Metabolic Fuels Notes PDF

Metabolic Fuels Instructor: Annamarie Dalton, PhD Assistant Professor, Dept. of Biochemistry and Molecular Biology Basic Science Building Room 535E Office telephone: 843-792-1495; [email protected] Outline: I. Types of dietary fuels A. Carbohydrates i. Types of carbohydrates ii. Regulation of blood...

Metabolic Fuels Instructor: Annamarie Dalton, PhD Assistant Professor, Dept. of Biochemistry and Molecular Biology Basic Science Building Room 535E Office telephone: 843-792-1495; [email protected] Outline: I. Types of dietary fuels A. Carbohydrates i. Types of carbohydrates ii. Regulation of blood glucose levels by insulin and glucagon iii. Glucose utilization in the liver, brain, red blood cells, and skeletal muscle B. Proteins i. Digestion of proteins into amino acids ii. Fates of amino acids iii. Protein requirement iv. Protein malnutrition C. Fats D. Alcohol II. Fuel stores in the body A. Types of fuel stores B. Fats C. Glycogen D. Muscle proteins E. Glucose, fatty acid, ketone body, and amino acid utilization during fasting III. Metabolic changes between fed and fasting states A. Fed state B. Brief fast C. Prolonged fast Metabolic Fuels-Dalton 1 Learning objectives: 1. List the four common dietary fuels. 2. Describe how dietary carbohydrates are processed and absorbed. 3. Describe the fates of glucose in the liver after a meal. 4. Explain how glucose is utilized as fuel in the brain, red blood cells, and skeletal muscle. 5. Outline how dietary proteins are converted into amino acids in the small intestine. 6. Describe the fates of dietary amino acids. 7. Specify two diseases associated with insufficient protein intake. 8. Describe how dietary fats are processed in the small intestine. 9. State the products of ethanol metabolism. 10. Compare the caloric (energy) contents of the four common fuels in the human diet. 11. Specify the three fuel stores in the body. 12. Explain why more energy can be stored as fat than as glycogen. 13. Specify the metabolic effects of insulin and glucagon and summarize their relative concentrations in the fed, brief fasting, and prolonged fasting states. 14. Explain the role of liver glycogen in buffering blood glucose. 15. Summarize the metabolic fates of carbohydrates, fats, and proteins during the fed state. 16. Specify the 3 non-carbohydrate sources utilized for gluconeogenesis. 17. Explain how fasting (brief and prolonged) affects the utilization of glycogen, fat, and protein stores and influences amino acid catabolism and the blood levels of glucose, ketone bodies, and fatty acids. Metabolic Fuels-Dalton 2 I. Types of dietary fuels Dietary fuels: •Carbohydrates •Proteins •Fats •Alcohol A. Carbohydrates i. Types of carbohydrates • • • • • • Starch, glycogen, sucrose, lactose, fructose, and glucose are the major dietary carbohydrates in the human diet. Sucrose and lactose are disaccharides. Fructose and glucose are monosaccharides. Glucose is the major carbohydrate in blood. Carbohydrates contain more oxygen, so yield less energy than fats. Caloric value: 4 Calories per gram. ii. Regulation of blood glucose levels by insulin and glucagon Elevated glucose after a meal promotes insulin secretion • High blood glucose stimulates insulin secretion. • Glucose and insulin suppress secretion of glucagon. • Insulin promotes uptake of glucose, utilization of glucose by glycolysis, and storage of excess glucose as glycogen • Glucagon is counterregulatory and promotes glycogenolysis Metabolic Fuels-Dalton 3 High blood glucose allows glucose uptake by b-cells of the pancreas via facilitated diffusion. This will stimulate the secretion of insulin. Once insulin is secreted, it binds to its receptors that are ubiquitous throughout the body, but are most notable on muscle, fat and liver cells. Insulin promotes uptake of glucose, utilization of glucose by glycolysis, and storage of excess glucose as glycogen and fat. Glucagon is counter-regulatory and promotes glycogenolysis, lipolysis, and proteolysis. Metabolic Fuels-Dalton 4 iii. Glucose utilization in the liver, brain, red blood cells, and