Metabolism of Lipids

Metabolism of Lipids

Fats have very low amounts of water and are the body’s most concentrated energy source. Fats yield approxi-mately 9 kilocalories (kcal) of energy per gram during catabolism. This is approximately twice the yield of proteins and carbohydrates (which is approximately 4 kcal of energy per gram each). When fat is digested, most of it is transported in lymph as fatty-protein droplets (chylomicrons). The lipids in these chylomi-crons are eventually hydrolyzed by enzymes from the capillary endothelium. The glycerol and fatty acids that result are absorbed by body cells for various types of processing.

Oxidation of Glycerol and Fatty Acids

Although there are many lipids, triglycerides are the only type regularly oxidized for energy. This requires separate catabolism of glycerol and fatty acid chains. Glycerol is easily converted by most body cells to glyceraldehyde- 3-phosphate. This acts as an interme-diate substance during glycolysis. It soon enters the Krebs cycle. The ATP energy that is gained from the complete oxidation of glyceraldehyde is about half of the energy gained from glucose (15 ATP/glycerol). Glyceraldehyde is, therefore, equal to half of one glucose molecule.

The initial phase of fatty acid oxidation is called beta oxidation, which occurs in the mitochondria. Fatty acid chains are broken into two-carbon frag-ments of acetic acid. Also, the FAD and NAD⁺ coen-zymes are reduced. The acetic acid molecules are fused to CoA. This forms acetyl CoA. During beta oxidation, carbon in the third (beta) position is oxi-dized. The fatty acid is cleaved between alpha and beta carbons. The acetyl CoA is picked up by oxaloacetic acid. This enters the aerobic pathways for oxidization to carbon dioxide and water. This is different from the oxidation of glycerol, which instead enters the glyco-lytic pathway. Acetyl CoA cannot be used for glucone-ogenesis because the metabolic pathway is irreversible past pyruvic acid.

Lipogenesis

Lipogenesis is also known as triglyceride synthesis. It occurs when ATP and glucose levels are high in the cells. Triglycerides in adipose tissue are continuously­ being cycled. New fats are stored for later use and stored fats are broken down to be released into the blood. Fat pockets on the body are always different, not containing the same fat molecules continuously.

Glycerol and fatty acids that come from the diet and are not needed quickly for energy are stored as triglycerides­. About half of these are found in the subcutaneous tissue, with the remainder located in other body fat deposits. Although acetyl CoA and glyceraldehyde 3-PO₄ would normally enter the Krebs cycle, excessive ATP leads them to accumu-late. However, when these two metabolites become excessive, they are moved into the pathways of tri-glyceride synthesis.

Fatty acid chains are formed by condensed ace-tyl CoA molecules. These chains lengthen at a rate of two carbons at a time, and nearly all fatty acids in the body have an even number of carbon atoms. Glucose is easily converted to fat because of acetyl CoA, which is an intermediate of glucose catabolism. This is because acetyl CoA is where fatty acid syn-thesis begins. Glyceraldehyde 3-PO₄ is converted to glycerol. This forms triglycerides when it condenses with fatty acids. Carbohydrates from the diet can always provide the basic materials needed to make triglycerides even when the diet is low in fat. High blood sugar causes lipogenesis to become the pri-mary activity in the adipose tissues. Lipogenesis is also an important function of the liver.

Lipolysis

Lipolysis is basically the opposite of lipogenesis. It is defined as the breakdown of stored fats into fatty acids and glycerol. These substances are then released to the blood, giving continuous supplies of fat fuels to the organs that are needed for aerobic respiration. Fatty acids are the preferred energy fuel for cardiac muscle, the liver, and the skeletal muscles when they are at rest. When carbohydrate intake is insufficient, lipoly-sis increases to fuel the body with fats. The availability of oxaloacetic acid to become a pickup molecule influ-ences the ability of acetyl CoA to enter the Krebs cycle. Oxaloacetic acid is converted to glucose to provide energy for the brain when carbohydrates are lacking. Without this acid, acetyl CoA accumulates because fat oxidation is incomplete.

The liver converts acetyl CoA molecules to ketones (ketone bodies) during the process known as ketogenesis. Ketones are defined as organic com-pounds containing the carbonyl group CdbondO, whose carbon atom is joined to two other carbon atoms, with the carbonyl group occurring within the carbon chain. The produced ketones are then released into the blood. Examples of ketones include acetone, acetoacetic acid, and β-hydroxybutyric acid. These are not the same as the keto acids that cycle through the Krebs cycle or the ketone bodies that are produced during fat metabolism.

1. What is glycogenolysis?

2. Explain gluconeogenesis.

3. Describe beta oxidation.

4. What is ketogenesis?