Title: The Nature of Activated Non-classical Hydrogen Bonds: A Case Study on Acetylcholinesterase-Ligand Complexes Berg L, Mishra BK, Andersson CD, Ekstrom F, Linusson A Ref: Chemistry, 22:2672, 2016 : PubMed
Molecular recognition events in biological systems are driven by non-covalent interactions between interacting species. Here, we have studied hydrogen bonds of the CHY type involving electron-deficient CH donors using dispersion-corrected density functional theory (DFT) calculations applied to acetylcholinesterase-ligand complexes. The strengths of CHY interactions activated by a proximal cation were considerably strong; comparable to or greater than those of classical hydrogen bonds. Significant differences in the energetic components compared to classical hydrogen bonds and non-activated CHY interactions were observed. Comparison between DFT and molecular mechanics calculations showed that common force fields could not reproduce the interaction energy values of the studied hydrogen bonds. The presented results highlight the importance of considering CHY interactions when analysing protein-ligand complexes, call for a review of current force fields, and opens up possibilities for the development of improved design tools for drug discovery.
Acetylcholinesterase (AChE) is an essential enzyme that terminates cholinergic transmission by rapid hydrolysis of the neurotransmitter acetylcholine. Compounds inhibiting this enzyme can be used (inter alia) to treat cholinergic deficiencies (e.g. in Alzheimer's disease), but may also act as dangerous toxins (e.g. nerve agents such as sarin). Treatment of nerve agent poisoning involves use of antidotes, small molecules capable of reactivating AChE. We have screened a collection of organic molecules to assess their ability to inhibit the enzymatic activity of AChE, aiming to find lead compounds for further optimization leading to drugs with increased efficacy and/or decreased side effects. 124 inhibitors were discovered, with considerable chemical diversity regarding size, polarity, flexibility and charge distribution. An extensive structure determination campaign resulted in a set of crystal structures of protein-ligand complexes. Overall, the ligands have substantial interactions with the peripheral anionic site of AChE, and the majority form additional interactions with the catalytic site (CAS). Reproduction of the bioactive conformation of six of the ligands using molecular docking simulations required modification of the default parameter settings of the docking software. The results show that docking-assisted structure-based design of AChE inhibitors is challenging and requires crystallographic support to obtain reliable results, at least with currently available software. The complex formed between C5685 and Mus musculus AChE (C5685*mAChE) is a representative structure for the general binding mode of the determined structures. The CAS binding part of C5685 could not be structurally determined due to a disordered electron density map and the developed docking protocol was used to predict the binding modes of this part of the molecule. We believe that chemical modifications of our discovered inhibitors, biochemical and biophysical characterization, crystallography and computational chemistry provide a route to novel AChE inhibitors and reactivators.
Representative scheme of ACHE structure and an image from PDBsum server
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