skeletal muscle Glucose utilization in liver Hepatocyte NADPH + Ribose 5-P Portal Vein I+ Glucose Glucose I+ Glycogen I+ Fat, Cholesterol Pancreas I+ ATP production VLDL Insulin # I+ = stimulated by insulin VLDL Tissues After a meal, dietary glucose absorbed by small intestinal mucosal cells travels to the liver via the portal vein. High glucose levels also promote the secretion of insulin by pancreatic b-islet cells. Glucose taken up by the liver is first utilized to meet the energy needs of liver. This process is stimulated by insulin. Insulin also promotes the conversion of excess glucose into glycogen, triacylglycerol, and cholesterol. In addition, glucose is used to make NADPH and ribose 5phosphate. NADPH is used in biosynthetic pathways and for regenerating reduced glutathione. Ribose 5-phosphate is used to synthesize nucleotides. Excess triacylglycerol and cholesterol are transported out of the liver as very low-density lipoprotein (VLDL) particles. VLDL delivers fatty acids and cholesterol to extrahepatic tissues. Glucose utilization in the brain •The brain is a major consumer of glucose, about 150 g per day. •The brain represents only 2% of body weight but consumes 20% of oxygen and 25% of glucose utilized. •Most ATP generated in the brain is consumed by Na+/K+ ATPase. •The brain can utilize ketone bodies under fasting conditions, when carbohydrates are lacking in the diet or in diabetes. Metabolic Fuels-Dalton 5 Red blood cells utilize only glucose • Red blood cells have an absolute requirement for glucose. • Red blood cells lack mitochondria, so they do not carry out mitochondrial oxidative phosphorylation to make ATP. • ATP generation in red blood cells is anaerobic, and lactate is the end-product of glycolysis. Glucose Lactate Glucose Lactate Red blood cells have an absolute requirement for glucose as the primary source of fuel because they do not have mitochondria. They rely on glycolysis to make ATP. The end product of glucose metabolism in red blood cells is lactate, which can be shipped to the liver to remake glucose. Glucose is also used to make NADPH via the pentose phosphate pathway, as you will later learn in Block 4. NADPH is used to regenerate reduced glutathione to keep hemoglobin reduced in red blood cells. Neither the brain, nor red blood cells, can use fatty acids for fuel. Glucose utilization in skeletal muscle •Glucose uptake in muscle is insulin dependent. •Exercising muscle uses both blood glucose and endogenous glycogen. Strenuous exercise leads to the production of lactate. •Glycogen is restored after a meal. Insulin regulates both glucose uptake and glycogen synthesis (glycogenesis). Note here that even though both the liver and skeletal muscle can convert excess glucose into glycogen, only liver glycogen can be used to buffer blood glucose. Metabolic Fuels-Dalton Marks’ Basic Medical Biochemistry, ©2013 Lippincott Williams & Wilkins •Muscle uses fatty acids as a major fuel. 6 B. Proteins i. Digestion of proteins into amino acids • Proteins contain linear chains of amino acids linked together by peptide linkages. • Proteins contain 16% by weight nitrogen. • Excess amino acids cannot be stored. They are utilized mainly for energy production. Nitrogen is excreted as urea. • Caloric value: 4 Calories per gram. Digestion of dietary proteins occurs initially in the stomach where the acidic pH promotes the denaturation of proteins and cleavage by pepsin. Protein degradation continues in the lumen of the small intestine via pancreatic proteases to form amino acids and oligopeptides. Aminopeptidases on the plasma membranes of the intestinal mucosal cells further break down oligopeptides into di- and tripeptides. Amino acids and di- and tripeptides are taken into the intestinal cells and additional peptidases cleave these peptides into amino acids. The resulting amino acids are released into the bloodstream. ii. Fates of amino acids Liver and other tissues Portal Vein Amino Acids Serum proteins (e.g., albumin) Liver proteins (enzymes, etc.) Heme (porphyrins) Hormones Neurotransmitters Purine and pyrimidine