Biochemistry 11 Lipid and Amino Acid Metabolism

Biochemistry 11: Lipid and Amino Acid Metabolism Lipid Digestion and Absorption Mechanical Mechanical digestion of lipids occurs primarily in the Digestion: mouth and stomach. Chemical Chemical digestion of lipids occurs in the small intestine Digestion: and is facilitated by bile, pancreatic lipase, colipase, and cholesterol esterase. Emulsification: Upon entry into the duodenum, emulsification occurs, which is the mixing of two normally immiscible liquids; in this case, fat and water. (A common example of an emulsion is oil-and-vinegar salad dressing). This increases the surface area of the lipid, which permits greater enzymatic interaction and processing. Emulsification is aided by bile salts. Micelles: Water-soluble spheres with a lipid-soluble interior. Digested lipids may form micelles to be carried to the intestinal epithelium where they are absorbed across the plasma membrane. Short vs. Long Short-chain fatty acids are absorbed across the intestine Chain Fatty Acids: into the blood. Long-chain fatty acids are absorbed as micelles and assembled into chylomicrons for release into the lymphatic system. Fatty Acids and Triacylglycerols Fatty Acids: Carboxylic acids with a long chain Saturated: No double bonds Unsaturated: One or more double bonds Fatty Acid Synthesized in cytoplasm from acetyl-CoA transported out of Synthesis: the mitochondria. Five steps: Activation, bond formation, reduction, dehydration, and a second reduction. Arachidonate: Precursor to eicosanoid signaling molecules: prostaglandins, prostacyclins, & thromboxanes. Also precursor to leukotrienes. Fatty Acid Oxidation occurs in the mitochondria following transport by the Oxidation: carnitine shuttle. b -oxidation uses cycles of oxidation, hydration, oxidation, and thiolysis cleavage. The fatty acid chain is shortened by two carbon atoms. FADH2, NADH, and acetyl CoA are generated. The carboxylic acid is the a end. The w carbon counting starts on the other end. Cis-Oleate, a cis-D 9 fatty acid The cis bond prevents tight packing, which lowers the melting point. a -Linolenate, an Omega-3 Fatty Acid (3rd carbon from the w end) Ketone Bodies Ketogenesis: Ketone bodies form via ketogenesis due to excess acetylCoA in the liver during a prolonged starvation state Ketolysis: Regenerates acetyl-CoA for use as an energy source in peripheral tissues Lipid Mobilization and Transport Lipid Lipids are mobilized from adipocytes by hormone-sensitive Mobilization: lipase. Lipids are mobilized from lipoproteins by lipoprotein lipase. Chylomicrons: Transport dietary triacylglycerols, cholesterol, & cholesteryl esters from intestine to tissues. Uses the lymphatic system. Lipoproteins: The transport mechanism for lipids. Very-low-density: Liver tissues. Intermediate-density: Transition particle. VLDL IDL VLDL Low-density: Bad. Moves cholesterol tissues. High-density: Good. Moves cholesterol liver, exits body. Apolipoproteins: Form the protein component of lipoproteins. They are receptor molecules that control interactions between li i Energy Source: The brain can derive up to 2/3 of its energy from ketone bodies during prolonged starvation Protein Catabolism Protein digestion occurs primarily in the small intestine. Catabolism of cellular proteins occurs only under conditions of starvation. Amino acids released from proteins usually lose their amino group through deamination. The remaining carbon skeleton can be used for energy. Glucogenic Can be converted into glucose through gluconeogenesis. Amino Acids: All but leucine and lysine. Ketogenic Can be converted into acetyl-CoA and ketone bodies. Amino Acids: Leucine and lysine are the only amino acids that are solely ketogenic. Cholesterol Metabolism Cholesterol may be obtained through dietary sources or through de novo synthesis in the liver HMG-CoASynthesizes mevalonate. This is the rate limiting step of Reductase: cholesterol synthesis LCAT:Catalyzes the formation of cholesteryl esters for transport with HDL CETP:Catalyzes the transition of IDL to LDL by transferring cholesteryl esters from HDL Fates of the amino acid carbon skeleton following protein catabolism