Phosphonylation and Activity Loss Mechanism of the Catalytic Triad of Fatty Acid Amide Hydrolase: Theoretical Study of a Model System

Methyl arachidonyl fluorophosphonate (MAFP) is an inhibitor of the fatty acid amide hydrolase (FAAH). We studied the phosphonylation reaction of the serine241 (Ser241)-serine217 (Ser217)-lysine142 (Lys142) catalytic triad of FAAH by MAFP, which leads to the loss of FAAH enzyme activity. This theoretical study was carried out by employing the B3LYP/6-311G(d,p) and MP2/6-311G(d,p) methods through a simplified model. Two reaction pathways were considered. Path A is a two. step addition. elimination process of the FAAH catalytic triad and the first step (addition process) is the rate. determining step and involves a zwitterionic trigonal bipyramidal intermediate. In this reaction pathway, both Ser217 and Lys142 in FAAH contribute to the base-catalyzed activation of the nucleophile Ser241 while Ser217 serves as a bridge between Lys142 and Ser241. In addition, one of the solvent water molecules performs a key role to act as a "hydrogen bridge" connecting the Lys142 residue and MAFP by donating and accepting protons to promote long-range proton transfer. Path B (after mutation of the Lys142 residue to alanine) is also a stepwise process. The bulk effect of water as a solvent was considered via the polarizable continuum model (PCM). The obtained results show that for this phosphonylation reaction, Path A is the most favorable mechanism with an activation free energy barrier of 64.9 kJ.mol(-1) in aqueous solution. We also conclude that the mutation of the FAAH catalytic triad at the Lys142 residue decreases the rate of phosphonylation. This is in good agreement with the experimental observations.