Temperature effects on S1- and S'1-enantioselectivity of alpha-chymotrypsin

The temperature dependence of E (enantiomeric ratio or enantioselectivity, a quantitative measure for enzyme stereospecificity) has been studied for the alpha-chymotrypsin catalysed hydrolysis of the enantiomeric N-Boc-L/D-TyrOMe, L/D-TyrOMe, Ac-L/D-PhgOMe, L/D-PhgOMe and for the kinetically controlled synthesis of the diastereomeric dipeptides N-Ac-L-Tyr-L/D-ArgNH2 and N-Ac-L-Tyr-L/D-ValNH2. The results show that the S1- and S'1-enantioselectivity can be modulated by the temperature (3-15 fold for the studied substrates in the range 5-45 degrees C). For L/D-PhgOMe a reversal in stereospecificity was found in this temperature interval. For the studied substrates both an increase or decrease of the enantiomeric ratio with increasing temperature was observed. For these processes the following relation for the temperature dependence of E has been derived ln E = ln(kL/kD) = -(delta deltaH# - delta deltaHb)/RT + (delta deltaS# - delta deltaSb)/R where kL and kD are apparent second order rate constants for the reactions with the L- and D-enantiomers, respectively. Delta delta denotes the differences between the thermodynamic parameters for transformation of the enantiomeric substrates. The subscript b applies for the binding of the substrate or the nucleophile and the superscript # for the formation of the transition state of the enzyme acylation or deacylation. For the studied processes either the enthalpy (delta deltaH# - delta deltaHb) or the entropy (delta deltaS# - delta deltaSb) term was found to control the discrimination. Thus, the enantioselectivity decreases or increases with temperature, respectively. The influence of ground-state interactions and transition-state stabilisation on enzyme enantioselectivity has been discussed.