Lipid Metabolism: Oxidation of Fatty Acids Lecture 13 - Biochemistry - Al-Esraa University, 2nd Year

‫ﻛﻠﯾﮫ اﻻﺳراء اﻟﺟﺎﻣﻌﺔ‬ Al-Esraa University collage Department of Dentistry Metabolism of Lipid: Oxidation of Fatty Acids Prepared by: Dr. Alaa Alnoori...

‫ﻛﻠﯾﮫ اﻻﺳراء اﻟﺟﺎﻣﻌﺔ‬ Al-Esraa University collage Department of Dentistry Metabolism of Lipid: Oxidation of Fatty Acids Prepared by: Dr. Alaa Alnoori Lecture 13: Biochemistry 2nd year 28/02/2024  Digestion and Absorption of Fats v Dietary fats Types of fats. Fats are of three types: 1. Simple fats or neutral fats, e.g. triglycerides and cholesterol. 2. Compound fats, e.g. phospholipids. 3. Associated fats, e.g. steroids and fat-soluble vitamins. Dietary fat is of both vegetable and animal origin. Mostly it is in the form of neutral fat (triglycerides). It also includes small amounts of phospholipids, cholesterol, some free fatty acids, lecithin, and cholesterol esters. Daily intake of fats in the diet varies widely, from about 25-160 gm.  Digestion of Fats v Site of digestion Although lipolytic enzymes are secreted in the mouth (lingual lipase) and stomach (gastric lipase), their action is so insignificant that practically digestion of all the dietary fats occurs in the small intestine.  Absorption of Fats Most of the fat absorption occurs in the duodenum; almost all the digested lipids are absorbed by the time the chyme reaches the mid-jejunum. The absorption of fats is accomplished by following the steps. 1. Transportation as micelles to the brush border membrane. 2. Diffusion of lipids across the enterocyte cell membrane. 3. Transport of lipids from inside the enterocytes to the interstitial space. 4. Transport of lipids into circulation.  Oxidation of fatty acids is a major energy source in many organisms Ø About one-third of our energy needs come from dietary triacylglycerols. Ø About 80% of the energy needs of the mammalian heart and liver are met by oxidation of fatty acids. Ø Many hibernating animals, such as grizzly bears, rely almost exclusively on fats as their source of energy. Ø Some animals (camels) store fat as an eventual source of water Ø β oxidation is major pathway of oxidation of fatty acids. Ø The oxidation of the hydrocarbon chain of Fatty acid by a sequential cleavage of two carbon atoms. Ø It is called β oxidation, because the oxidation and splitting of two carbon units occur at the beta-carbon atom. Preparative Steps for Beta Oxidation Ø Before oxidation fatty acids require to be – 1. Activated 2. Transported into mitochondria  Fatty Acids Activation Ø Fatty acids are activated to their co-enzyme A (CoA) derivative. Ø This activation is taking place in cytoplasm Ø Enzyme : thiokinase or fatty acyl CoA synthetase Ø ATP is hydrolysed to AMP and Ppi using energy of two high energy bonds. Ø Two inorganic phosphates are used Ø Three different enzymes, one each for short chain, medium chain and long chain fatty acids have been identified.  Transport of Activated Fatty acid Ø For transport two molecules are required 1. Carnitine 2. Translocase Ø Carnitine is beta-hydroxy gamma trimethyl ammonium butyrate. Carnitine Ø It is synthesized from lysine and methionine in the liver and kidney. Ø Translocase is a general term for a protein that assists in moving another molecule, usually across a cell membrane. These enzymes catalyze the movement of ions or molecules across membranes or their separation within membranes  Role of Carnitine Ø Fatty acids are activated in the cytoplasm, but the beta-oxidation is in mitochondria. Ø Carnitine a transporter is involved in the transfer of a long chain of fatty acids. Ø Medium-chain and short-chain fatty acids do not require carnitine for transport across the inner mitochondrial membrane. Ø So medium-chain and short-chain fatty acids are easily oxidized.  