The importance of the second hairpin loop of cystatin C for proteinase binding. Characterization of the interaction of Trp-106 variants of the inhibitor with cysteine proteinases

The single Trp of human cystatin C, Trp-106, is located in the second hairpin loop of the proteinase binding surface. Substitution of this residue by Gly markedly altered the spectroscopic changes accompanying papain binding and reduced the affinity for papain, actinidin, and cathepsins B and H by 300-900-fold. The decrease in affinity indicated that the side chain of Trp-106 contributes a similar free energy, -14 to -17 kJ·mol-1, to the binding to all four cysteine proteinases, corresponding to about 20-30% of the total binding energy. Replacement of Trp-106 by Phe led to a smaller (30-120-fold) decrease in affinity for the four enzymes than Gly substitution. The binding energy of the Phe residue corresponded to 20-45% of that of Trp, showing that a phenyl group can only partly substitute for the indole ring. The reduced affinities of the cystatin C Trp-106 variants for all proteinases studied were due almost exclusively to increased dissociation rate constants. The second hairpin loop thus contributes to the binding primarily by keeping cystatin C anchored to the proteinase once the complex has been formed. This role is partly in contrast to that of the N-terminal region, which increases the affinity of cystatin C for cathepsin B by increasing the association rate constant. Removal of the N-terminal region of the Trp-106Gly variant by proteolytic cleavage substantially weakened the binding to papain and cathepsin B. The resulting affinity indicated that the first hairpin loop (the "QVVAG-region"), which is the only region of the proteinase binding surface remaining intact in the truncated variant, contributes 40-60% of the total free energy of binding of cystatin C to both proteinases