bases Amino acids released from the small intestine are taken up by other tissues. In the liver, they are used to synthesize proteins. Virtually all serum proteins, the major one being albumin, are synthesized by the liver. Serum proteins include the globulins associated with the immune system. The liver makes large amounts of heme (from glycine; Block 9) that is incorporated into mitochondrial cytochromes and the endoplasmic reticular cytochrome P450 enzymes. In other tissues, amino acids serve as precursor for peptide hormones such as insulin, glucagon, and growth hormones. Many of the neurotransmitters, such as gamma-amino butyric acid (GABA) and dopamine are made from amino acids (GABA is made by decarboxylating glutamate and dopamine is made from tyrosine; Block 5). The synthesis of purine and pyrimidine nucleotides also requires carbon and nitrogen atoms from certain amino acids (Block 4). Metabolic Fuels-Dalton 7 Excess amino acids are converted to glucogenic or ketogenic compounds Biochemistry 5th Ed. ã 2002 W.H. Freeman and Company •Acetyl CoA and acetoacetyl CoA are ketogenic (leucine and lysine). •Pyruvate, oxaloacetate, fumarate, succinyl CoA, and a-ketoglutarate are glucogenic. Excess amino acids are not stored by the body but are broken down. The amino groups of amino acids are converted to urea for disposal via the urea cycle. The carbon skeletons are converted into 7 metabolic intermediates that can be used either to synthesize glucose or ketone bodies, or oxidized by the TCA cycle for energy production, depending on the metabolic needs of the body. Amino acids that are used to make glucose are known as glucogenic amino acids while amino acids that are used to make ketone bodies are known as ketogenic amino acids. Only leucine and lysine are solely ketogenic amino acids. Some amino acids, such as phenylalanine, isoleucine, tryptophan and tyrosine, are both ketogenic and glucogenic. The three primary ketone bodies are acetoacetate, beta-hydroxybutyrate and acetone. Metabolic Fuels-Dalton 8 iii. Protein requirement •For an “average” adult human, the minimum protein requirement is normally 56 grams per day. •This assumes that the biological value of the protein is 70, the average biological value of the American diet (Egg white protein, or ovalbumin, is defined as having a value of 100). •If the biological value is 40, as would be the case with certain plant proteins, which are short of some essential amino acids, the requirement would be (56 g X 70/40) = 98 g. iv. Protein malnutrition •Protein malnutrition will severely compromise health. • The synthesis of antibodies, hormones, enzymes, neurotransmitters, RNA, DNA, etc., would be diminished. • Kwashiorkor and marasmus: two protein deficiency disorders in children. The health of a person who is deprived of dietary protein is going to be severely compromised. Insufficient consumption of protein, but with sufficient calorie intake, gives rise to kwashiorkor. It is characterized by edema (distended abdomen) and ulcerating dermatosis. The lack of micronutrients and antioxidants is also believed to contribute to this form of malnutrition. Marasmus, on the other hand, results from insufficient intake of both calories and protein. Children with this condition are emaciated and they suffer many developmental disorders. Metabolic Fuels-Dalton 9 C. Fats • Fats are triacylglycerols (triglycerides; TAG). • Triacylglycerols contain three fatty acids and one glycerol moiety. • Fats contain far less oxygen than carbohydrates or proteins and are more reduced. • Caloric value: 9 Calories per gram. Triacylglycerol is the primary form of fat found in food. In the small intestine, dietary fats (triacylglycerols; TAG) are emulsified by bile salts and then acted on by pancreatic lipases to form free fatty acids (FFA) and monoacylglycerols (MAG). FFA & MAG are reassembled into TAG in the intestinal epithelial cells (mucosal cells). TAG, along with proteins, phospholipids, and other components are assembled into lipoprotein particles called chylomicrons and released into the bloodstream by the way