DAGL has been studied in multiple domains of life, including bacteria, fungi, plants, insects, and mammals.[4] By searching with BLAST for the previously sequenced microorganism DAGL,[5] Bisogno et al discovered two distinct mammalian isoforms, designated DAGLα (DAGLA) and DAGLβ (DAGLB).[1] Most animal DAGL enzymes cluster into the DAGLα and DAGLβ isoforms.[4]
Mammalian DAGL is a crucial enzyme in the
regulation of homeostasis and disease.[6] As a result, much effort has been made toward investigating the mechanisms of action and the therapeutic potential of the system's receptors, endogenous ligands, and enzymes like DAGLα and DAGLβ.[6]
Structure
While both DAGLα and DAGLβ are extensively homologous (sharing 34% of their sequence[4]), DAGLα (1042 amino acids) is much larger than DAGLβ (672 amino acids) due to the presence of a sizeable C-terminal tail in the former.[1][7]
Between β strands 7 and 8 is a 50-60 residue regulatory loop that is believed to act as a well-positioned "lid" controlling access to the catalytic site.[7] Numerous phosphorylation sites have been identified on this loop as evidence of its regulatory nature.[7]
Mechanism
Diacylglycerol lipase uses a Serine-Aspartate-Histidine catalytic triad to hydrolyze the ester bond of an acyl chain from diacylglycerol (DAG), generating a monoacylglycerol (MAG), and a free fatty acid.[9][10] This hydrolytic cleavage mechanism for DAGLα and DAGLβ is more selective for the sn-1 position of DAG over the sn-2 position.[1]
Initially, histidine deprotonates serine forming a strong nucleophilic alkoxide, which attacks the carbonyl of the acyl group at the sn-1 position of DAG.[1] A tetrahedral intermediate briefly forms before the instability of the oxyanion collapses the tetrahedral intermediate to re-form the double bond while cleaving the ester bond.[11] The monoacylglycerol product, which in this case is 2-arachidonoylglycerol, is released leaving behind an acyl-enzyme intermediate.[11]
An incoming water molecule is deprotonated, and the hydroxide ion attacks the ester linkage generating a second tetrahedral intermediate.[12] The instability of the negative charge once again collapses the tetrahedral intermediate, this time displacing the serine.[12] The second product (a fatty acid) is released from the catalytic site.
Biological function
DAGLα and DAGLβ have been identified as the enzymes predominantly responsible for the biosynthesis of the endogenous
proinflammatory signaling in neuroinflammation and pain.[16][17][18][19]
Disease relevance
Diacylglycerol lipase has been identified as a tunable target in the endocannabinoid system.[6] It has been the subject of extensive preclinical research, and many propose that disease states, including inflammatory disease, neurodegeneration, pain, and metabolic disorders may benefit from drug discovery.[6] However currently, the conversion of these preclinical findings into viable approved therapeutics for disease remains elusive.[6]
DAGLα inhibition in mice has also been shown to reduce neuroinflammatory response due to the reduction of overall 2-AG, a precursor to the synthesis of proinflammatory prostaglandins. Therefore DAGLα inhibition has been identified as an approach to treating neurodegenerative diseases.[10] Indeed, rat models of Huntington's disease show the neuroprotective nature of DAGLα inhibition.[20]
In vivo experiments show that selectively inhibiting DAGLβ has the potential to be a powerful
tumor necrosis factor α in macrophages and dendritic cells.[16][17][18] As a consequence, DAGLβ inhibition has been identified as a potential therapy for pathological pain that does not impair immunity.[10][17]