Introduction
DAGAT1 (diacylglycerol O-acyltransferase 1) is an integral membrane enzyme that catalyzes the final step in triacylglycerol (TAG) synthesis, converting diacylglycerol (DAG) and acyl-CoA into TAG and CoA. The enzyme is encoded by the DGAT1 gene in humans and has orthologs in a wide range of eukaryotes, from yeast to mammals. DGAT1 is a member of the diacylglycerol acyltransferase family, which also includes DGAT2 and other related enzymes. Its activity is essential for lipid storage, membrane lipid remodeling, and cellular energy homeostasis.
Gene and Protein Structure
Genomic Organization
The DGAT1 gene is located on chromosome 6q25.3 in humans. It spans approximately 18 kilobases and contains 12 exons. Alternative splicing generates two main transcript variants that encode proteins differing at the N‑terminal region. The human DGAT1 locus shows strong conservation of key regulatory motifs across vertebrates, indicating selective pressure to maintain functional integrity.
Protein Topology
DAGAT1 is a 430‑amino‑acid protein that is predicted to contain six transmembrane domains. The N‑terminal domain is cytosolic and contains the HPHG catalytic motif that is essential for enzymatic activity. The C‑terminal domain is also cytosolic and harbors additional conserved residues involved in substrate recognition and enzyme stability. Multiple glycosylation sites are present in the extracellular loops, which may contribute to protein folding and trafficking to the endoplasmic reticulum (ER).
Homology and Conserved Domains
Sequence alignment with orthologs reveals a highly conserved HPHG sequence and a DXXH motif that are characteristic of the acyltransferase superfamily. The presence of a signature Cys residue suggests potential regulation by redox state, although the functional significance remains under investigation. Phylogenetic analyses place DGAT1 in a distinct clade that is separate from DGAT2 and other acyltransferases, reflecting divergent evolution of TAG biosynthetic pathways.
Function and Biochemistry
Enzymatic Mechanism
The catalytic reaction proceeds via an acyl‑transfer mechanism: the fatty acyl group of acyl‑CoA is transferred to the hydroxyl group of DAG, forming TAG and releasing CoA. The HPHG motif coordinates a nucleophilic attack on the carbonyl carbon of the acyl‑CoA. The enzyme does not exhibit significant activity toward lysophospholipids, distinguishing it from other acyltransferases involved in phospholipid remodeling.
Substrate Specificity
DGAT1 preferentially utilizes saturated and monounsaturated fatty acyl‑CoAs, such as palmitoyl‑CoA and oleoyl‑CoA. Experimental kinetic studies have shown that unsaturated substrates are processed more efficiently than polyunsaturated ones. The enzyme also accepts DAG species with varying sn‑1 and sn‑2 positions, indicating flexibility in substrate recognition.
Regulation
DGAT1 activity is regulated at multiple levels:
- Transcriptional control: Nuclear receptors such as PPARγ and SREBP-1c enhance DGAT1 expression in response to nutritional cues.
- Post‑translational modifications: Phosphorylation at serine residues within the cytosolic loops has been shown to modulate enzyme activity in vitro.
- Allosteric effectors: Elevated levels of diacylglycerol act as both substrate and potential feedback regulator, while the presence of CoA derivatives can influence enzyme turnover.
- Protein‑protein interactions: Association with the ER membrane protein complex OSBP‑related proteins (ORPs) may stabilize DGAT1 in the lipid droplet formation microenvironment.
Physiological Roles
Lipid Storage and Adipose Tissue
In adipocytes, DGAT1 is critical for the conversion of DAG to TAG, facilitating the expansion of lipid droplets. Knockdown of DGAT1 in adipose tissue results in reduced TAG accumulation and a lean phenotype in animal models. The enzyme also participates in adipocyte differentiation by providing a reservoir of TAG that can be mobilized during lipolysis.
Energy Homeostasis
During fasting, DGAT1 contributes to hepatic TAG synthesis, influencing circulating triglyceride levels. Overexpression of DGAT1 in the liver has been linked to hepatic steatosis in rodents, whereas inhibition leads to decreased lipid accumulation and improved insulin sensitivity.
