Abstract
The crystal structure of neurotoxin a from the venom of Philippines sea snake Laticauda semifasciata has been determined at 2.5 A resolution by x-ray diffraction. Comparison with the structure of neurotoxin b from the same source indicates that the two toxins differ only by substitution at His 26. Earlier chemical work had suggested a difference in chain length and considerable differences in amino acid composition. The difference between our a and b toxins is the same as that reported from sequence analysis for the related erabutoxins a and b from Japanese sea snake, and suggests that the Philippines toxin may be identical to erabutoxin. The replacement of His 26 by a shorter side-chain in toxin a has no effect on the structure of the rest of the molecule. In particular, the protruding loop, which we believe interacts with the acetylcholine receptor, is not affected, even though His 26 is in the loop.