Reaction pathways and free energy profiles for cholinesterase-catalyzed hydrolysis of 6-monoacetylmorphine

Abstract

As the most active metabolite of heroin, 6-monoacetylmorphine (6-MAM) can penetrate into the brain for the rapid onset of heroin effects. The primary enzymes responsible for the metabolism of 6-MAM to the less potent morphine in humans are acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). The detailed reaction pathways for AChE- and BChE-catalyzed hydrolysis of 6-MAM to morphine have been explored, for the first time, in the present study by performing first-principles quantum mechanical/molecular mechanical free energy calculations. It has been demonstrated that the two enzymatic reaction processes follow the similar catalytic reaction mechanism, and the whole catalytic reaction pathway for each enzyme consists of four reaction steps. According to the calculated results, the second reaction step associated with the transition state TS2a/TS2b should be rate-determining for the AChE/BChE-catalyzed hydrolysis, and the free energy barrier calculated for the AChE-catalyzed hydrolysis (18.3 kcal/mol) is 2.5 kcal/mol lower than that for the BChE-catalyzed hydrolysis (20.8 kcal/mol). The free energy barriers calculated for the AChE- and BChE-catalyzed reactions are in good agreement with the experimentally derived activation free energies (17.5 and 20.7 kcal/mol for the AChE- and BChE-catalyzed reactions, respectively). Further structural analysis reveals that the aromatic residues Phe295 and Phe297 in the acyl pocket of AChE (corresponding to Leu286 and Val288 in BChE) contribute to the lower energy of TS2a relative to TS2b. The obtained structural and mechanistic insights could be valuable for use in future rational design of a novel therapeutic treatment of heroin abuse.

Kinetic characterization of cholinesterases and a therapeutically valuable cocaine hydrolase for their catalytic activities against heroin and its metabolite 6-monoacetylmorphine

“…The reaction pathways of cholinesterase-catalyzed hydrolyses of heroin and 6-MAM were studied in our previous computational studies 44–45 through first-principles quantum mechanics and molecular mechanics-free energy (QM/MM-FE) simulations. The optimized reactant complexes (e.g.…”

“…Our previous molecular dynamics (MD) simulations on the structures of enzyme-substrate complexes started from the X- Ray crystal structures deposited in the protein databank (PDB) (AChE: code 1B41; BChE: 2XQF and 1P0P). Molecular docking and subsequent optimization were carried out using a similar protocol described previously 44 . Briefly, the acetyl group of the substrate (heroin or 6-MAM) was positioned in the oxyanion hole (consisting of Gly116, Gly117, and Ala199 in BChE, or Gly121, Gly122, and Ala204 in AChE, or Gly116, Gly117, and Ser199 in CocH1), and the positively charged amino-group of the substrate (heroin and 6-MAM) was placed in the choline-binding site near Trp82 in BChE and CocH1 or Trp86 in AChE.…”

Unexpected Reaction Pathway for butyrylcholinesterase-catalyzed inactivation of “hunger hormone” ghrelin

“…In addition, the geometry optimization at the QM/MM(B3LYP/6-31G*:CHARMM27) level was followed by single-point energy calculation at the QM/MM(B3LYP/6-311++G**:CHARMM27) level, which may be denoted as the QM/MM(B3LYP/6-311++G**:CHARMM27)//QM/MM(B3LYP/6-31G*:CHARMM27) level for convenience. Our previous QM/MM calculations on BChE-involved enzyme reactions 59 60 61 62 63 64 65 66 revealed that these QM levels (B3LYP/6-31G* for geometry optimization and B3LYP/6-311++G** for subsequent single-point energy calculations) are adequate for the QM/MM calculations on these enzymatic reaction systems.…”

In silico Study of Potential Non-oxime Reactivator for Sarin-inhibited Human Acetylcholinesterase

“…hAChE residue in the catalytic triad, Ser203, will attack the phosphoryl group from sarin. This nucleophilic attack involves the oxygen atom of Ser203 (Qiao et al, 2014). The bond between phosphorus and fluorine becomes weaker as the partial bond forms between the oxygen in serine and the phosphorus in sarin.…”