Lipid Protein Catabolism

Lipid & Protein Catabolism Lipids Most energy reserves in the body are stored as triacylglycerides, which can be hydrolyzed to glycerol and fatty acids through the action of lipases: Glycerol can be metabolized by glycolysis upon oxidation to DHAP. Both electrons are fed into the electron transport chain. Fatty acids follow a different pathway: β-oxidation, which takes place in the mitochondrion. Before entering the mitochondrion, fatty acids must be activated. The activation reaction happens in the cytoplasm, and it consists on the transformation of the fatty acid into acetyl-CoA derivatives. The fatty acid is essentially broken at every two carbons in the hydrocarbon chain. These molecules then enter the Krebs’ cycle. Proteins Proteins can be used for energy as well. Polypeptide chains are broken down via hydrolysis into individual amino acids. Amino acids are generated by the digestion of proteins in the intestine or by the degradation of proteins within the cell. Cellular proteins are constantly being degraded and resynthesized. Some proteins are incredibly stable, others are very short lived. If amino acids are required for synthesis of ‘new’ proteins, the amino acids are utilized for that purpose. Otherwise they are degraded. In mammals, amino acid degradation occurs in the liver. The first step is to deaminate the amino acids to form α-ketoacids. The amino group is converted to ammonia, and enzymes then convert it so it is lost as urea in the urine. The α-ketoacids are metabolized so that the carbon skeletons can enter metabolism as citric acid cycle intermediates, depending on their R-group.