Substrate inhibition of Pseudomonas aeruginosa elastase by 3-(2-furyl)acryloyl-glycyl-L-phenylalanyl-L-phenylalanine

Product inhibition applications

A product inhibition study provides important insight into the binding mechanism of an enzyme, especially in the identification of abortive complexes, but seldom is it the only tool required to solve the mechanism completely. Always keep in mind that more than one mechanism may be consistent with the patterns, and several alternative schemes should be analyzed by including abortive complexes and, as a last resort, isomerization steps or slow product release steps in the interpretation. Support for the proposed mechanism should be garnered from other data, such as kinetic studies with alternative substrates and competitive inhibitors, positional and molecular isotope exchange studies, binding studies, and isotope effects. A well-characterized binding mechanism complete with the identification of abortive complexes is even more important as rational drug design becomes more prevalent. Product inhibition studies represent an important tool that is relatively easy to apply to gain significant information about the binding mechanism of most enzymes.

Substrate analogs as mechanistic probes for the bifunctional chorismate synthase from Neurospora crassa

Analogs of EPSP (4-8) have been prepared, and their activity as substrates for the chorismate synthase from Neurospora crassa has been characterized kinetically. The enzyme appears to show strict discrimination against substitution at the Z-position of the enol ether side chain as well as against substitution at the S-position of the reduced analogs. Both the glycolyl and (R)-lactyl analogs 4 and (R)-5 are good substrates, with (R)-5 having a higher V value than the natural substrate. Three substrates, including EPSP, have been found to show significant substrate inhibition with this enzyme, which at present can be explained by a noncompetitive model involving formation of a catalytically incompetent, ternary ES2 complex. A significant secondary kinetic isotope effect on V of 1.10 +/- 0.02 has been observed at C-3 with EPSP, indicating that C-O bond cleavage is kinetically significant at saturating substrate concentration; this effect is severely depressed at limiting substrate, with D(V/K) = 0.97 +/- 0.02. A similar effect is found for the primary deuterium isotope effect at C-6R, as observed previously [Balasubramanian, S., Davies, G. M., Coggins, J. R., & Abell, C. (1991) J. Am. Chem. Soc. 113, 8945-8946]. The primary isotope effects at C-6R with reduced analogs (R)-5 and (S)-6 are significantly larger than those with EPSP. The larger values of V and DV for (R)-5, when compared to EPSP, are evidence that release of chorismate is partially rate-limiting under saturating conditions. Incubation of the enzyme with reduced 5-deazaFMN does not result in any observable formation of chorismate, consistent with previous results indicating that reduced flavin is chemically involved in the synthesis of chorismate from EPSP [Ramjee, M. N., Balasubramanian, S., Abell, C., Coggins, J. R., Davies, G. M., Hawkes, T. R., Lowe, D. J., & Thorneley, R. N. F. (1992) J. Am. Chem. Soc. 114, 3151-3153].