Shifts in Backbone Conformation of Acetylcholinesterases upon Binding of Covalent Inhibitors, Reversible Ligands and Substrates

Zoran Radić

doi:10.3390/cryst11121557

Abstract

The influence of ligand binding to human, mouse and Torpedo californica acetylcholinesterase (EC 3.1.1.7; AChE) backbone structures is analyzed in a pairwise fashion by comparison with X-ray structures of unliganded AChEs. Both complexes with reversible ligands (substrates and inhibitors) as well as covalently interacting ligands leading to the formation of covalent AChE conjugates of tetrahedral and of trigonal-planar geometries are considered. The acyl pocket loop (AP loop) in the AChE backbone is recognized as the conformationally most adaptive, but not necessarily sterically exclusive, structural element. Conformational changes of the centrally located AP loop coincide with shifts in C-terminal α-helical positions, revealing interacting components for a potential allosteric interaction within the AChE backbone. The stabilizing power of the aromatic choline binding site, with the potential to attract and pull fitting entities covalently tethered to the active Ser, is recognized. Consequently, the pull can promote catalytic reactions or relieve steric pressure within the impacted space of the AChE active center gorge. These dynamic properties of the AChE backbone inferred from the analysis of static X-ray structures contribute towards a better understanding of the molecular template important in the structure-based design of therapeutically active molecules, including AChE inhibitors as well as reactivators of conjugated, inactive AChE.

Highlights

Acetylcholinesterase (EC 3.1.1.7; AChE) is an essential regulatory enzyme in cholinergic neurotransmission of vertebrates
The similarity of conjugated backbone conformations with those of apo AChEs suggest that active center geometries of native, apo human AChE (hAChE) appear quite adequate for achieving
The similarity of conjugated backbone conformations with those of apo AChEs suggest that active center geometries of native, apo hAChEs appear quite adequate for achieving ffaasstt ccaattaallyyttiicc ttuurrnnoovveerrss ooff tthhee bboouunndd pphhyyssiioollooggiiccaall ssuubbssttrraattee wwiitthhoouutt aaddddiittiioonnaall ssuubbssttrraattee-iinndduucceedd aaddjjuussttmmeennttss

Summary

Introduction

Acetylcholinesterase (EC 3.1.1.7; AChE) is an essential regulatory enzyme in cholinergic neurotransmission of vertebrates It has evolved, consistent with its important function, into near-perfect biological catalyst [1]. Because of the size of the ~70 kDa catalytic subunit, only approaches capable of resolving “static” structural snapshots have been successful in obtaining atomic structures of AChE [2]. Those structural snapshots, documented in more than 200 PDB-deposited X-ray structures, have not revealed any significant conformational outliers and seem to largely reveal one dominant conformation of the backbone [3]. Because macromolecular structural dynamics and the catalytic activity of AChE can be detected even in the crystalline state [6,7], analysis of low-level conformational diversity in AChE X-ray structures could provide a glimpse of a larger-scale flexibility requirement for physiological function in solution

Methods

Results

Conclusion