Structure and Conformational Heterogeneity of a Weak Toxin from the Cobra Naja kaouthiaVenom

Resonances in the two-dimensional ¹H NMR spectra of a weak toxin (WTX) from the venom of cobra Naja kaouthiafor all 65 amino acid residues were assigned. The amino acid sequence of WTX, determined by the sequential assignment of spin systems, was found to be similar to that of the CM-9a toxin from the N. kaouthiavenom. Unlike CM-9a, WTX contains an additional Trp36 residue; Lys50 and Tyr52 are interchanged; and there is a Thr residue in place of Arg2. For some residues of WTX, the presence of two components of approximately equal intensities in the spectra was shown, which is explained by the conformational heterogeneity of the polypeptide owing to the cis–transisomerization of the peptide bond Arg32–Pro33. The data (contacts of the nuclear Overhauser effect, constants of spin–spin coupling of protons, and rates of exchange of amide protons for deuterium of the solvent) made it possible to determine the secondary structure of two forms of WTX, which is characterized by the presence of two antiparallel -sheets, one of which consists of two strands (regions 1–5 and 13–17) and the other, of three strands (regions 23–28, 38–43, and 55–59).