Synthesis and evaluation of TAC-based inhibitors of papain as mimics of cystatin b

An approach for mimicking protein-protein interactions by using a discontinuous epitope to construct a mimic that is about one tenth of the size of a natural inhibitor of papain, namely, cystatin B, is described. The discontinuous epitope of cystatin B, which is involved in the interaction with papain, was mimicked by synthesis of a tripodal molecular construct by using the triazacyclophane (TAC) scaffold. The mimic contains three peptide arms: one that mimics the N terminus, one that mimics the C terminus, and one that mimics the beta-hairpin loop structure of cystatin B. These peptide sequences were assembled on the TAC scaffold. The resulting cystatin mimic, CysTACtin 9, showed excellent inhibition of papain with a K, of 12 nM, which approaches the inhibitory potency of cystatin B (K-i = 0.12 nM). Experiments with molecular constructs that contained one or two arms or a mixture of the nonscoffolded peptides showed that both scaffolding and the presence of the three peptide arms are crucial for a successful mimic.