Proton‐Nuclear‐Magnetic‐Resonance Study on Molecular Conformations of Long Neurotoxins

Abstract

The 270-MHz proton N M R spectra were analyzed of tlie long neurotoxins a-bungarotoxin from Bungcrru.~ nzulzicincius and Toxin B from Nuju nuju. The aromatic proton resonances were completely assigned to individual nuclei for a-bungarotoxin and in part for toxin B. The pH dependences of proton chemical shifts were analyzed by the nonlinear least-square method, for obtaining pK, values and protonation shifts. The pK, values of Tyr-25, an invariant residue of neurotoxins, are 12.1 for x-bungarotoxin and 11.3 for toxin B, suggesting the presence of a strong hydrogen bond involving Tyr-25 in a-bungarotoxin. The Trp-29 residues of both toxins show a common titration shift due to the carboxylate group of Asp-31 and a similar structural arrangement of functionally invariant pair of Trp-29 and Asp-31 is implied. From the temperature dependences of the chemical shifts of His-68 and a methyl group of x-bungarotoxin, the local structure around His-68 near the tail part is shown to be more flexible than the other part. The six main-chain amide protons of a-bungarotoxin exchange most slowly with solvent deuterons and are found by interproton nuclear Overhauser effects to be in the /]-sheet near the aromatic ring of Tyr-25 residue. Hydrogen --$ deuterium exchange rates in 2 H 2 0 solution at 37 "C were measured of slowly exchanging amide protons of 3-bungarotoxin, toxin B, and two short neurotoxins, namely cobrotoxin and erabutoxin b. The two long neurotoxins have amide protons with relatively long half-times spanning as long as 10-100 h, but the two short neurotoxins do not have amide protons with half-times longer than 3 h. The distributions of the half-times of amide proton exchange indicate the structural rigidity of neurotoxins in the order, Neurotoxic proteins from the venom of Hydrophid and Elapid snakes block the neuromuscular transmission at the post-synaptic level by the specific binding to the acetylcholine receptor protein [I]. They are classified into two groups, short neurotoxins with 60-62 amino acid residues and long neurotoxins with 71 -74 residues. The three-dimensional structures in crystals have been elucidated of both types of neurotoxins, iximi.1) erabutoxin b from Luricaudu sern(fusciuzu [2-51 and x-cobratoxin from Nuja naju siumensis [6]. The solution. structures have been investigated more intensively for short neurotoxins than for long neurotoxins by physico-chemical methods including nuclear magnetic resonance (NMR) spectroscopy [7 -131. However, appreciable differences in the affinity to acetylcholine receptor [14,15] and in the effect of chemical modifications [16-181 have been found between the groups of short and long neurotoxins. Accordingly, it is of interest to compare the solution structures of long neurotoxins with those of short neurotoxins and to elucidate the common structural characteristics responsible for neurotoxicity.a-Bungarotoxin and toxin B are long neurotoxins isolated from the venom of Bungarus rnulticinctus and Naja nuja, respectively (see Table 1 for the primary structures) [...

Conversion of spin state and protein structure in Rhodospirillum rubrum ferric cytochrome c' coupled with dissociation of dimer

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Structural studies of .alpha.-bungarotoxin. 1. Sequence-specific proton NMR resonance assignments

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A comparative investigation of snake venom neurotoxins and their triplet-state tryptophan-disulfide interactions using phosphorescence and optically detected magnetic resonance

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