Structure and conformational heterogeneity of a weak toxin from the cobra Naja kaouthia venom

Resonances in the two-dimensional H-1 NMR spectra of a weak toxin (WTX) fro m the venom of cobra Naja kaouthia for all 65 amino acid residues were assi gned. The amino acid sequence of WTX, determined by the sequential assignme nt of spin systems, was found to be similar to that of the CM-9a toxin from the N. kaouthia venom. Unlike CM-9a, WTX contains an additional Trp36 resi due; 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 approx imately equal intensities in the spectra was shown, which is explained by t he: conformational heterogeneity of the polypeptide owing to the cis-trans isomerization of the peptide bond Arg32-Pro33. The data (contacts of the nu clear Overhauser effect, constants of spin-spin coupling of protons, and ra tes of exchange of amide protons for deuterium of the solvent) made it poss ible to determine the secondary structure of two forms of WTX, which is cha racterized by the presence of two antiparallel beta -sheets, one of which c onsists of two strands (regions 1-5 and 13-17) and the other, of three stra nds (regions 23-28, 38-43, and 55-59).