The glycerolipid metabolism pathway describes the synthesis of glycerolipids such as monoacylglycerols (MAGs), diacylglycerols (DAGs), triacylglycerols (TAGs), phosphatidic acids (PAs), and lysophosphatidic acids (LPAs). The process begins with cytoplasmic 3-phosphoglyceric acid (a product of glycolysis). This molecule is dephosphorylated via the enzyme glycerate kinase to produce glyceric acid. Glyceric acid is then transformed to glycerol (via the action of aldehyde dehydrogenase and aldose reductase). The free, cytoplasmic glycerol can then be phosphorylated to glycerol-3-phosphate through the action of glycerol kinase. Glycerol-3-phosphate can then enter the endoplasmic reticulum where glycerol-3-phosphate acyltransferase (GPAT) may combine various acyl-CoA moieties (which donate acyl groups) to form lysophosphatidic (LPA) or phosphatidic acid (PA). The resulting phosphatidic acids can be dephosphorylated via lipid phosphate phosphohydrolase (also known as phosphatidate phosphatase) to produce diacylglycerols (DAGs). The resulting DAGs can be converted into triacylglycerols (TAGs) via the addition of another acyl group (contributed via acyl-CoA) and the action of 1-acyl-sn-glycerol-3-phosphate acyltransferase. Extracellularly, the triacylglycerols (TAGs) can be converted to monoacylglycerols (MAGs) through the action of hepatic triacylglycerol lipase. In addition to this cytoplasmic route of glycerolipid synthesis, another route via mitochondrial synthesis also exists. This route begins with glycerol-3-phosphate, which can be either derived from dihydroxyacetone phosphate (DHAP), a product of glycolysis (usually in the cytoplasm of liver or adipose tissue cells) or from glycerol itself. Glycerol-3-phosphate in the mitochondria is first acylated via acyl-coenzyme A (acyl-CoA) through the action of mitochondrial glycerol-3-phosphate acyltransferase to form lysophosphatidic acid (LPA). Once synthesized, lysophosphatidic acid is then acylated with another molecule of acyl-CoA via the action of 1-acyl-sn-glycerol-3-phosphate acetyltransferase to yield phosphatidic acid. Phosphatidic acid is then dephosphorylated to form diacylglycerol. Specifically, diacylglycerol is formed by the action of phosphatidate phosphatase (also known as lipid phosphate phosphohydrolase) on phosphatidic acid coupled with the release of a phosphate. The phosphatase exists as 3 isozymes. Diacylglycerol is a precursor to triacylglycerol (triglyceride), which is formed in the addition of a third fatty acid to the diacylglycerol by the action of diglyceride acyltransferase. Since diacylglycerol is synthesized via phosphatidic acid, it will usually contain a saturated fatty acid at the C-1 position on the glycerol moiety and an unsaturated fatty acid at the C-2 position. When the body uses stored fat as a source of energy, glycerol and fatty acids are released into the bloodstream. Fatty acids, stored as triglycerides in humans, are an important and a particularly rich source of energy. The energy yield from a gram of fatty acids is approximately 9 kcal/g (39 kJ/g), compared to 4 kcal/g (17 kJ/g) for carbohydrates. Since the hydrocarbon portion of fatty acids is hydrophobic, these molecules can be stored in a relatively anhydrous (water-free) environment. Fatty acids can hold more than six times the amount of energy than sugars on a weight basis. In other words, if you relied on sugars or carbohydrates to store energy, then you would need to carry 67.5 lb (31 kg) of glycogen to have the energy equivalent to 10 lb (5 kg) of fat.

Glycerolipid Metabolism References