Refined homology model of monoacylglycerol lipase: toward a selective inhibitor

Abstract

Monoacylglycerol lipase (MGL) is primarily responsible for the hydrolysis of 2-arachidonoylglycerol (2-AG), an endocannabinoid with full agonist activity at both cannabinoid receptors. Increased tissue 2-AG levels consequent to MGL inhibition are considered therapeutic against pain, inflammation, and neurodegenerative disorders. However, the lack of MGL structural information has hindered the development of MGL-selective inhibitors. Here, we detail a fully refined homology model of MGL which preferentially identifies MGL inhibitors over druglike noninhibitors. We include for the first time insight into the active-site geometry and potential hydrogen-bonding interactions along with molecular dynamics simulations describing the opening and closing of the MGL helical-domain lid. Docked poses of both the natural substrate and known inhibitors are detailed. A comparison of the MGL active-site to that of the other principal endocannabinoid metabolizing enzyme, fatty acid amide hydrolase, demonstrates key differences which provide crucial insight toward the design of selective MGL inhibitors as potential drugs.

Fig. 1

Modeling alignment for the sequences of human MGL (Q99685) and RsbQ from Bacillus subtilis (PDB ID: 1WOM). Aligned query and template residues that are identical are highlighted in black; conserved residues (according to the BLOSUM62 scoring matrix), in grey. The predicted secondary structure for MGL and the experimentally determined secondary structure for RsbQ are shown above and below the sequence, respectively. β sheets are denoted by an arrow, and α helices by a block. The residues of the catalytic triad are marked with an inverted triangle (▼)

Fig. 2

Tertiary structure and schematic diagram of MGL. The catalytic triad S122, D239 and H269 is shown in stick representation. The enzyme takes theα/β hydrolase fold, with four helices (D’₁-D’₄) forming a lid covering the active site. Due to lack of sequence homology, a structure prediction prior to Y34 is not possible. The coloring of the structure is equivalent for both representations

Fig. 3

Plot reflecting the CαRMSD fluctuations between the equilibrium structure of MGL (solid line) and template structure of RsbQ (PDB ID 1WOM; dotted line) and respective trajectory snapshots versus time

Fig. 4

Average backbone structure for MGL snapshots during molecular dynamics simulations. A red, wider tube indicates greater RMSD across the trajectory, whereas a narrow blue tube shows greater stability

Fig. 5

The MGL binding pocket is shown in gray, and the predicted binding modes of 2-AG (b), AM6701 (c) and JZL184 (d) are shown in stick representation. Residues that may be important for hydrogen bonding are shown in stick representation (green carbons). Design of more potent and selective MGL inhibitors could be facilitated by taking advantage of hydrogen-bonding opportunities or by occupying the sub-pocket

Fig. 6

The binding channel of MGL (a) and binding channels of FAAH (b) calculated by CAVER [59]