Alterations in the Topography of Acetylcholinesterase Active Site Gorge after Binding of Peripheral Anionic Site Ligands

Abstract

The active site of AChE is located in a pocket- or gorge-like structure, as demonstrated eighteen years ago by electron paramagnetic resonance (EPR) (Šentjurc et al., 1976a, b). EPR results were supported later by topographic studies using polyclonal and monoclonal antibodies (Ogert et al., 1990). Final confirmation came from x-ray crystal-lographic analysis (Sussman et al., 1991) that additionally provided a detailed three-dimensional structure of the gorge. The peripheral anionic site is located at the lip of the gorge (Shafferman et al., 1992; Radić et al., 1993). Ligands binding specifically to this site are known to inhibit AChE catalytic activity. Their mechanism of action, however, is still unknown. In order to investigate this mechanism we studied and compared the effects on the microtopography of the active site gorge of fetal bovine serum AChE of two peripheral anionic site-binding ligands, monoclonal antibody 25B1 (Fab fragments) and propidium. The EPR approach, which allowed detection of conformational alterations in the active site gorge, was used in our experiments.