Cystatin inhibition of cathepsin B requires dislocation of the proteinase occluding loop. Demonstration by release of loop anchoring through mutationof His110

Cystatins A and C were both shown to inhibit cathepsin B by a two-step mech anism, involving an initial weak interaction followed by a conformational c hange. Disruption of the major salt bridge anchoring the occluding loop of cathepsin B to the main body of the enzyme by mutation of His110 to Ala con verted the binding to an apparent one-step reaction, The second step of cys tatin binding to cathepsin B must therefore be due to the inhibitor having to alter the conformation of the enzyme by displacing the occluding loop to allow a tight complex to be formed. Cystatin A mas appreciably less effect ive in displacing the loop than cystatin C, resulting in a considerably low er overall inhibition rate constant. (C) 2000 Federation of European Bioche mical Societies. Published by Elsevier Science B.V. All rights reserved.