Effect of Osmolytes and Guanidinium Chloride on the Enzymatic Properties of Bovine α-Chymotrypsin

We have studied the effect of guanidinium chloride (Gdn.Cl) and different osmolytes such as betaine, trimethylamine-N-oxide (TMAO) and urea on the rate of chymotrypsin catalyzed reaction. The rates were measured using three synthetic chromogenic substrates, succinyl-ala-ala-pro-arg-pNA (AAPR), succinyl-ala-ala-pro-leu-pNA (AAPL), and succinyl-ala-ala-pro-phe-pNA (AAPF). Qualitatively, the results with the three substrates were identical. Guanidinium chloride and urea produced a linear decrease while TMAO produced a linear increase in the rate with increase in osmolyte concentration. Betaine had practically no effect on the rate of enzyme catalyzed reaction up to a concentration of 1.2 M. However, quantitatively the rate change per molar concentration of osmolyte (or Gdn.Cl) was significantly larger for AAPR that has a polar and cationic reactive site residue than the two substrates (AAPL and AAPF) that have non-polar reactive site residues. These results suggest that the chemical nature of the substrate (and presumably the active site of the enzyme) plays an important role in determining the effect of osmolytes in enzyme catalyzed reactions.