Triacylglycerols (triglycerides) (TGs) are the major storage molecules of metabolic energy and FAs in most living organisms. Excessive accumulation of TGs, however, is associated with human diseases, such as obesity, diabetes mellitus, and steatohepatitis. The final and the only committed step in the biosynthesis of TGs is catalyzed by acylCoA:diacylglycerol acyltransferase (DGAT) enzymes. The genes encoding two DGAT enzymes, DGAT1 and DGAT2, were identified in the past decade, and the use of molecular tools, including mice deficient in either enzyme, has shed light on their functions. Although DGAT enzymes are involved in TG synthesis, they have distinct protein sequences and differ in their biochemical, cellular, and physiological functions. Both enzymes may be useful as therapeutic targets for diseases. Here we review the current knowledge of DGAT enzymes, focusing on new advances since the cloning of their genes, including possible roles in human health and diseases.-Yen, C-L. E., S. J. Stone, S. Koliwad, C. Harris, and R. V. Farese, Jr. DGAT enzymes and triacylglycerol biosynthesis. J. Lipid Res. 2008. 49: 2283-2301. Supplementary key words triacylglycerolsTriacylglycerols (triglycerides) (TGs), a major type of neutral lipid, are a heterogeneous group of molecules with a glycerol backbone and three FAs attached by ester bonds. The physical and chemical properties of TG differ based on chain length and the degree to which their FAs are desaturated. TGs serve multiple important functions in living organisms. Chief among these, they are the major storage molecules of FA for energy utilization and the synthesis of membrane lipids. Because they are highly reduced and anhydrous, TGs store 6-fold more energy than the same amount of hydrated glycogen (1). In plants, TGs are a major component of seed oils, which are valuable resources for dietary consumption and industrial uses. TG from plants and microorganisms can also be used for biofuels. In animals, energy stores of TG are concentrated primarily in adipocytes, although TGs are also found prominently in myocytes, hepatocytes, enterocytes, and mammary epithelial cells. In addition to energy storage, TG synthesis in cells may protect them from the potentially toxic effects of excess FA. In the enterocytes and hepatocytes of most mammals, TGs are synthesized for the assembly and secretion of lipoproteins, which transport dietary and endogenously synthesized FA between tissues. Also, TGs in secreted lipids acts as a component of the skinʼs surface water barrier, and collections of TG in adipose tissue provide insulation for organisms.Although TGs are essential for normal physiology, the excessive accumulation of TG in human adipose tissue results in obesity and, in nonadipose tissues, is associated with organ dysfunction. For example, excessive TG deposition in skeletal muscle and the liver is associated with insulin resistance, in the liver with nonalcoholic steatohepatitis, and in the heart with cardiomyopathy (2, 3). Owing to worldwide increases in the p...
Studies involving the cloning and disruption of the gene for acyl-CoA:diacylglycerol acyltransferase (DGAT) have shown that alternative mechanisms exist for triglyceride synthesis. In this study, we cloned and characterized a second mammalian DGAT, DGAT2, which was identified by its homology to a DGAT in the fungus Mortierella rammaniana. DGAT2 is a member of a gene family that has no homology with DGAT1 and includes several mouse and human homologues that are candidates for additional DGAT genes. The expression of DGAT2 in insect cells stimulated triglyceride synthesis 6-fold in assays with cellular membranes, and DGAT2 activity was dependent on the presence of fatty acyl-CoA and diacylglycerol, indicating that this protein is a DGAT. Activity was not observed for acyl acceptors other than diacylglycerol. DGAT2 activity was inhibited by a high concentration (100 mM) of MgCl 2 in an in vitro assay, a characteristic that distinguishes DGAT2 from DGAT1. DGAT2 is expressed in many tissues with high expression levels in the liver and white adipose tissue, suggesting that it may play a significant role in mammalian triglyceride metabolism.
The synthesis of triglycerides is catalyzed by two known acyl-CoA:diacylglycerol acyltransferase (DGAT) enzymes. Although they catalyze the same biochemical reaction, these enzymes share no sequence homology, and their relative functions are poorly understood. Gene knockout studies in mice have revealed that DGAT1 contributes to triglyceride synthesis in tissues and plays an important role in regulating energy metabolism but is not essential for life. Here we show that DGAT2 plays a fundamental role in mammalian triglyceride synthesis and is required for survival. DGAT2-deficient (Dgat2 ؊/؊ ) mice are lipopenic and die soon after birth, apparently from profound reductions in substrates for energy metabolism and from impaired permeability barrier function in the skin. DGAT1 was unable to compensate for the absence of DGAT2, supporting the hypothesis that the two enzymes play fundamentally different roles in mammalian triglyceride metabolism.Triglycerides (triacylglycerols) are the major storage form of energy in eukaryotic organisms. However, excessive deposition of triglycerides in white adipose tissue (WAT) 1 leads to obesity and in non-adipose tissues (such as pancreatic  cells, skeletal muscle, and liver) is associated with tissue dysfunction referred to as lipotoxicity (1, 2). Therefore, an understanding of the processes that mediate triglyceride synthesis is of significant biomedical importance.Triglycerides are synthesized from diacylglycerol and activated forms of fatty acids (fatty acyl-CoAs) in a reaction catalyzed by acyl-CoA:diacylglycerol acyltransferase (DGAT) enzymes (3-5). The genes for two DGAT enzymes, DGAT1 and DGAT2, have been identified (6, 7). Both DGAT1 and DGAT2 are ubiquitously expressed, with the highest levels of expression found in tissues that are active in triglyceride synthesis, such as WAT, small intestine, liver, and mammary gland (6, 7). Both enzymes are intrinsic membrane proteins, although DGAT1 has 6 -12 putative transmembrane domains, whereas DGAT2 has one. Both also have similarly broad fatty acyl-CoA substrate specificities in in vitro assays (7). However, despite their ability to catalyze similar reactions, DGAT1 and DGAT2 belong to different gene families that share neither DNA nor protein sequence similarity. DGAT1 is homologous to the acylCoA:cholesterol acyltransferase enzymes, ACAT1 and ACAT2, which are involved in cholesterol ester biosynthesis (6), whereas DGAT2 shares homology with acyl-CoA:monoacylglycerol acyltransferase enzymes (8 -13). This raises the question of why two different types of DGAT enzymes have emerged from convergent evolution.Insights into the functions of DGAT1 and DGAT2 in triglyceride metabolism have been provided by studies in yeast. Through deletion and overexpression studies, several groups have demonstrated that DGA1, the yeast homologue of DGAT2, is the major DGAT enzyme contributing to triglyceride synthesis and storage in yeast (14 -16). In contrast, ARE2, a yeast homologue of DGAT1, plays a minor role in triglyceride synthesis. Intere...
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