Molecular conformation and function of erabutoxins as studied by nuclear magnetic resonance.

The 270-MHz proton NMR spectra of erabutoxins a, b and c from Laticauda semifasciata in 2H2O solution were observed together with [15-N6-acetyllysine]erabutoxin b, [27-N6-acetyllysine]-erabutoxin b and [47-N6-acetyllysine]erabutoxin b. The lysine epsilon-methylene proton resonances of erabutoxin b are assigned to individual residues. The epsilon-methylene proton resonance of Lys-27 is significantly broad, indicating that the mobility of this residue is restricted. Upon acetylation of Lys-27 of erabutoxin b, the pKa values of three other lysine residues are lowered by about 0.2, indicating long-range interactions among lysine residues. All the methyl proton resonances are assigned to amino acid types, primarily by the spin-echo double-resonance method. The pH dependences of proton chemical shifts were analyzed by the nonlinear least-square method, for obtaining pKa values and protonation shifts. The interproton nuclear Overhauser effect enhancements were measured for elucidating the spatial proximity of methyl-bearing residues and aromatic residues. On the basis of these NMR data and with the crystal structures by Low et al. and by Petsko et al., the methyl proton resonances of all the valine, leucine, and isoleucine residues and Thr-45 have been identified. The microenvironments of Tyr-25, His-26, Trp-29, four lysines and eight methyl-bearing residues have been elucidated. The addition of the paramagnetic hexacyanochromate ion causes broadening of the proton resonances of Thr-45, Lys-47, Ile-50, Trp-29 and Ile-36 residues located on one end of the molecule of erabutoxin b. The positively charged invariant residues of Lys-47 and Arg-33 at this part of the molecule are probably involved in the binding to the receptor protein.