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

The chatacteristic feature of the crystal structure of erabutoxin b, a short ncurotoxin from Laticuuda semifasciata , and α‐cobratoxin, a long neurotoxin from Naja naja siamensis , is the presence of a triple‐stranded antiparallel pleated β‐sheet structure formed by the central and the third peptide loops. In the present study, wc have studied the assignment of slowly exchangeable amide protons of Laticauda semifasciata III from L.semifaciata using nuclear Overhauser effects (NOE) and spin‐dccoupling methods. The results show that nearly all of the slowly exchangeable amide protons are to be assigned to the back‐bone iimide protons, involved in the triple‐stranded antiparallel pleated β‐sheet structure, indicating that this sheet is stable in 2 H 2 O solution. In contrast, the amide protons in short neurotoxins are readily exchangeable under the same experimental condition, suggesting that long neurotoxins have a more rigid sheet structure than short ones. This rigidity may come from the hydrophobic and hydrogen bond interaction between the central loop and the tail, which is not present in short neurotoxins. Since the functionally important residues are located on this β‐sheet, the different kinetic properties of the neurotoxins are well correlated with the difference in the rigidity of the β‐sheet.