Toxicodynamics of Organophosphates with Human Acetylcholinesterase Interaction at Novel Site Trp-86 for Antidote Action

In-silico and in-vitro evaluation of human acetylcholinesterase inhibition by organophosphates

Organophosphates (OP) inhibit the acetylcholinesterase (AChE) activity and devastate the nervous system of pest however its mode of action is ubiquitous and acts similarly on human AChE (hAChE). Screening of OP was carried out by molecular docking with hAChE using Glide docking module of Schrodinger suite as the structural information of hAChE and OP together as co-crystal structure is rarely available. The docking was done at three different precision levels, high throughput virtual screening (HTVS), standard precision and extra precision. The ranking was done using over all binding energy i.e. dock score and molecular modelling generalized born surface area (MM-GBSA). Investigation reported Tryptophan (Trp86) residue involved in most interactions by forming a π-cation interaction apart from Ser203 on anionic subsite of hAChE. The top rank ligand was Phoxim ethyl phosphonate (PEP) interacting with Trp86, Gly121 and Ser203. However contact with Gly121 was lost during simulation and Asp74 appeared and sustained. Molecular dynamic simulation (GROMACS 4.5.5) of hAChE-PEP complex for 4 × 10⁴ pico-second with SPC16 water system at 310 K temperature explained the evident role of Trp86 in stabilizing the ligand at P-site of the enzyme. Asp74 and Tyr124 were noticed in conveying H-bonds. Trp86 has shown consistent and better stability of bond based on distance between residues and ligand. The top ranked OP i.e. PEP was used to establish a dose response relationship between OP and hAChE. PEP inhibits half of the enzyme activity at concertation of 29.99 μM (calculated by sigmoid plot) at R² = 0.996 and P < 0.0001.