Biosynthesis of Triacylglycerols:-Triacylglycerols and Glycerophospholipids Are Synthesized from the Same Precursors
Animals can synthesize and store large quantities of tri acylglycerols, to be used later as fuel (see Box 17–1). Humans can store only a few hundred grams of glycogen in liver and muscle, barely enough to supply the body’s energy needs for 12 hours. In contrast, the total amount of stored triacylglycerol in a 70-kg man of average build is about 15 kg, enough to support basal energy needs for as long as 12 weeks (see Table 23–5). Triacylglycerols have the highest energy content of all stored nutrients—more than 38 kJ/g. Whenever carbo hydrate is ingested in excess of the organism’s capacity to store glycogen, the excess is converted to triacyl glycerols and stored in adipose tissue. Plants also manufacture triacylglycerols as an energy-rich fuel, mainly stored in fruits, nuts, and seeds.
In animal tissues, triacylglycerols and glycerophospho lipids such as phosphatidylethanolamine share two precursors (fatty acyl–CoA and L-glycerol 3-phosphate) and several biosynthetic steps. The vast majority of the glycerol 3-phosphate is derived from the glycolytic intermediate dihydroxyacetone phosphate (DHAP) by the action of the cytosolic NAD-linked glycerol 3-phosphate dehydrogenase; in liver and kidney, a small amount of glycerol 3-phosphate is also formed from glycerol by the action of glycerol kinase (Fig. 21–17). The other precursors of triacylglycerols are fatty acyl–CoAs, formed from fatty acids by acyl-CoA synthetases, the same enzymes responsible for the activation of fatty acids for β oxidation (see Fig. 17–5). The first stage in the biosynthesis of triacylglycerols is the acylation of the two free hydroxyl groups of L glycerol 3-phosphate by two molecules of fatty acyl–CoA to yield diacylglycerol 3-phosphate, more commonly called phosphatidic acid or phosphatidate (Fig. 21–17). Phosphatidic acid is present in only trace amounts in cells but is a central intermediate in lipid biosynthesis; it can be converted either to a triacylglycerol or to a glycerophospholipid. In the pathway to triacylglycerols, phosphatidic acid is hydrolyzed by phosphatidic acid phosphatase to form a 1,2-diacylglycerol (Fig. 21–18). Diacylglycerols are then converted to triacylglycerols by transesterification with a third fatty acyl–CoA.
FIGURE 21–17 Biosynthesis of phosphatidic acid. A fatty acyl group is activated by formation of the fatty acyl–CoA, then transferred to ester linkage with L-glycerol 3-phosphate, formed in either of the two ways shown. Phosphatidic acid is shown here with the correct stere ochemistry at C-2 of the glycerol molecule. To conserve space in sub sequent figures (and in Fig. 21–14), both fatty acyl groups of glycerophospholipids, and all three acyl groups of triacylglycerols, are shown projecting to the right.
