Solution structure of alpha-conotoxin SI

The nuclear magnetic resonance solution structure of alpha-conotoxin SI has been determined at pH 4.2. The 36 lowest energy structures show that cw-co notoxin SI exists in a single major solution conformation and is stabilized by six hydrogen bonds. Comparisons are made between the SI solution struct ure and the solution and crystal structures of cc-conotoxin GI, Surprisingl y, a high degree of similarity between the backbone conformations of the GI crystal and the SI solution structures is seen in the region of lowest seq uence homology, namely residues Gly-8 to Ser-12, This similarity is more su rprising when considering that in SI a proline replaces the Arg-9 found in GI, The correspondence in conformation in this region provides the definiti ve evidence that it is the loss of the arginine basic charge at residue 9 w hich determines the differences in toxicity between GI and SI, rather than any changes in conformation induced by the cyclic proline residue. (C) 2000 Federation of European Biochemical Societies. Published by Elsevier Scienc e B.V. All rights reserved.