of lymph. Fatty acids from TAG of chylomicrons can be used by various tissues for membrane lipid synthesis or for energy production. Excess fatty acids can be stored as triacylglycerol in adipose tissues. Fats are more reduced than carbohydrates or proteins because they contain far less oxygen atoms. Oxidation of 1 gram of fat will yield 9 Calories. D. Alcohol Ethanol is primarily metabolized in the liver by alcohol dehydrogenase and acetaldehyde dehydrogenase. In the process, acetate and NADH are formed. Acetate generated from ethanol oxidation can be converted into acetyl CoA. Acetyl CoA can enter the TCA cycle to make GTP, NADH, and FADH2. NADH and FADH2 can be utilized to form ATP via oxidative phosphorylation in mitochondria. Alcohol (Ethanol, ETOH) • • • Ethanol is oxidized to acetate. Acetate is then converted into acetyl CoA, which can be used for energy production. Oxidation of ethanol produces the intermediate acetaldehyde, which is harmful in high levels. Caloric value: 7 Calories per gram. CH3CH2OH + NAD+ Ethanol CH3CHO + NADH + H+ Alcohol Dehydrogenase CH3CHO + NAD+ + H2O Acetaldehyde Acetaldehyde CH3COO- + NADH + H+ Acetaldehyde Dehydrogenase Metabolic Fuels-Dalton Acetate 10 II. Fuel stores in the body A. Types of fuel stores • Fat from adipose tissues • Glycogen • Muscle proteins Percentages and kilograms of stored energy after an overnight fast in a 70 kg man B. Fats • About 85% of stored energy is fat. • Fat yields more than twice the energy per gram than carbohydrates or proteins. • Adipose tissue contains only about 15% water, and it does not hydrate like glycogen. • Insulin promotes fat synthesis and storage during the fed state while glucagon promotes fat breakdown during fasting. • Muscle can utilize fatty acids as energy source. • Fatty acids can be used to synthesize ketone bodies during fasting. Fat is primarily stored in adipose tissues as triacylglycerol. Fat can be catabolized to produce large amounts of free fatty acids, which are transported in blood by albumin to cells throughout the body where they can be used as an energy source. The major consumers of fatty acids are skeletal and heart muscles. In fact, they prefer fatty acids over glucose except in short-term emergency situations. Fatty acids are broken down into acetyl CoA via b-oxidation in mitochondria. Acetyl CoA then enters the TCA cycle for energy production. The brain and red blood cell are exceptions. They cannot utilize free fatty acids as source of energy. Metabolic Fuels-Dalton 11 C. Glycogen • Virtually all cells contain glycogen. • Carbohydrates (i.e., glycogen and glucose) are the only fuel that can be used for energy in the absence of oxygen. • Liver glycogen buffers blood glucose. • Muscle glycogen does not buffer blood glucose. • When well-fed, the liver holds about 200 g of glycogen. • Insulin promotes glycogen synthesis during the fed state, while glucagon promotes glycogen breakdown during the fasting state. Glycogen is the primary storage form of glucose in our body. Glycogen synthesis and breakdown are highly regulated. Glycogen storage in the liver lasts about 18-20 hours. After liver glycogen store is exhausted, gluconeogenesis from non-carbohydrate sources, such as glucogenic amino acids, lactate, and glycerol, is carried out by the liver to maintain blood glucose homeostasis. More energy can be stored as fat than as glycogen How many kg of glycogen would store the same amount of energy as 200,000 kcal stored as fat? • 200,000 kcal is equivalent to about 22 kg of pure fat or 26 kg of adipose tissue (15% water) 26 kg of adipose tissue = 200 kg (441 lbs) of glycogen! •200,000 kcal/4 kcal per g = 50 kg glycogen •Glycogen is hydrated to four times its weight by water. •50 kg X 4 = 200 kg of glycogen More energy can be stored as fat than as glycogen because fat is more reduced and is also far less hydrated than glycogen. For adipose tissue, it contains 15% water. For glycogen, it contains 75% water. Metabolic Fuels-Dalton 12 D. Muscle proteins •Protein mainly serves structural, movement, enzymatic, and transport roles. •Protein