Key Enzyme of Lipid Formation

 Lipid is formed by 

Two convergent pathways for triacylglycerol biosynthesis. The stepwise acylation of glycerol through the two pathways for triacylglycerol synthesis are shown. Abbreviations: fatty acid (FA), glycerol-3-phosphate (G-3-P), G-3-P acyltransferase (GPAT), lysophosphatidic acid (LPA), acylglycerol-3-phosphate acyltransferase (AGPAT), phosphatidic acid (PA), monoacylglycerol (MAG), MAG acyltransferase (MGAT), diacylglycerol (DAG), DAG acyltransferase (DGAT), and triacylglycerol (TAG).

Acyl-CoA: monoacylglycerol acyltransferase (MGAT) 

Acyl-CoA: monoacylglycerol acyltransferase (MGAT) catalyzes the synthesis of diacylglycerols from monoacylglycerols and long chain fatty acyl-CoAs, a first step in the monoacylglycerol pathway that is major route of triacylglycerol synthesis. This  pathway contributes more than 80% of glycerolipid synthesis in the intestinal mucosa.

MGAT isoforms and expression MGAT 1 Mouse MGAT-1, identified by its sequence homology to acyl-CoA:diacylglycerol acyltransferase-2 (DGAt-2),is a predicted 335 aa protein with at least one 

transmembrane domain near the amino terminus . MGAT-1 overexpressed in Sf9  insect cells acylates all stereoisomers of monoacylglycerol equally and uses a variety of acyl-CoAs. Highest activity is measured using 20:4-, CoA, followed by 18:1- and 

18:2- CoAs. Expression with ERCOS-7 cells shows an immunocytochemistry pattern consistent with ER staining. In mouse tissues m-RNA expression is observed in stomach, kidney, uterus, liver, and white and brown adipose tissues.

MGAT 2
MGAT-2 was cloned based on sequence homology to the mouse MGAT-1 and MGAT-2.

MGAT-2 is 52% identical to MGAT-1, and is predicted to have 334 aa and to contain at least one transmembrane domain near the N-terminus .Highest mRNA 

expression occurs in the human liver , small intestine , stomach ,kidney, colon and white adipose tissue. Mouse MGAT-2 has been variably reported to be expressed only in small intestine or primarily in small intestine but also in kidney, adipose, stomach, liver, skeletal muscle and spleen.

Expression of MGAT-2 in COS-7 cells showed immunocytochemistry staining consistent with ER and possibly Golgi, although specific subcellular markets were not used. Overexpression of human 

MGAT-2 in SF9 insect cells and in mammalian cells results in marked increases in MGAT activity, as well as a low DGAT activity.

MGAT 3 

Human MGAT 3 was cloned based on similarity to DGAT-2 and has 49% identity to DGAT-2, 44% identity to MGAT-1, and 46% identity to MGAT-2. Overexpression of MGAT 3 in Sf9 insect cells shows that MGAT 3 encodes for a 36 kDa protein which 

is predicted to have as many as five transmembrane domains. Human MGAT 3 is 

highly expressed in the gastrointestinal tract, particularly in ileum, but with 3- to 4-fold lower expression in duodenum, jejunum, saecum, colon, rectum, and liver. 

The high expression in ileum is surprising because dietary lipid is almost completely absorbed in the duodenum and jejunum unless fat intake is very high.MGAT-3 expressed in Sf9 insect cells prefers sn-2-monoacylglycerol and uses a broad range of 

acyl-CoAs, but highest activity is observed with 16:0- and 18:1-CoA.

Role of MGAT

MGAT role  in biosynthesis of triacylglycerol 

Dietary fat triacylglycerol is digested by pancreatic lipase to 2-monoacylglycerol and free fatty acids prior to its absorption in the intestinal lumen. The adsorbed 2-monoacylglycerols and free fatty acids are resynthesized to triacylglycerol by enterocytes.Monoacylglycerol pathway and glycerol 3 phosphate pathway are two pathway for synthesizing diacylglycerol. from that tracylglycerol is synthesized with the help of DGAT. In the monoglyceryl pathway, MGAT produces diacylglycerol, the precursor of triacylglycerol and certain phospholipids, by covalently joining a fatty 

acyl moiety to monoacylglycerol. In the glycerol phosphate pathway, diacylglycerol is derived from the dephosphorylation of phosphatic acid (PA), which is produced by sequential acyaltion of glycerol 3-phosphate.

Role of MGAT inhibitors Oleate and sphingosine are MGAT inhibitors. 

Inhibition of MGAT leads to inhibition of triacylglycerol biosynthesis. Inhibition of MGAT lead to inhibition of signaling pathway since its enzymatic product diacylglycerol is an activator of protein kinase C as well as an intermediate in the 

synthesis of phospholipids