Individual Assignments of the Amide Proton Resonances Involved in the Triple‐Stranded Antiparallel Pleated β‐Sheet Structure of a Long Neurotoxin, Laticauda Semifasciata I11 from Laticauda semifasciata

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Abstract
The characteristic feature of the crystal structure of erabutoxin b, a short neurotoxin from L. semifasciata, and .alpha.-cobratoxin, a long neurotoxin from Naja naja siamensis, is the presence of a triple-stranded antiparallel pleated .beta.-sheet structure formed by the central and 3rd peptide loops. The assignment of slowly exchangeable amide protons of Laticauda semifasciata III from L. semifasciata was studied using nuclear Overhauser effects (NOE) and spin-decoupling methods. Most of the slowly exchangeable amide protons were assigned to the back-bone amide protons involved in the triple-stranded antiparallel pleated .beta.-sheet structure, indicating that this sheet was stable in 2H2O solution. The amide protons in short neurotoxins were readily exchangeable under the same experimental condition, suggesting that long neurotoxins had a more rigid sheet structure than short ones. This rigidity may come from the hydrophobic and H-bond interaction between the central loop and the tail, which was not present in short neurotoxins. Since the functionally important residues were located on this .beta.-sheet, the different kinetic properties of the neurotoxins were well correlated with the difference in the rigidity of the .beta.-sheet.