys-87 on the basis of the kinetic isotope effect results and of the observation that the competitive inhibitors glycine and oxindolyl-L-alanine display single pK(i) values of 7.3. The residue with this pK(a) (betaLys-87) must be unprotonated for binding glycine or oxindolyl-L-alanine, and, by inference, L-serine. Investigations of the temperature dependence of the pK(a) values support the assignment of pK(a)2 to betaLys-87 and suggest that the ionizing residue with pK(a)1 could be a carboxylate, possibly betaAsp-305, and that the residue associated with a conformational change at pK(a)3 may be betaLys-167. The occurrence of a closed to open conformational conversion at high pH is supported by investigations of the effects of pH on reaction specificity and on the equilibrium distribution of enzyme-substrate intermediates.

A comparative study of the kinetics and stereochemistry of the serine hydroxymethyltransferase- and tryptophan synthase-catalysed exchange of the pro-2R and pro-2S protons of glycine

The stereospecificity of the serine hydroxymethyltransferase (EC 2.1.2.1)- and tryptophan synthase (EC 4.2.1.20)- catalysed exchange of the pro-2R and pro-2S alpha-protons of glycine was investigated by using 13C n.m.r. The exchange process is described in terms of a minimal four-step mechanism, and a method for analysing the exchange process by complete progress curves is presented. It is shown that serine hydroxymethyltransferase does not have absolute stereospecificity for the pro-2S-proton of glycine, but it catalyses the exchange of this proton 7400 times faster than the pro-2R proton of glycine. Tryptophan synthase is shown preferentially to catalyse the exchange of the pro-2R proton of glycine at a rate 380 times faster than the pro-2S proton of glycine. The exchange rates for the rapidly exchanged alpha-protons of glycine are similar for both enzymes. However, the exchange rates of the slowly exchanged alpha-protons differ by an order of magnitude. The structural features that may be responsible for the differences in the stereospecificity of the two enzymes are discussed.

A proton-magnetic-resonance study of hydrogen-exchange reactions of yeast tryptophan synthase

1H n.m.r. was used to observe tryptophan formation from indole and L-serine, proton exchange at C-2 of L-tryptophan, and proton exchange at C-2 of L-serine, catalysed by yeast tryptophan synthase in the presence of 2H2O. Tryptophan synthesis took place with compulsory replacement of C-2 hydrogen by solvent hydrogen. The exponential decay rate (kobs) of the serine exchange reaction was insensitive to serine concentration in the range 2-20mM and was used to calculate kcat./Km values. However, kobs. was very sensitive to pH* values in the range 6.5-8.5 and the data require that the free enzyme is active in the base form resulting from two inseparable ionizations of pKa 7.3, and inactive after a third ionization controlled by a pKa of 7.5. Initial rates measured by u.v. absorbance and colorimetric procedures were used to calculate kinetic parameters of the tryptophan synthesis reaction. From pH 6.5 to 7, kcat./Km values for L-serine in the tryptophan synthesis and hydrogen exchange reactions were indistinguishable and increased rapidly under the control of two acid-base groups of pKa 6.7 and 7.2. Above pH 7, this equivalence breaks down because the exchange reaction alone is responsive to the third pKa value of the free enzyme. The pH dependence of the catalytic constant for tryptophan synthesis was qualitatively similar to that of the kobs. for serine exchange. A mechanism to explain the results is contrasted with recent proposals for the Escherichia coli system.