Enhancement of chymotrypsin-inhibitor/substrate interactions by 3 M NaCl

A series of 16 bovine pancreatic trypsin inhibitor variants mutated at the P(1) position of the binding loop and seven tetrapeptide p-nitroanilide (pNa) substrates of the general formula: suc-Ala-Ala-Pro-Aaa-pNa (where Aaa denotes either: Phe, Arg, Lys, Leu, Met, Nva, Nle) were used to investigate the influence of high salt concentration on the activity of bovine chymotrypsin. The increase of the association constant (K(a)) and the specificity index (k(cat)/K(m)) in the presence of 3 M NaCl highly depends on the chemical nature of the residue at the P(1) position. The highest increase was observed for inhibitors/substrates containing the basic side chains at this site. Surprisingly, for the remaining 13 residues the observed salt effect is not correlated with any side chain properties. In particular, there is a lack of correlation between the accessible non-polar surface area and the magnitude of the salt effect. It suggests that salt-induced increase of the K(a) and k(cat)/K(m) values is not caused by the enhancement of the hydrophobic interactions in chymotrypsin-inhibitor/substrate complex. Moreover, the increase of the K(a) and k(cat)/K(m) values occurs only in the presence of Na(+) ions, while K(+) and Li(+) ions do not change the activity of chymotrypsin. Additionally, the activities of two other proteinases: bovine trypsin and Streptomyces griseus proteinase B were tested in the presence of 3 M NaCl using their specific substrates. The activity of both enzymes was almost not affected by the presence of high NaCl concentration.