is a vital source of glucose when there is a dietary lack of carbohydrate (also in diabetes and other metabolic disorders). •Protein is not considered a store of energy. •Only a limited amount of protein (about 8%) can be degraded before bodily functions are impaired. E. Glucose, fatty acid, ketone body, and amino acid utilization during fasting Fuel utilization during fasting Marks’ Basic Medical Biochemistry, ©2013 Lippincott Williams & Wilkins During the fed state, the body uses glucose and fatty acids as primary sources of energy. During fasting, the blood glucose level drops initially, but is then maintained at a lowered level by gluconeogenesis from non-carbohydrate sources. Fatty acids from the breakdown of triacylglycerols in adipocytes are then used by various tissues. During prolonged fasting, fatty acids are converted into ketone bodies by the liver, which are picked up by the brain for energy as a replacement for glucose. Metabolic Fuels-Dalton 13 Protein utilization during fasting Muscle protein proteolysis, gluconeogenesis Ketogenesis, sparing glc & muscle proteins Ketogenesis, sparing glc & muscle proteins •Urea excretion is very low when consuming glucose only. •It increases markedly in the initial phase of fasting, because muscle proteins are supplying amino acids for gluconeogenesis. It decreases after several days, because ketone bodies are used as fuel, sparing glucose (and muscle proteins). Urea is a byproduct of amino acid breakdown. Therefore, excretion of urea can be used as a measure of amino acid catabolism. When glucose is abundant, low amounts of amino acids are broken down, so urea excretion is low. During the initial phase of fasting, muscle proteolysis occurs to generate amino acids for conversion into glucose. Thus, more urea is being formed and excreted. During prolonged fasting, as ketogenesis is ramped up to replace glucose utilization, muscle proteolysis diminishes to preserve muscle proteins. Metabolic Fuels-Dalton 14 III. Metabolic changes between fed and fasting states A. Fed state Fed state Marks’ Basic Medical Biochemistry, ©2013 Lippincott Williams & Wilkins The following processes occur during the fed state: 1. Carbohydrates are converted into glucose which is then absorbed by the small intestinal cells and released into the bloodstream. 2. Triacylglycerol molecules (as fatty acids and monoacylglycerol) taken up by small intestinal mucosal cells are repackaged as chylomicrons and released into the bloodstream. 3. Proteins are converted into amino acids in the small intestine. They are then released into the bloodstream. 4. Increased blood glucose elevates insulin and decreases glucagon secretion. 5. Insulin promotes glycolysis in liver cells. This forms acetyl CoA which then enters the TCA cycle to meet the energy needs of liver cells. Carbon dioxide is produced in the process. 6. Insulin promotes the synthesis of glycogen from excess glucose in the liver. 7. Insulin promotes the synthesis of fatty acids from acetyl CoA. Fatty acids are then used to make triacylglycerol. Excess triacylglycerol is exported from the liver as VLDL. 8. Glucose is taken up by the brain and is converted to acetyl CoA via glycolysis and the pyruvate dehydrogenase complex. Acetyl CoA then enters the TCA cycle where it is oxidized to form carbon dioxide. Subsequently, ATP is generated via oxidative phosphorylation. 9. Glucose is also taken up by red blood cells. Glucose metabolism is strictly anaerobic in these cells and lactate is formed as a result. Lactate is released into the bloodstream and is picked up by the liver. Metabolic Fuels-Dalton 15 10. Insulin promotes the uptake of glucose by adipocytes. Glucose is used to make glycerol 3phosphate for triacylglycerol synthesis. 11. Insulin promotes glucose uptake by muscle cells. Insulin stimulates muscle cell glycolysis. Excess glucose is stored as glycogen. 12. Triacylglycerols in chylomicrons and VLDL are acted on by lipoprotein lipase to form fatty acids and glycerol. Fatty acids are then taken into various cells including adipocytes. 