Inhibitor Report for: N-Methyl-acridinium

The acetylcholinesterase (AChE) active site consists of a gorge 2 nm deep that is lined with aromatic residues. A serine residue near the base of the gorge defines an acylation site where an acyl enzyme intermediate is formed during the hydrolysis of ester substrates. Residues near the entrance to the gorge comprise a peripheral site where inhibitors like propidium and fasciculin 2, a snake neurotoxin, bind and interfere with catalysis. Like certain other cationic ligands that bind specifically to the acylation site, N-methylacridinium can still interact with the acylation site in the AChE-fasciculin 2 complex. At 310 K (37 degrees C), the equilibrium dissociation constant KL' for N-methylacridinium binding to the complex was 4.0 +/- 0.7 microM, less than an order of magnitude larger than the KL = 1.0 +/- 0.3 microM for N-methylacridinium interaction with human AChE in the absence of fasciculin 2. To assess whether fasciculin 2 can sterically block access of a ligand to the acylation site, thermodynamic and kinetic constants for the interaction of N-methylacridinium with AChE in the presence and absence of fasciculin 2 were measured by fluorescence temperature jump relaxation kinetics. During progressive titration of the enzyme with increasing concentrations of N-methylacridinium, a prominent relaxation in the 0.1-1 ms range was observed in the absence of fasciculin 2. When excess fasciculin 2 was added, the prominent relaxation shifted to the 0.3-1 s range. Estimates of total AChE concentrations, KL, or KL' from analyses of relaxation amplitudes agreed well with those from equilibrium fluorescence, confirming that the relaxations corresponded to the bimolecular reactions of interest. Further analysis of the relaxation times in the absence of fasciculin 2 gave estimates of the N-methylacridinium association rate constant k12 = 8 x 10(8) M-1 s-1 and dissociation rate constant k21 = 750 s-1 at 310 K (37 degrees C). For the AChE-fasciculin 2 complex, the corresponding constants were k12' = 1.0 x 10(5) M-1 s-1 and k21' = 0.4 s-1. Thus the rate constants decreased by more than 3 orders of magnitude when fasciculin 2 was bound, consistent with a pronounced steric blockade of N-methylacridinium ingress to and egress from the acylation site.

By examining inhibitor interactions with single and multiple site-specific mutants of mouse acetylcholinesterase, we have identified three distinct domains in the cholinesterase structure that are responsible for conferring selectivity for acetyl- and butyrylcholinesterase inhibitors. The first domain is the most obvious; it defines the constraints on the acyl pocket dimensions where the side chains of F295 and F297 primarily outline this region in acetylcholinesterase. Replacement of these phenylalanine side chains with the aliphatic residues found in butyrylcholinesterase allows for the catalysis of larger substrates and accommodates butyrylcholinesterase-selective alkyl phosphates such as isoOMPA. Also, elements of substrate activation characteristic of butyrylcholinesterase are evident in the F297I mutant. Substitution of tyrosines for F295 and F297 further alters the catalytic constants. The second domain is found near the lip of the active center gorge defined by two tyrosines, Y72 and Y124, and by W286; this region appears to be critical for the selectivity of bisquaternary inhibitors, such as BW284C51. The third domain defines the site of choline binding. Herein, in addition to conserved E202 and W86, a critical tyrosine, Y337, found only in the acetylcholinesterases is responsible for sterically occluding the binding site for substituted tricyclic inhibitors such as ethopropazine. Analysis of a series of substituted acridines and phenothiazines defines the groups on the ligand and amino acid side chains in this site governing binding selectivity. Each of the three domains is defined by a cluster of aromatic residues. The two domains stabilizing the quaternary ammonium moieties each contain a negative charge, which contributes to the stabilization energy of the respective complexes.