| Summary: | Acetylcholinesterase is an enzyme which plays an important part in neural transmission. This enzyme, however, has not been extensively investigated, especially at the molecular level, due to the difficulties in obtaining pure and well characterized forms of the enzyme. This thesis describes a method for preparing such a form of AchE from Electrophorus electricus, followed by NMR studies on the interaction between this form and various inhibitors. Inhibitors range in size from a small trimethylammonium ion to a large atropine molecule, but all contain a quaternary ammonium group. In contrast to previously reported large line broadenings of inhibitor NMR resonances upon interaction with natural AchE, the present results showed no significant line broadening with any except one of the inhibitors in the same concentration range. In order to further investigate the binding site in the active center of the enzyme, the active esteratic site was permanently blocked by sulfonylation. Inhibitors interacting with this modified AchE indeed show line broadenings, giving rise to bound linewidths which are well within the expected theoretical limit. The cause of the differences in observed linewidths of inhibitors between natural and modified AchE is discussed in terms of changes in exchange rates and flexibility of bound inhibitors. The present overall results confirm the importance of prior characterization of the enzyme hence explaining the discrepancy between the line broadenings of inhibitors interacting with natural AchE observed by another worker and ones observed in current work, and also indicate that the active region of natural AchE is likely to be large, the effective size being at least the size of atropine, and/or located on the surface of the enzyme . === Science, Faculty of === Chemistry, Department of === Graduate
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