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Evidence for preorganization in quinonoid intermediate formation from L-Trp in H463F mutant Escherichia coli tryptophan indole-lyase from effects of pressure and pH.
MedLine Citation:
PMID:  22852771     Owner:  NLM     Status:  Publisher    
Abstract/OtherAbstract:
The effects of pH and hydrostatic pressure on the reaction of H463F tryptophan indole-lyase (TIL) have been evaluated. The mutant TIL shows very low activity for elimination of indole, but is still competent to form a quinonoid intermediate from L-tryptophan (Phillips, R. S., Johnson, N. and Kamath, A. V. (2002) Biochemistry 41, 4012-4019). Stopped-flow measurements show that the formation of the quinonoid intermediate at 505 nm is affected by pH, with a bell-shaped dependence for the forward rate constant, kf, and dependence on a single basic group for the reverse rate constant, kr, with pKa1 = 8.14 ± 0.15, pKa2 = 7.54 ± 0.15, kf,min = 18.1 ± 1.3 s-1, kf,max = 179 ± 46.3 s-1, kr,min = 11.4 ± 1.2 s-1 and kr,max = 33 ± 1.6 s-1. The pH effects may be due to ionization of Tyr-74 as the base and Cys-298 as the acid influencing the rate constant for deprotonation. High pressure stopped-flow measurements were performed at pH 8, which is the optimum for the forward reaction. The rate constants show an increase with pressure up to 100 MPa, and a subsequent decrease above 100 MPa. Fitting the pressure data gives kf,0 = 15.4 ± 0.8 s-1, ∆V‡ = -29.4 ± 2.9 cm3 mol-1, ∆β ‡ = -0.23 ± 0.03 cm3 mol-1 MPa-1 for the forward reaction and kr,0 = 20.7 ± 0.8 s-1, ∆V‡ = -9.6 ± 2.3 cm3 mol-1, ∆β ‡ = -0.05 ± 0.02 cm3 mol-1 MPa-1 for the reverse reaction. The primary kinetic isotope effect on quinonoid intermediate formation at pH 8 is small (~2) and is not significantly pressure-dependent, suggesting that the effect of pressure on kf may be due to perturbation of an active site preorganization step. The negative activation volume is also consistent with preorganization of the ES complex prior to quinonoid intermediate formation, and the negative compressibility may be due to the effect of pressure on the enzyme conformation. These results support the conclusion that the preorganization of the H463F TIL Trp complex, which is probably dominated by motion of the L-Trp indole moiety of the aldimine complex, contributes to quinonoid intermediate formation.
Authors:
Robert S Phillips; Ukoha Kalu; Sam Hay
Publication Detail:
Type:  JOURNAL ARTICLE     Date:  2012-8-1
Journal Detail:
Title:  Biochemistry     Volume:  -     ISSN:  1520-4995     ISO Abbreviation:  Biochemistry     Publication Date:  2012 Aug 
Date Detail:
Created Date:  2012-8-2     Completed Date:  -     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  0370623     Medline TA:  Biochemistry     Country:  -    
Other Details:
Languages:  ENG     Pagination:  -     Citation Subset:  -    
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