Abstract
The aim of the present study is to understand the structural features responsible for the lethal activity of snake venom cardiotoxins. Comparison of the lethal potency of the five cardiotoxin isoforms isolated from the venom of Taiwan cobra (Naja naja atra) reveals that the lethal potency of CTX I and CTX V are about twice of that exhibited by CTX II, CTX III, and CTX IV. In the present study, the solution structure of CTX V has been determined at high resolution using multidimensional proton NMR spectroscopy and dynamical simulated annealing techniques. Comparison of the high resolution solution structures of CTX V with that of CTX IV reveals that the secondary structural elements in both the toxin isoforms consist of a triple and double-stranded antiparallel beta-sheet domains. Critical examination of the three-dimensional structure of CTX V shows that the residues at the tip of Loop III form a distinct "finger-shaped" projection comprising of nonpolar residues. The occurrence of the nonpolar "finger-shaped" projection leads to the formation of a prominent cleft between the residues located at the tip of Loops II and III. Interestingly, the occurrence of a backbone hydrogen bonding (Val27CO to Leu48NH) in CTX IV is found to distort the "finger-shaped" projection and consequently diminish the cleft formation at the tip of Loops II and III. Comparison of the solution structures and lethal potencies of other cardiotoxin isoforms isolated from the Taiwan cobra (Naja naja atra) venom shows that a strong correlation exists between the lethal potency and occurrence of the nonpolar "finger-shaped" projection at the tip of Loop III. Critical analysis of the structures of the various CTX isoforms from the Taiwan cobra suggest that the degree of exposure of the cationic charge (to the solvent) contributed by the invariant lysine residue at position 44 on the convex side of the CTX molecules could be another crucial factor governing their lethal potency.