Characterization of potato proteinase inhibitor II reactive site mutants

Potato proteinase inhibitor II (PI-2) is composed of two sequence repeats. It contains two reactive site domains. We developed an improved protocol fo r the production of PI-2 using the yeast Pichia pastoris as the expression host. We then assessed the role of its two reactive sites in the inhibition of trypsin and chymotrypsin by mutating each of the two reactive sites in various ways. From these studies it appears that the second reactive site s trongly inhibits both trypsin (K-i = 0.4 nm) and chymotrypsin (K-i = 0.9 nm ), and is quite robust towards mutations at positions P2 or P1'. In contras t, the first reactive site inhibits only chymotrypsin (K-i = 2 nm), and thi s activity is very sensitive to mutations. Remarkably, replacing the reacti ve site amino acids of domain I with those of domain II did not result in i nhibitory activities similar to domain II. The fitness for protein engineer ing of each domain is discussed.