Cystatin inhibition of cathepsin B requires dislocation of the proteinase occluding loop. Demonstration by release of loop anchoring through mutation of His110
Cystatins A and C were both shown to inhibit cathepsin B by a two-step mechanism, involving an initial weak interaction followed by a conformational change. 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 converted the binding to an apparent one-step reaction. The second step of cystatin binding to cathepsi