Mechanistic origins of the substrate selectivity of serine proteases.
Serine proteases catalyze the hydrolysis of amide bonds of their protein and peptide substrates through a mechanism involving the intermediacy of an acyl-enzyme. While the rate constant for formation of this intermediate, k(2), shows a dramatic dependence on peptide chain length, the rate constant for the intermediate's hydrolysis is relatively insensitive to chain length. To probe the mechanistic origins of this phenomenon, we determined temperature dependencies and solvent isotope effects for the alpha-chymotrypsin-catalyzed hydrolysis of Suc-Phe-pNA (K(s) = 1 mM, k(2) = 0.04 s(-)(1), and k(3) = 11 s(-)(1)), Suc-Ala-Phe-pNA (K(s) = 4 mM, k(2) = 0.9 s(-)(1), and k(3) = 42 s(-)(1)), and Suc-Ala-Ala-Pro-Phe-pNA (K(s) = 0.1 mM, k(2) = 98 s(-)(1), and k(3) = 71 s(-)(1)). We found that while the van't Hoff plots for K(s) and the Eyring plots for k(3) are linear for all three reactions, the Eyring plots for k(2) are convex, indicating that the process governed by k(2) is complex, possibly involving a coupling between active site chemistry and protein conformational isomerization. This interpretation is strengthened by solvent isotope effects on k(2) that are largely temperature-independent. Furthermore, the dependence of k(2) on peptide length is manifested entirely in the enthalpy of activation, suggesting a mechanism of catalysis by distortion. Taken together, this analysis of acylation suggests that extended substrates which can engage in subsite interactions are able to efficiently trigger the coupling mechanism between chemistry and a conformational isomerization that distorts the substrate and thereby promotes nucleophilic attack.
Case
April
A
Laboratory for Drug Discovery in Neurodegeneration, Harvard Center for Neurodegeneration and Repair, 65 Landsdowne Street, Fourth Floor, Cambridge, Massachusetts 02139, USA.
Stein
Ross L
RL
D002384:Catalysis; D002918:Chymotrypsin / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007554:Isotopes; D007700:Kinetics; D012697:Serine Endopeptidases / Q000378:metabolism*; D012997:Solvents; D013379:Substrate Specificity; D013816:Thermodynamics
D007554:Isotopes; D012997:Solvents; D012697:Serine Endopeptidases; D002918:Chymotrypsin; C117608:alpha-chymotrypsin