Proton‐Nuclear‐Magnetic‐Resonance Study of the Conformation of Neurotoxin II from Middle‐Asian Cobra (Naja naja oxiana) Venom

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Abstract
A proton NMR study at 100 and 300 MHz of neurotoxin II from the venom of Middle-Asian cobra N. naja oxiana was performed in 2H2O and H2O solutions. By means of chemical modification and double resonance all the aromatic residue resonances were assigned. From the NMR titration curves, pK values of histidine 4 and histidine 31 residues were determined. For 1 of the 2 neighboring tryptophan residues pH dependence (in the 2-8 pH range) of the chemical shifts of indole protons was revealed. According to the different sensitivity of the linewidth of indole NH resonances to pH in H2O solution, the accessibility of each of the tryptophan residues was estimated. Temperature dependence was observed for the linewidth of the aromatic resonances of the tyrosine 24 residue. Deuterium exchange rates were measured for amide protons and for C(2)H histidine resonances. The NMR data obtained suggest that the 2 histidine residues and 1 of the tryptophan residues should be localized on the surface of the protein globule, that arginine residues should be present in the environment of histidine 4, that histidine 31 and the buried tryptophan are possibly localized in close spatial proximity and that the side chain of tyrosine 24 is buried within the protein globule.