Modulation of structured polypeptide specificity

A peptide ligand specific for human Kallikrein comprising a polypeptide comprising at least three reactive groups, separated by two loop sequences, and a molecular scaffold which forms covalent bonds with the reactive groups of the polypeptide such that two polypeptide loops are formed on the molecular scaffold, wherein the loops of the peptide ligand comprise five amino acids and wherein a first loop comprises the motif GrxWx1Ax2Gr (SEQ ID NO:

• 3) or GrxFx1Yx2Gr (SEQ ID NO:
• 6) and a second loop comprises the motif GrA(ψCH2NH)HQ/TXLGr (SEQ ID NO:
• 212), wherein Gr at the C-terminal of the motif in the first loop is the Gr at the N-terminal of the motif in the second loop, and wherein x is any amino acid, x1 is proline or azetidine carboxylic acid, x2 is arginine, N-methyl arginine, homoarginine, or guanidylphenylalanine, and Gr represents cysteine.