Structure
Bertrand, et al, created the first crystal structure of MGL in its . MGL is a part of the α-β hydrolase family of enzymes. This category of proteins contains an eight-stranded beta sheet, specifically containing seven parallel and one antiparallel constituent strand (), surrounded by alpha-helices. [1]
MGL has a characteristic lid domain comprised of two large loops that surround . This region of the enzyme is the putative membrane-interacting moiety of the protein, which is consistent with its amphipathic nature and outward-facing hydrophobic residues. It has been proposed that 2-AG and other lipids suspended in the hydrophobic section of the cell membrane associate with this region before entering the active tunnel. Interestingly, MGL’s lid domain may be more flexible than its analogs in other α-β hydrolases, due to the various conformations it assumed in crystallographic studies. [1] Currently, there is no consensus about the quaternary arrangement of MGL. Some researchers claim that it is found as a monomer [1] [2], whereas others believe it to be a physiologically active dimer. [3]
MGL contains an active site tunnel roughly 25Å long and 8Å wide residing beneath its lid region. Like its substrate, 2-AG and other monoacylglycerols, the tunnel is largely amphipathic. Hydrophobic residues dominate the tunnel except for the terminal occluded region, which houses the catalytic triad. In its apo form, the catalytic region is not solvent-exposed, unlike the wide opening of the tunnel. [1][3] A unique structural motif in MGL is a 5Å solvent-exposed hole connecting the exterior to the catalytic site. It is proposed to act as an “exit hole” through which the glycerol product leaves MGL. The fatty acid product, namely arachidonic acid, presumably travels back through the active site tunnel. [1][2][3]
MGL’s serine hydrolase chemistry is executed by a (Ser132-His279-Asp249) and seems to utilize the same mechanism as the much-studied chymotrypsin. In this mechanism, an activated serine nucleophile cleaves the ester bond of the substrate. The subsequent tetrahedral intermediate is stabilized by the , formed by the main-chain nitrogens of Ala61 and Met (or Se-Met) 133.
Biological/Medical Relevance
2-AG activates the same cannabinoid receptors (CB1 and CB2) for both anandamide and the main psychoactive compound found in Cannabis sativa, Δ9-Tetrahydrocannabinol (THC), via retrograde signaling. It is the most abundant endocannabinoid found in the brain, and it is believed to possess analgesic, anti-inflammatory, immunomodulating, neuroprotective, and hypotensive effects, as well as being capable of inhibiting growth of cancer cells in prostate and breast tissue. [3][4]
Studies have shown that around 85% of 2-AG in the rat brain is metabolized by MGL, while other lipases such as fatty acid amide hydrolase (FAAH) process the remainder of the metabolite (Blankman). This evidence indicates that MGL is the primary enzyme for the metabolism of 2-AG in humans, making it a highly desirable target molecule for the modulation of 2-AG concentration in the body. Most MGL is found in the cell membrane, although it has been discovered in the cytosol as well. (Bertrand, Labar, Schalk) Although the most-studied role of MGL is the degradation of 2-AG in the brain, MGL may also play a role in adipose tissue to complete the hydrolysis of triglycerides into fatty acids and glycerol as well as working in the liver to mobilize triglycerides for secretion. (Labar, Schalk)
Three general MGL inhibitor classes have been observed: noncompetitive, partially irreversible inhibitors such as URB602; irreversible serine-reactive inhibitors such as JZL184 and SAR629 (SAR-629); and cysteine-reactive inhibitors such as N-arachidonoylmaleimide (NAM). (Bertrand) Despite the existence of such compounds, there is a strong demand for the creation of more highly-specific and more potent inhibitors that could be used as anti-pain drugs for their ability to keep 2-AG active in the neuronal synapses. (Labar)
MGL is an enzyme of immense interest for cancer research, with the potential to shed light on the role of fatty acids in malignancy, the varying efficacy of endocannabinoids as anti-cancer agents in different body tissues, and the multifarious influences on the PI-3k/Akt signaling pathway in carcinogenesis.
MGL exerts a twofold influence on cancer growth; endocannabinoids such as 2-AG have been shown to have anti-tumorigenic properties (Labar, Nomura) and a high fatty-acid concentration may play a role in the promotion of cancer aggressiveness and malignancy. One study showed that in aggressive breast, melanoma, ovarian, and prostate cancer cells, MGL activity was higher than in nonaggressive malignant cells. (Nomura) Subsequently, the creation of effective MGL inhibitors may help to treat highly aggressive cancers in addition to their proposed use as analgesics.
Contrary to this evidence, however, a recent study found that in lung, breast, ovary, stomach, and colorectal cancer, MGL expression was reduced. It was found that in addition to the previously-discussed control over the 2-AG degradation and fatty acid synthesis pathways, MGL also interacted with key phospholipids (specifically, the 3-phosphorylated phosphoinositide products of PI-3K) in the PI3K/Akt signaling and tumor growth pathway. In this role, it serves as a negative effector. Indeed, decreased concentrations of MGL were found to increase Akt phosphorylation. (Hong)
Further research into MGL’s role in different body tissues is necessary to more fully elucidate its complex role in cancer pathology. A specific research topic for exploration is MGL’s effect on exogenous cannabinoid medications given to cancer patients as a palliative medication. (Nomura)
