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Effects of α-Deuteration and of Aza and Thia Analogs of l-Tryptophan on Formation of Intermediates in the Reaction of Escherichia coli Tryptophan Indole-lyase

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journal contribution
posted on 17.12.1996 by Milton J. Sloan, Robert S. Phillips
Tryptophan indole-lyase catalyzes the hydrolytic cleavage of l-tryptophan to indole and ammonium pyruvate. After the enzyme is mixed with l-tryptophan in the rapid-scanning stopped-flow spectrophotometer, there is an absorbance increase at 505 nm in the pre-steady state attributed to formation of a quinonoid intermediate, which occurs in at least three consecutive first-order phases. Reaction with [α-2H]-l-tryptophan results in significant primary kinetic isotope effects on the first two phases, and there is a significant isotope effect on the amplitude of the absorbance increase in the second phase. This result suggests that proton transfer to carbon to form the indolenine intermediate is relatively slow and is probably at least partially rate-determining. Reaction of l-tryptophan in the presence of benzimidazole results in a rapid increase in absorbance in the first phase, followed by a decrease in absorbance in the second phase, with rate constants very similar to those observed without benzimidazole. We have also examined aza and thia analogs of l-tryptophan, with the benzene ring of the indole replaced by pyridine or thiophene. Both 4,5-thiatryptophan and 6,7-thiatryptophan form quinonoid intermediates in the reaction with tryptophan indole-lyase; however, 6,7-thiatryptophan is a better substrate (kcat/Km = 32% of l-trp) for tryptophan indole-lyase than is 4,5-thiatryptophan (kcat/Km = 4% of l-trp). Benzimidazole affects the pre-steady-state reaction of 6,7-thiatryptophan in a way similar to l-tryptophan, while benzimidazole does not affect the pre-steady-state reaction of 4,5-thiatryptophan. 4-Aza-, 5-aza-, 6-aza-, and 7-aza-l-tryptophan are all very slow substrates (kcat < 1% of l-trp) for Escherichia coli tryptophan indole-lyase. β-Indazolyl-l-alanine is a relatively good substrate and exhibits a quinonoid intermediate in its reaction with tryptophan indole-lyase. 6-Aza- and 7-azatryptophan accumulate quinonoid intermediates in the reaction with tryptophan indole-lyase, whereas 4-aza- and 5-azatryptophans do not significantly accumulate quinonoid intermediates, and these latter compounds exhibit very high Km values. Addition of benzimidazole does not change the rapid-scanning stopped-flow spectra of 6-aza- and 7-azatryptophan. This suggests that the rate-determining step in the reaction changes depending on the position and type of heteroatom substitution. For 6-aza- and 7-azatryptophan, the very slow rates of elimination may be due to slow C-protonation of the azaindole, while for 4,5-thiatryptophan, the elimination of thienopyrrole is probably slow. Of all analogs examined, 6,7-thiatryptophan is most similar to tryptophan in its reaction with E. coli tryptophan indole-lyase.