Studies on the Mechanism of Action of Penicillopepsin

Several high resolution structures of aspartic proteinases are now known, (see Gilliland et al.1 for a recent list and earlier contributions in this volume) and detailed information has been obtained on the binding of substrate analogue inhibitors from the crystallographic analysis of their complexes with the enzymes. Several proposals for their mechanism of action have been made.2–7 Yet we still do not fully understand how these enzymes function. The crystallographic data are insufficient to rationalize the extensive experimental information available from studies on specificity,8 steady-state and presteady-state kinetics,8 isotope exchange experiments,9,10 transpeptidation reactions11–13 and low-temperature kinetics.14 Thus, we do not know much about the molecular events that are responsible for the so-called “secondary specificity”,8 that is, the very large increases in catalytic efficiency with increasing length of substrates, nor do we understand the structural details of transpeptidation reactions. It is also not clear what the rate-determining steps of the reactions are. In our recent studies we have attempted to throw some more light on these phenomena.