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J. Biol. Chem., Vol. 276, Issue 7, 4622-4633, February 16, 2001

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Interaction Kinetics of Reversible Inhibitors and Substrates with Acetylcholinesterase and Its Fasciculin 2 Complex^*

Zoran Radi and Palmer Taylor

From the Department of Pharmacology, University of California San Diego, La Jolla, California 92093-0636

Fasciculin 2 (Fas2), a three-fingered peptide of 61 amino acids, binds tightly to the peripheral site of acetylcholinesterases (AChE; EC 3.1.1.7), occluding the entry portal into the active center gorge of the enzyme and inhibiting its catalytic activity. We investigated the mechanism of Fas2 inhibition by studying hydrolysis of cationic and neutral substrates and by determining the kinetics of interaction for fast equilibrating cationic and neutral reversible inhibitors with the AChE·Fas2 complex and free AChE. Catalytic parameters, derived by eliminating residual Fas2-resistant activity, reveal that Fas2 reduces k_cat/K_m up to 10⁶-fold for cationic substrates and less than 10³-fold for neutral substrates. Rate constants for association of reversible inhibitors with the active center of the AChE·Fas2 complex were reduced about 10⁴-fold for both cationic and neutral inhibitors, while dissociation rate constants were reduced 10²-to 10³-fold, compared with AChE alone. Rates of ligand association with both AChE and AChE·Fas2 complex were dependent on the protonation state of ionizable ligands but were also markedly reduced by protonation of enzyme residue(s) with pK_a of 6.1-6.2. Linear free energy relationships between the equilibrium constant and the kinetic constants show that Fas2, presumably through an allosteric influence, markedly alters the position of the transition state in the reaction pathway. Since Fas2 complexation introduces an energetic barrier for hydrolysis of substrates that exceeds that found for association of reversible ligands, Fas2 influences catalytic parameters by a more complex mechanism than simple restriction of diffusional entry and exit from the active center. Conformational flexibility appears critical for facilitating ligand passage in the narrow active center gorge for both AChE and the AChE·Fas2 complex.

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