The storage lipids in adipose tissue are triacylglycerols: glycerol esterified with three molecules of fatty acids. As discussed in section 4.3.1, the three fatty acids in a triacylglycerol molecule are not always the same, and the fatty acid at carbon-2 is usually unsaturated.
Triacylglycerols are synthesized mainly in the liver, adipose tissue and small intestinal mucosa, as well as lactating mammary gland. As shown in Figure 5.29, the substrates for triacylglycerol synthesis are fatty acyl CoA esters (formed by reaction between fatty acids and CoA, linked to the conversion of ATP to AMP plus pyrophosphate), and glycerol phosphate. The main source of glycerol phosphate is by reduction of dihydroxyacetone phosphate (an intermediate in glycolysis; see Figure 5.10); the liver, but not adipose tissue, can also phosphorylate glycerol to glycerol phosphate.
Two molecules of fatty acid are esterified to the free hydroxyl groups of glycerol phosphate, by transfer from fatty acyl CoA, forming monoacylglycerol phosphate and then diacylglycerol phosphate (or phosphatidate). Diacylglycerol phosphate is then hydrolysed to diacylglycerol and phosphate before reaction with the third molecule of fatty acyl CoA to yield triacylglycerol. (The diacylglycerol phosphate can also be used for the synthesis of phospholipids; section 188.8.131.52.)
It is obvious from Figure 5.29 that triacylglycerol synthesis incurs a considerable ATP cost; if the fatty acids are being synthesized from glucose, then overall some 20% of the energy yield of the carbohydrate is expended in synthesizing triacylglycerol reserves. The energy cost is lower if dietary fatty acids are being esterified to form triacylglycerols.
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 184.108.40.206) and very low-density lipoprotein in the liver (section 5.6.2).
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