Membrane Composition
Although its primary role is in neutral lipid synthesis, DGAT1 indirectly affects membrane phospholipid composition by modulating the DAG pool available for phospholipid reacylation pathways. Changes in DGAT1 activity can therefore alter membrane fluidity and signaling processes that depend on lipid rafts.
Genetic Disorders
Congenital DGAT1 Deficiency
Loss‑of‑function mutations in DGAT1 cause a rare autosomal recessive disorder characterized by severe neonatal enteropathy, failure to thrive, and recurrent infections. The hallmark clinical presentation includes chronic diarrhea, steatorrhea, and a high frequency of respiratory complications. Histological examination of intestinal biopsies reveals villous atrophy and reduced TAG content in enterocytes.
Mechanistic Insights
Mutations such as p.Gly226Ser and p.Arg341Cys disrupt the HPHG catalytic motif or destabilize transmembrane helices, leading to loss of enzymatic activity. Patients with these mutations often exhibit normal DGAT2 activity, underscoring the non‑redundant role of DGAT1 in enterocyte lipid metabolism. The severity of the phenotype correlates with the residual activity measured in patient-derived cell lines.
Treatment Approaches
Current management relies on dietary modification with medium‑chain triglycerides (MCTs) that bypass DGAT1-dependent pathways. MCTs are readily absorbed and utilized by the liver, providing an energy source without requiring TAG synthesis. Nutritional support and periodic monitoring of growth parameters remain the cornerstone of therapy.
Animal Models
DGAT1 Knockout Mice
Global deletion of DGAT1 in mice results in a lean, hypermetabolic phenotype with decreased adiposity and increased energy expenditure. These animals display elevated fatty acid oxidation in skeletal muscle and reduced plasma triglycerides. Importantly, they maintain normal glucose tolerance, suggesting a protective effect against diet‑induced insulin resistance.
Tissue‑Specific Models
- Adipocyte‑specific DGAT1 knockout: Leads to reduced fat mass and protection from high‑fat diet‑induced obesity.
- Liver‑specific DGAT1 knockout: Alleviates hepatic steatosis in models of non‑alcoholic fatty liver disease.
- Intestinal DGAT1 knockout: Mimics aspects of the human congenital deficiency, including steatorrhea and villous blunting.
Comparative Studies in Lower Eukaryotes
Yeast (Saccharomyces cerevisiae) possesses a single DGAT homolog, Dga1p, which is essential for TAG accumulation under nutrient‑rich conditions. Dga1p shares functional similarities with mammalian DGAT1 but differs in regulatory mechanisms and tissue distribution. Studies in Dga1p knockout strains have provided insight into the basic principles of TAG biosynthesis that are conserved across species.
Inhibition and Therapeutic Use
Pharmacological Inhibitors
Several small‑molecule inhibitors have been developed to target DGAT1. Notable examples include:
- GP-183 (also known as T863) – a potent, reversible inhibitor with a Ki in the low nanomolar range.
- A-939, an orally active compound that reduces plasma triglycerides in rodent models.
- Tetracene derivatives – which exhibit selectivity for DGAT1 over DGAT2 but are limited by solubility issues.
These inhibitors have been evaluated in preclinical models of obesity, type 2 diabetes, and atherosclerosis, showing reductions in hepatic lipid accumulation and improved metabolic profiles. However, gastrointestinal side effects, including diarrhea and steatorrhea, have limited their clinical translation.
Clinical Trials
Early-phase trials with DGAT1 inhibitors were terminated due to intolerable gastrointestinal adverse events. Nonetheless, the safety data provide valuable information for the design of next‑generation compounds with improved tolerability. Combination strategies that pair DGAT1 inhibition with modulators of lipid absorption or bile acid metabolism are currently under investigation.
Structural Studies
Crystallographic Data
High‑resolution crystal structures of DGAT1 from Thermotoga maritima have been solved at 2.2 Å, revealing the arrangement of the HPHG motif and the active‑site pocket. These structures have illuminated the binding mode of acyl‑CoA and suggested a two‑step catalytic mechanism involving a covalent acylated intermediate.
