Comparative study of Heat-stable enterotoxin analogs in crystal and solution states

摘要

Comparative study of heat-stable enterotoxin analogs from Escherichia coli porcine strain (STp) in crystal and solution states was performed. Structures of 3-mercaptopropionic acid (Mpr) analogs, [Mpr~5] STp (5-17) and [Mpr5, Leu13]STp(5-17) were determined in crystal state. Structures of STp (5-17) and [Leu~(13)]STp(5-17) were those in solution state. In the crystalline state, the replacement of Ala~(13)with Leu~(13) resulted in the change of torsion angles (phi) at amino acid residues included in the second beta-turn (Asn~(11)-Cys~(14)), except for a change of the side chain. While in the solution state, the replacement at 13 th position did not cause notable changes in the chemical shift of protons and the distribution of NOEs. Almost all inter-residual NOEs observed for NH, C_(alpha)H, and C_(beta)H of STp (5-17) were also detected in the proton pairs of [Leu~(13)] STp (5-17). ~3J_((alpha)N) values for residues included in the second beta-turn were similar to each other between these analogs. These results strongly suggest that the backbone structure of ST is not affected by the replacement of Ala~(13) with Leu~(13) in the solution state. Above results demonstrated that the change of torsion angles, observed in the crystalline state by the replacement of Ala~(13) with Leu~(13), might be caused by the change of the molecular packing in crystal. Results in this study also confirmed that the ST molecule possesses the flexibility at the region of the second beta-turn. Taking into account that STloses its toxicity by the replacement of Ala~(13) with Leu~(13), the second beta-turn region is the important part of ST to generate its toxicity. Therefore, the flexibility at this region is expected to play an important role in the recognition of ST byits receptor protein.