Inhibitory activity and structural characterization of a C-terminal peptide fragment derived from the prosegment of the proprotein convertase PC7

Mammalian proprotein convertases (PCs) belong to the family of recently dis covered serine proteases responsible fur the processing of a large number o f precursor proteins into their active forms. The enzymatic activities of t he convertases have been implicated in a variety of disease states, such as cancer and infectious and inflammatory diseases. Like many other proteases , PCs are also synthesized as inactive proenzymes with N-terminal extension s as their prosegments. Here, we present the inhibitory activities of a num ber of "putative" interfacial peptide fragments derived from the proregion of PC7, We found that a peptide fragment corresponding to the C-terminal re gion (residues 81p-104p, or C24, E-1-A-V-L-A-K-H-E-A-V-R-W-H-S-E-Q-R-L-L-K- R-A-K-R-24) of the PC7 prosegment displays a strong inhibition (K-i = 7 nM) of the PC7 enzyme comparable to that of the full-length (104 residue) pros egment. The same 24 residue peptide shows significantly populated helical c onformations in an aqueous solution close to the physiological condition. S tructure calculations driven by NOE distance restraints revealed a slightly kinked helical conformation for the entire peptide, characterized by many side-chain/side-chain interactions including those involving charged residu es E8-R11-E15 and hydrophobic residues W12 and L19. These results suggest t hat the C-terminal region of the prosegment of PC7 may play a dominant role in conferring the inhibitory potency to the cognate enzyme and this strong inhibitory activity may be a direct consequence of the folded conformation of the peptide fragment in solution. We surmise that such a structure-func tion correlation for an inhibitory peptide could lead to the design and dis covery of molecules mimicking the specific interactions of the PC prosegmen ts for their cognate proteases.