Fat as Fuel - 4MBBS101 Nutrition and Metabolism PDF

Faculty of Life Sciences & Medicine Dr. Lauren 4MBBS101 Nutrition and Metabolism Albee Department of Biochemistry Fat as Fuel  Fat as Fuel Dr. Lauren Albee Biochemist...

Faculty of Life Sciences & Medicine Dr. Lauren 4MBBS101 Nutrition and Metabolism Albee Department of Biochemistry Fat as Fuel  Fat as Fuel Dr. Lauren Albee Biochemistry and Molecular Biology Chapter 14 Available as an e-textbook at https://bibliu.com/app/#/signinPage 2  Learning outcomes After this lecture you should be able to: a) Indicate the importance of triglyceride fat for long term fuel storage. b) Describe the role of adipose tissue lipase in the breakdown of triglyceride into fatty acids and glycerol. c) Describe how fatty acids are activated to their CoA esters, and how they are transported into mitochondria via the carnitine shuttle system. d) Describe the enzyme reactions of the β-oxidation pathway that yield acetyl-CoA, NADH and FADH2. e) Summarise the factors regulating fatty acid oxidation. f) Explain the term ‘ketone bodies’ and outline the significance of ketogenesis in starvation. 3  Biological Functions of Lipids 1. Components of cell membranes Lipid (phospholipids & cholesterol) bilayer (5 nm) 2. Precursors of hormones cholesterol steroid hormones Lipid molecule Protein molecule arachidonic acid prostaglandins 3. Long term fuels (triglycerides) 4  Efficiency of Triglycerides as Fuel Compact storage - triglycerides stored as large fat droplets in the fat cells of adipose Fat droplet tissue Large body stores - 70 kg adult has: 11 kg fat (as TG) 120 g glycogen in liver 10 g glucose Efficiency on weight basis – 1 g fat yields 38 kJ Adipose tissue 1 g protein 21 kJ 1 g carbohydrate 17 kJ 5  Structure of Triglyceride fat (triacylglycerols) Glycerol component of triacylglycerol Glycerol Common fatty acids: palmitic acid C 16:0 stearic acid C 18:0 oleic acid C 18:1 linoleic acid C 6 18:2  Breakdown of stored triglyceride fat in adipose tissue Lipase Triacylglycerol activated by Triacylglycerol lipase (active) adrenaline Fatty acid & glucagon Diacylglycerol DAG lipase Free fatty acids travel in plasma Fatty acid bound to albumin Monoacylglycerol Glycerol MAG lipase diffuses in blood stream Fatty acid Act as fuels to all tissues Glycerol for muscles, heart & liver 7  Metabolism of glycerol Glycerol is water soluble & is taken up by all tissues Glycerol in liver most tissues in starvation Enters glycolysis Enters glycolysis pathway for conversion pathway and is to pyruvate, then converted to glucose by into TCA cycle for gluconeogenesis oxidation to CO2 Fatty acid metabolism by -oxidation pathway all reactions occur in the mitochondrial matrix – (* transport across membrane) intermediates present as CoA thioesters biological energy of fatty acid molecule is conserved as the transfer of 2 H atoms to the cofactors NAD+ and FAD to form NADH & FADH2 (no direct ATP synthesis) series of four enzyme reactions results in removal of two carbon unit as acetyl CoA 9 Activation of long chain fatty acids O CH3 – (CH2)n – CH2 – CH2 – C – O – H CoASH ATP Fatty acyl-CoA synthetase (cytosol) AMP + PPi H H O CH3 – (CH2)n – C – C – C – S-CoA H H Fatty acyl-CoA 10  Coenzyme A forms thioester bonds with carboxylic acids Blob you don’t need to worry about CoA-SH RCH2 C-O-H + CoA-SH RCH2 C-S-CoA O O 11 Transport of fatty acyl-CoA into mitochondria: carnitine shuttle 1) Fatty acyl-CoA freely 5) Carnitine transported back into diffuses across the outer the intermembrane space. mitochondrial membrane 4) Carnitine is switched back for CoA by carnitine acyltransferase II, recreating fatty acyl-CoA. 3) Fatty acyl-carnitine crosses the 2) Fatty acid group transferred to inner mitochondrial membrane carnitine by carnitine acyltransferase I, via a translocase. creating fatty acyl-carnitine. The process is energetically neutral. Transport of fatty acids into mitochondria - Stage 1 13 Transport of fatty acids into mitochondria - Stage 2 14 Overview of β-oxidation pathway Fatty acyl-CoA with 10 carbons β-oxidation + Fatty acyl-CoA with 8 carbons acetyl-CoA Called β-oxidation because the β-carbon undergoes oxidation to produce a carbonyl group (carbon double-bonded to oxygen). One round of β-oxidation produces acetyl-CoA and a fatty acyl-CoA that is 2 carbons shorter - the 2 carbons are now carried by acetyl-CoA. Overview of β-oxidation pathway Remove 2 hydrogens Cleave bond and add CoASH + Add oxygen (in water) Remove 2 hydrogens Reaction 1 - Removal of 2 H atoms H H O CH3 – (CH2)n – C – C – C – S-CoA H H Fatty acyl-CoA FAD acyl-CoA 1 dehydrogenase FADH2 H O CH3 – (CH2)n – C = C – C – S-CoA H 17 Reaction 2 – Addition of water H O CH3 – (CH2)n – C = C – C – S-CoA H Enoyl-CoA 2 H2O Enoyl-CoA hydratase H O CH3 – (CH2)n – C – CH2 – C – S-CoA OH 18 Hydroxyacyl-CoA Reaction 3 – Removal of 2 H atoms H O CH3 – (CH2)n – C – CH2 – C – S-CoA OH Hydroxyacyl-CoA NAD+ 3 Hydroxyacyl-CoA dehydrogenase NADH + H+ O O CH3 – (CH2)n – C – CH2 – C – S-CoA 19 -Ketoacyl-CoA Reaction 4 - Removal of 2 C units O O CH3 – (CH2)n – C – CH2 – C – S-CoA -Ketoacyl-CoA 4 CoA-SH -Ketoacyl-CoA thiolase O O CH3 – (CH2)n – C – S-CoA + CH3 – C – S-CoA Fatty acyl-CoA Acetyl-CoA (2 C atoms shorter) TCA cycle 20 CO2 Same reaction pattern seen in the TCA cycle Remove 2 hydrogens Add oxygen (in water) Remove 2 hydrogens Shorter fatty acid re enters reactions 1 - 4 O CH3 – (CH2)n – C – S-CoA Fatty acyl-CoA (2 C atoms shorter) 2C 1 2H unit 4 3 2 H2O 2H 22 Summary of -oxidation pathway- 1 Activation stage C16 mitochondrial membranes matrix 23  Summary of -oxidation pathway -2 Fatty acid with 16 C atoms will pass through 7 repeats of -oxidation pathway C16 producing 7 NADH & 7 FADH2 C2 C14 2H Acetyl C12 CoA C10 H2O 2H 24 Summary of -oxidation pathway C16 Fatty acid with 16 C atoms will give rise C2 C14 2H to 8 Acetyl C12 acetyl CoA which CoA C10 enter the TCA cycle H2O 2H 25 Energy yield from fatty acid oxidation Fatty acid with 16 C atoms goes through 7 repeats of -oxidation producing 7 NADH & 7 FADH2 ATP yield = 7 x 2.5 + 7 x 1.5 = 28 Fatty acid with 16 C atoms produces 8 acetyl CoA ATP yield from complete oxidation of acetyl CoA by TCA cycle = 8 x 10 ATP = 80 TOTAL = 80 + 28 = 108 – 1 = 107 26  Regulation of fat metabolism Release of fatty acids from adipose tissue adrenaline & glucagon activate lipase enzyme Rate of entry into mitochondria via carnitine shuttle Rate of reoxidation of cofactors NADH & FADH2 by Electron Transport Chain 27 Metabolism of odd numbered fatty acids -oxidation will result in the following: C15 C13 C11 C9 C7 C5 C3 28  Metabolism of odd numbered fatty acids – dealing with the last 3 carbons CH3 CH2 – C – S-CoA ATP O CO2 ADP +Pi Propionyl-CoA-carboxylase OOC – CH – C – S-CoA - CH3 O Methylmalonyl-CoA mutase OOC – CH2 – CH2 – C – S-CoA - O Succinyl-CoA TCA cycle  Ketone body formation ‘Ketogenesis’ occurs when fat metabolism is the main source of energy: – in starvation – in Type I diabetes No need to learn these Fatty acid oxidation in hepatocytes leads to high structures concentrations of Acetyl Co A - exceeds capacity of the TCA cycle. Excess Acetyl CoA is converted into ‘ketone bodies’ in liver acetoacetate and β hydroxybutyrate are released into the bloodstream  Ketone bodies can be utilised for energy by most (but not all) tissues acetoacetate and β hydroxybutyrate are released into the bloodstream. In most cell types they can be converted back into TCA cycle intermediates (acetyl CoA and succinate). Most tissues oxidise a mixture of fatty acids and ketone bodies Liver cannot utilise ketone bodies – WHY? Brain cannot utilise fatty acids – WHY? – uses glucose and small amount of ketone bodies (‘emergency fuel’) Red blood cells cannot utilise fatty acids or ketone bodies, use glucose only – WHY? Summary of mitochondrial events  MCQ 1 1. How are free fatty acids transported in the blood? They are: A. carried by albumin. B. carried by carnitine. C. carried by chylomicrons. D. carried by low- and high-density lipoproteins (LDLs and HDLs) E. freely soluble and do not require a carrier. 33  MCQ 2 2. What are the three main biological function of lipids? A. Components of ribosomes, precursor for steroid hormones, and long-term fuel storage. B. Components of membranes, precursor for steroid hormones, and short-term fuel storage. C. Components of membranes, precursor for steroid hormones, and long-term fuel storage. D. Components of the nucleic acids, precursor for peptide hormones, and long-term fuel storage. E. Components of membranes, precursor for peptide hormones, and long-term fuel storage. 34  MCQ 3 3. Fatty acids with an odd number of carbons undergo β- oxidation until three carbons of the fatty acid remain. This three-carbon unit (propionyl-CoA) is then converted to: A.succinyl-CoA. B. acetyl-CoA. C. enoyl-CoA. D.pyruvate. E. glycerol. 35  MCQ 4 4. A reaction sequence that takes place in mitochondria when fatty acids are broken down for energy can be described as: A.an oxidation followed by a hydration followed by a reduction. B. an oxidation followed by an oxidation followed by a decarboxylation. C. an oxidation followed by an oxidation followed by a hydrolysis. D.an oxidation followed by a reduction followed by a hydrolysis. E. an oxidation followed by a hydration followed by an oxidation. 36  MCQ 5 5. A deficiency that prevents someone from synthesising carnitine could cause: A.Accumulation of lipid droplets in the mitochondrial matrix of liver cells B. Accumulation of lipid droplets in the cytosol of liver cells C. A decrease in free long chain fatty acids circulating in the blood. D.Increased beta -oxidation and reduced storage of long chain fatty acids in liver cells E. Increased conversion of long chain fatty acids to glucose in the liver. 37