Cryo‑EM Analyses
Recent cryo‑electron microscopy studies of mammalian DGAT1 in nanodiscs have provided a near‑atomic view of the enzyme in a membrane context. These analyses indicate that DGAT1 operates as a monomeric enzyme, though dimerization has been observed under certain detergent conditions, potentially influencing substrate access.
Computational Modeling
Molecular dynamics simulations have identified critical residues that interact with the phosphopantetheine arm of acyl‑CoA, providing a basis for rational drug design. The predicted binding energies correlate with experimental inhibition constants, validating the computational approach.
Evolutionary Perspective
Gene Duplication and Divergence
The presence of both DGAT1 and DGAT2 in vertebrates suggests a gene duplication event in the common ancestor of vertebrates. Subsequent divergent evolution allowed specialization: DGAT1 is predominantly involved in TAG synthesis for storage, while DGAT2 is more closely associated with lipid remodeling in the ER.
Conserved Motifs Across Kingdoms
Sequence alignments across fungi, plants, insects, and mammals reveal a conserved HPHG motif and a DXXH motif, underscoring the essential catalytic architecture. Variations in the number of transmembrane domains are noted in plants, where DGAT1 proteins often possess additional N‑terminal domains that may mediate lipid droplet targeting.
Functional Conservation
Despite sequence divergence, the fundamental reaction catalyzed by DGAT1 - conversion of DAG to TAG - remains conserved. Functional complementation studies in yeast demonstrate that mammalian DGAT1 can rescue TAG synthesis in dga1Δ strains, confirming biochemical conservation.
Industrial Applications
Food Industry
DGAT1 is exploited in the production of high‑oleic sunflower and soybean oils with improved oxidative stability. Overexpression of DGAT1 in oilseed crops enhances TAG accumulation, yielding higher oil yields and altered fatty‑acid profiles that meet consumer demands for healthier fats.
Biofuel Production
Microalgae engineered to overexpress DGAT1 show increased lipid content, making them attractive candidates for biodiesel production. DGAT1-mediated TAG accumulation provides a simple genetic target for improving lipid yields in photosynthetic microorganisms.
Pharmaceutical Lipid Formulations
In drug delivery, DGAT1 is used to create stable lipid nanoparticles for oral bioavailability of lipophilic drugs. By modulating DGAT1 activity in cell culture systems, researchers can tailor the TAG content of lipid carriers, optimizing particle size and release kinetics.
Research Tools
Expression Vectors
Codon‑optimized DGAT1 constructs are available in plasmids such as pcDNA3.1 and pCMV, facilitating overexpression in mammalian cell lines. Mutagenesis kits allow site‑directed changes within the catalytic motif to study structure‑function relationships.
Antibodies
Commercially available monoclonal and polyclonal antibodies against DGAT1 recognize conserved epitopes in the N‑terminal cytosolic domain. These antibodies are used in Western blotting, immunoprecipitation, and immunofluorescence assays to assess protein expression and localization.
Reporter Systems
Luciferase reporter constructs driven by the DGAT1 promoter have been used to screen for transcriptional regulators. Reporter assays in adipocyte and hepatocyte models have identified nuclear receptors and signaling pathways that modulate DGAT1 transcription.
Future Directions
Targeted Gene Editing
CRISPR‑Cas9 mediated editing of the DGAT1 gene in livestock aims to produce animals with altered fat composition, improving meat quality and reducing saturated fat content. Precise base editing could also correct pathogenic mutations in patients with congenital DGAT1 deficiency.
Selective Inhibitor Development
Research focuses on designing inhibitors that selectively target DGAT1 without affecting DGAT2, thereby reducing side effects associated with global TAG suppression. Structural insights into the active‑site pocket guide the synthesis of high‑affinity, selective compounds.
Systems Biology Approaches
Integrative omics studies combining transcriptomics, proteomics, and lipidomics are expanding our understanding of DGAT1 regulation in metabolic networks. Systems models predict compensatory pathways that might be activated upon DGAT1 inhibition, informing combination therapies.
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