title: Local conformational transition of toxin B from Naja naja as studied by nuclear magnetic resonance and circular dichroism.
abstract: The 270-MHz proton NMR spectra of toxin B, a long neurotoxin from Naja naja, in 2H2O solution were analyzed. From the analysis of the discontinuous pH dependence of His-21 C-2 proton chemical shift a 23 degrees C, toxin B is found to exhibit a slow-exchange conformational transition between the states A and B with the transition midpoint at pH 5.75. Separate peaks due to the states A and B were also observed for His-21 C-4 proton, Tyr-25 ring protons, phenylalanine ring protons and two C-alpha protons. This local conformational transition as induced by the protonation of His-21 residue is further confirmed by the measurement of the pH dependence of circular dichroism. In state A, the pKa value of His-21 is 6.6-6.9, indicating that His-21 residue is not significantly perturbed. The His-21 residue in state B is accessible to solvent, as found by the analysis of hydrogen leads to deuterium exchange rate. The low pKa value (4.6-4.9) of His-21 in stat B is probably due to the formation of a hydrogen bond of the imidazole ring. The conformational transition rate at 23 degrees C is obtained as 2.5 s-1 at the transition midpoint (pH 5.75), by the saturation transfer method. The conformational transition at 60 degrees C corresponds to a fast exchange case for the Tyr-25 ring proton resonances and to an intermediate exchange case for the His-21 C-2 and C-4 proton resonances. The transition rate at the midpoint is about 300 s-1 at 60 degrees C.
mesh_terms: D002942:Circular Dichroism; D004546:Elapid Venoms / Q000032:analysis*; D006639:Histidine; D006863:Hydrogen-Ion Concentration; D009682:Magnetic Resonance Spectroscopy; D009498:Neurotoxins / Q000302:isolation & purification*; D011487:Protein Conformation
chemical_list: D004546:Elapid Venoms; D009498:Neurotoxins; D006639:Histidine