2-Monoacylglycerols triacylglycerol synthesis

Hansen, H.O., Jensen, S.S., Knudsen, J. 1986. Absence of monoacylglycerol pathway for triacylglycerol synthesis in goat mammary gland. Biochem. J. 238, 173-176. 

In the intestine, triacylglycerol synthesis also occurs in the ER membrane, but fatty adds are esterified with 2-monoacylglycerol, as follows ... 

Figure 6-5. Digestion of triacylglycenols. TG triacylglycerols bs = bile salts FA = fatty add 2-MG=2-monoacylglycerol. B. Synthesis of chylomicrons...

Figure 4.10 Monoacylglycerol pathway for synthesis of triacylglycerol and formation of chylomicrons within the enterocyte.

Correct answer = A. Pancreatic lipase hydrolyzes dietary triacylglycerol primarily to 2-monoacylglycerol plus two fatty acids. These products of hydrolysis can be absorbed by the intestinal mucosal cells. Bile salts do not inhibit release of fatty acids from triacylglycerol, but rather are necessary for the proper solubilization and hydrolysis of dietary triacylglycerol in the small intestine. Short- and medium-chain length fatty acids enter the portal circulation after absorption from the small intestine. Synthesis of apolipoproteins, especially apo B-48, is essential for the assembly and secretion of chylomicrons. 

Direct synthesis of triacylglycerols from 2-monoacylglycerols, arising from Upid digestion in the intestine, takes place in the intestinal mucosa of higher animals. 

The synthesis by route C is an alcoholysis reaction. The reaction proceeds as an emulsion due to the immiscibility of triacylglycerol and glycerol. Under this emulsified condition, the equilibrium concentration of the monoacylglycerol is 30% (Yamane et al., 1986). However, when the reaction is run at temperatures slightly below the melting point of the fat used, the concentration is favourably increased to 70-90% (McNeill et al., 1990 McNeill and Yamane, 1991). The reaction rate is affected by moisture content. Relatively high moisture leads to high initial reaction rates (McNeill et al., 1991) but excessive levels result in hydrolysis and production of free fatty acids. Compared to the synthetic pathways of A and B, route C seems a cheaper alternative since the primary reactive components do not include free fatty acids reactants that otherwise would have to be produced by other methods to make them available for the process. 

The enzymes for phosphatidate synthesis, acyl CoA synthetase, glycerol 3-phosphate acyltransferase and monoacylglycerol acyltransferase, are on both the outer mitochondrial membrane and the endoplasmic reticulum membrane. Diacylglycerol acyltransferase is only on the endoplasmic reticulum it may use either diacylglycerol phosphate synthesized on the endoplasmic reticulum or that synthesized on the mitochondrion. Triacylglycerol synthesized on the endoplasmic reticulum membrane may then enter lipid droplets either in the cytosol or, in the liver and intestinal mucosa, the lumen of the endoplasmic reticulum for assembly into lipoproteins - chylomicrons in the intestinal mucosa (section 4.3.2.2) and very low-density lipoprotein in the liver (section 5.6.2). 

Irimescu, R, Furihata, K, Hata, K, Iwasato, Y and Yamane, T (2001) Two-step enzymatic synthesis of docosahexaenoic acid-rich symmetrically structured triacylglycerols via 2-monoacylglycerols. J. Am. Oil Chem. Soc., 78, 743-748. 

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