13. Insulin promotes the synthesis of triacylglycerol in adipocytes from fatty acids and glycerol. 14. Dietary amino acids are taken up by various tissues to make proteins and various biologically important compounds. Excess amino acids are broken down for energy production. B. Brief fast Brief fasting state The following processes occur during brief fasting: 1. After a brief fast (12 h after eating), blood glucose level goes down. This decreases insulin secretion and increases glucagon secretion. 2. The increased level of glucagon triggers hepatic glycogen breakdown to form glucose. Glucose is then released into the bloodstream. 3. Glucose is taken up by the brain and is converted to acetyl CoA via glycolysis. Acetyl CoA is used for ATP synthesis via the TCA cycle and electron transport chain. 4. Glucose is also taken up by red blood cells. Glucose metabolism is strictly anaerobic in these cells and lactate is formed as a result. Metabolic Fuels-Dalton 16 5. High glucagon levels promote the lipolysis of triacylglycerol in adipocytes via the activation of several lipases. This results in the release of glycerol and fatty acids from adipocytes into the bloodstream. 6. Fatty acids released from adipocytes are taken up by muscle cells and are then converted into acetyl CoA via b-oxidation. Acetyl CoA then enters the TCA cycle for ATP production. 7. Fatty acids taken up by liver cells are converted into acetyl CoA via b-oxidation. Acetyl CoA is used to make ketone bodies. 8. Ketone bodies are released into the bloodstream and are then taken up by muscle cells where they are converted back into acetyl CoA for energy production. 9. A brief fast promotes the breakdown of muscle proteins into amino acids. Glucogenic amino acids are taken up by liver cells to make glucose via gluconeogenesis. The resulting glucose is then released into the bloodstream. 10. The breakdown of amino acids in the liver leads to the formation of urea via the urea cycle. Urea released into the bloodstream is picked up by the kidneys for excretion. 11. Lactate formed from anaerobic glycolysis in red blood cells is sent to the liver for conversion into glucose via gluconeogenesis. 12. Glycerol released from adipocytes is picked up by the liver for conversion into glucose via gluconeogenesis. C. Prolonged fast Prolonged fasting state Marks’ Basic Medical Biochemistry, ©2013 Lippincott Williams & Wilkins Metabolic Fuels-Dalton 17 The following processes occur during prolonged fasting: 1. After three to five days of fast, the lowered levels of blood glucose will decrease insulin secretion and promote glucagon secretion. 2. All the hepatic glycogen store is exhausted in prolonged fasting. 3. Less amount of glucose is being utilized by the brain for energy production. 4. Glucose is continuously being taken up by red blood cells for anaerobic glycolysis. 5. Lipolysis of triacylglycerols in adipocytes now provides the main source of fuel for the body. Glycerol and fatty acids from adipocytes are released into the bloodstream. 6. Fatty acids released from adipocytes are taken up by muscle cells and are then converted to acetyl CoA for energy production. 7. Fatty acids taken up by liver cells are converted into acetyl CoA via b-oxidation. Ketone body formation from acetyl CoA is ramped up. 8. Ketone bodies are released into the bloodstream and become the primary fuel source for the brain. This reduces glucose uptake by the brain. 9. Formation of large amounts of ketone bodies during prolonged fasting reduces the breakdown of muscle proteins into amino acids to conserve muscle proteins. 10. As amino acid utilization by the liver is reduced, less urea is produced. 11. Lactate formed from anaerobic glycolysis in red blood cells is sent to the liver for conversion into glucose via gluconeogenesis. 12. Glycerol released from adipocytes is picked up by the liver for conversion into glucose via gluconeogenesis. Acknowledgment: Special thanks to Dr. Yi-Te Hsu and Dr. Bill Stillway for providing the syllabus template. Many figures presented in this lecture were generated using Biorender.com. Metabolic Fuels-Dalton 18