Mutagenesis Studies on the N-terminus and Thr54 of Naja naja atra (Taiwan cobra) Chymotrypsin Inhibitor

Abstract

Ala-screening mutagenesis studies on Arg1, Pro2, Arg3, Phe4 and Thr54 of Naja naja atra (Taiwan cobra) chymotrypsin inhibitor showed that inhibitory potency and gross conformation of the mutants were not significantly different from those of wild-type inhibitor. Nevertheless, the R1A mutant had an appreciable decrease in the structural stability underlying thermal unfolding and urea-induced denaturation. Alternatively, deleting the first three residues at the N-terminus caused a reduction in structural stability as well as inhibitory potency. In sharp contrast to wild-type and other mutated inhibitors, R1A mutant and truncated mutant completely lost their inhibitory activity when the inhibitors were incubated with chymotrypsin for periods of up to 3 h. The loss of activity correlated with chymotryptic cleavage of inhibitors as evidenced by SDA-PAGE. Taken together, these results reflect that the globally structural rigidity of N. naja atra chymotrypsin inhibitor functionally affects the sustainable period in inhibiting chymotrypsin activity, and that the intact N-terminus might contribute to this event.