描述
This pathway depicts the synthesis of glycerolipids such as diacylglycerol (DAG), triacylglycerols (TAGs), phosphatidic acids, lysophosphatidic acids and several glycolipids. The synthesis of diacylglycerol (DAG) begins with glycerol-3-phosphate, which is derived primarily 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 is first acylated with acyl-coenzyme A (acyl-CoA) to form lysophosphatidic acid. Once synthesized, lysophosphatidic acid is then acylated with another molecule of acyl-CoA to yield phosphatidic acid. Phosphatidic acid is then de-phosphorylated to form diacylglycerol. Specifically, diacylglycerol is formed by the action of phosphatidate phosphatase 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 under the catalysis 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. The glycerol component can be converted to glucose by the liver and provides energy for cellular metabolism. The fatty acids go through beta-oxidation which splits the long carbon chains of the fatty acid into acetyl CoA, which can eventually enter the TCA cycle. The energy yield from a gram of fatty acids is approximately 9 Kcal (39 kJ), 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. Carbohydrates, on the other hand, are more highly hydrated. For example, 1 g of glycogen can bind approximately 2 g of water, which translates to 1. 33 Kcal/g (4 Kcal/3 g). This means that fatty acids can hold more than six times the amount of energy. 5 lb (31 kg) of hydrated glycogen to have the energy equivalent to 10 lb (5 kg) of fat. Also shown in this pathway is the synthesis of various mono-and diglycosyldiacylglyerols. These relatively rare lipids exist in both diacyl and alkyl acyl forms, and contain mainly saturated and monoenoic fatty acid components, with 16:0, 18:0 and 18:1 comprising 90% or more of the total. The carbohydrate moieties in these glycolipids may consist of galactose or glucose. The biosynthesis of the galactosyldiacylglycerols has been studied in vitro and there appears to be an enzyme that catalyses the transfer of galactose from UDP-galactose to diacylglycerol. The function of the galactolipids is still a matter for conjecture. They probably have a role in myelination, and may also have a function in cell differentiation and intracellular signalling. The glucolipids in saliva and related secretions may be involved in a defense mechanism against microbial attack.