A ligand-induced, temperature-dependent conformational Change in penicillopepsin. Evidence from nonlinear Arrhenius plots and from circular dichroism studies.

The effect of temperature on the rate constants of hydrolysis of various substrates by penicillopepsin is dependent on the length of the substrate. For the series Ac-(Ala)m-Lys-Nph-(Ala)n-amide (where Ac- is acetyl- and Nph- is p-nitrophenylalanyl-), where m and n = 0-2, substrates lacking both P'2 and P3 residues give linear Arrhenius plots with an energy of activation of about 55 kJ.mol-1. The Arrhenius plots of substrates in which an alanine residue occupies P'2 show a sharp break at an average transition temperature of 10.5 degrees C. The activation energies are approximately 90 kJ.mol-1 below and approximately 54 kJ.mol-1 above the transition temperature, respectively. For substrates in which P3 is occupied, the average transition temperature is 14.2 degrees C. In this case, the activation energies are 66 kJ.mol-1 below and from 26 to 39 kJ.mol-1 above the transition point. The most probable explanation of these phenomena is that substrate interaction at subsites S3 and/or S'2 of the enzyme induces a temperature-dependent conformational change. Physical evidence for this comes from the observation that the temperature dependence of a CD absorption band at 242 nm of a penicillopepsin-pepstatin complex shows a sharp break that corresponds to those observed in the Arrhenius plots of substrates with alanine at P'2 and P3, whereas the same CD band in the free enzyme is linearly dependent on temperature.