Carnitine Acyl Transferase Ø The enzyme carnitine acyl transferase-I (CATI/CPT -I) will transfer the fatty acyl group to the hydroxyl group of carnitine to form acylcarnitine Ø The reaction occurs on the cytosolic side of the inner mitochondrial membrane.  Translocase Ø A protein translocase will carry the acyl carnitine across the membrane to the matrix of mitochondria. Ø On the matrix side of the membrane another enzyme, carnitine acyl transferase-II (CAT-II) will transfer the acyl group back to co-enzyme A molecule Ø Carnitine is returned to the cytosolic side by the translocase.  Beta Oxidation Ø After the penetration of the acyl-CoA into mitochondria, it undergoes β-oxidation. Ø A saturated acyl-CoA is degraded by a repeated sequence of four reactions 1. Oxidation by FAD 2. Hydration 3. Oxidation by NAD 4. Cleavage.  Step 1:Oxidation by FAD Ø The first reaction is the oxidation of acyl-CoA by an acyl-CoA dehydrogenase to give an Δ2-trans enoyl-CoA Ø The coenzyme for the dehydrogenase is FAD which is converted to FADH2 Ø FADH2 when oxidized in t h e electron transport chain will produce 1.5 ATP molecules. Step 2: Hydration Ø The next step is the hydration (addition of water) of the double bond between C2 and C3 by Δ2 –enoyl-CoA hydratase to form β-hydroxy acyl-CoA.  Step 3:Oxidation by NAD Ø The β-hydroxy derivative undergoes a second oxidation reaction catalyzed by β hydroxy acyl-CoA dehydrogenase to form β- ketoacyl-CoA and generates NADH. The NADH when oxidized in the electron transport chain will generate 2.5 ATPs. Step 4:Cleavage Ø Finally, β-ketoacyl-CoA is split at the β-carbon by thiolase to yield acetyl-CoA and an acyl CoA which is shorter by two carbon atoms. Ø The new acyl-CoA, containing two carbons less than the original, re- enters the β-oxidation pathway at a reaction catalyzed by acyl- CoA dehydrogenase. Ø The process continues till the fatty acid degrades completely to acetyl-CoA. Oxidation of Unsaturated Fatty Acids (Remember they are cis!) Multiple points of unsaturation can require energy to get them through β-Oxidation  β-Oxidation of Odd Numbered Fatty Acids Results in Propionyl-SCoA β-oxidation of Odd Numbered Fatty Acids … last round produces 1 Ac-SCoA and 1 Propionyl-SCoA  Energetics of Beta Oxidation (ATP Yield) Ø Palmitic acid (16 C) needs 7 cycles of beta-oxidation. Ø It gives rise to 8 molecules of acetyl CoA. Ø Every molecule of acetyl CoA when oxidized in the TCA cycle gives 10 molecules of ATP. Each molecule of Ø The energy yield from one molecule of palmitate is calculated as: Ø 8 acetyl CoA × 10 = 80 ATP Ø 7 FADH2 × 1.5 = 10.5 ATP Ø 7 NADH × 2.5 = 17.5 ATP Ø Gross total = 108 ATP Ø Net yield = 108–2 = 106 ATP Ø In the initial activation reaction, the equivalents of 2 high energy bonds are utilized.  Regulation of Beta Oxidation Ø The availability of free fatty acid (FFA) regulates the net utilization through beta- oxidation. Ø The level of FFA is controlled by the glucagon: insulin ratio. Ø Glucagon increases FFA levels and insulin has the opposite effect. Ø CAT-I is the regulator of the entry of fatty acid into mitochondria. Ø Malonyl-CoA is an inhibitor of CAT-I.(intermediate of fatty acid synthesis) Ø In well-fed conditions the concentration of malonyl CoA increases which inhibits CAT-I and leads to a decrease in fatty acid oxidation. Ø In starvation, the reverse occurs i.e. decreased malonyl-CoA will remove CAT I inhibition allowing more AcylCoA to be oxidized. Thank you for your listening