Conformation and inhibitory properties of peptides based on the tissue kallikrein-aprotinin complex

A series of peptides encompassing the primary binding segment (residues 12-19) of aprotinin has been synthesized and tested for their ability to inhibit porcine pancreatic kallikrein. A minimum sequence of five amino acids spanning residues 12-16 of aprotinin is necessary for inhibition of porcine pancreatic kallikrein. An octapeptide homologous with the binding segment of aprotinin has a Ki-value of 1.2 x 10(-4) M. The solution structure of the octapeptide was studied by one- and two-dimensional NMR methods for comparison with the known structure of the segment of aprotinin that contacts tissue kallikrein. NMR experiments suggest that the peptide is either a random coil or that it samples several conformations on the NMR time scale. Analysis of the molecular dynamics trajectory of the octapeptide also suggests that the peptide is highly flexible. Thus, inhibition by the octapeptide occurs because of its homology with residues 12-19 of aprotinin. Moreover, the absence of a stable solution conformation similar to that of the binding segment of aprotinin is consistent with the 150,000-fold increase in Ki of the octapeptide compared to intact aprotinin.