Carbohydrate Metabolism - Glycolysis PDF

Metabolism of Carbohydrates- Glycolysis By Ajilore B.S. (MBChB, PhD) Overall Objectives:  By the end of this course, you should: 1) understand how carbohydrate metabolism normally responds in the fed state, the fasting state, and during exercise. 2) understand how carbohyd...

Metabolism of Carbohydrates- Glycolysis By Ajilore B.S. (MBChB, PhD) Overall Objectives:  By the end of this course, you should: 1) understand how carbohydrate metabolism normally responds in the fed state, the fasting state, and during exercise. 2) understand how carbohydrate metabolism is altered by diabetes 2 Introduction to Metabolism  Thousands of chemical reactions are taking place inside a cell in an organized, well coordinated, and purposeful manner; all these reactions are collectively called metabolism.  Metabolism is the sum total of all chemical reactions in living cells; It is the overall process through which living systems acquire and utilize free energy to carry out their functions  Metabolism (syn = intermediary metabolism)  Is the totality of the chemical reactions in the cell catalyzed by enzymes  The intermediates, substrates or products of these enzyme-catalyzed reactions are called metabolites  Sequence of enzymatic reactions that produce specific products are called metabolic pathways There are 3 types of metabolic pathways: i. Catabolism or catabolic pathway: “down” ii. Anabolism or anabolic pathway: “up” iii. Amphibolism or amphibolic pathway: “cross road” Metabolic pathways  Catabolism: is the breakdown of large molecules to small molecules. It yields energy  Anabolism: is the formation of big molecules from small molecules. It consumes energy  Amphibolism: is the pathway that involves in both breakdown and build up of molecules. It is described as “cross road” between anabolic and catabolic pathways e.g CAC - Chemical energy is obtained from the degradation of energy rich nutrients. - When energy rich complex macromolecules are degraded into smaller molecules, energy release during this process is trapped as chemical energy, usually as ATP i.e. catabolism. - The cells need this energy to synthesize complex molecules from simple precursors i.e. anabolism. - The degradation of foodstuffs occurs in 3 stages: 1. Digestion in the GIT to convert the macromolecules into small units e.g. proteins are digested to amino acids. This is called primary metabolism. 2. Absorption of these products, followed by degradation/ catabolism to smaller units, and ultimately oxidized to CO 2. During this stage, reducing equivalents are generated in the mitochondria by the common oxidative pathway (i.e. amphibolic) known as citric acid cycle (CAC). In this process, NADH or FADH2 are generated. This called secondary or 3. Finally, these reduced equivalents enter into electro transport chain, ETC (Respiratory chain) where energy is released. This is tertiary metabolism or internal respiration (or cellular respiration). Digestion of Carbohydrates  In the diets, carbohydrates are present as complex polysaccharides (starch, glycogen), and in some cases as disaccharides (sucrose and lactose).  They are hydrolysed to monosaccharide units (glucose, galactose and fructose) in GIT. a. Digestion in mouth: - Digestion of carbohydrates starts in the mouth, where they come in contact with saliva during mastication. - Saliva contains a carbohydrate splitting enzyme called salivary amylase (ptyalin). - The enzyme hydrolyzes α-1 → 4 glycosidic linkage inside polysaccharide molecule like starch, glycogen and dextrins, producing smaller molecules maltose, glucose and b. Digestion in stomach - Practically no action. No carbohydrate splitting enzymes available in gastric juice. c. Digestion in duodenum - Food bolus reaches the duodenum from stomach where it mixes with the pancreatic juice. - Pancreatic juice contains a carbohydrate-splitting enzyme pancreatic amylase (also called amylopsin) similar to salivary amylase. - The enzyme hydrolyses α-1→4 glycosidic linkage in polysaccharide molecule. d. Digestion in Small Intestine - Intestinal juice (succus entericus) contains amylase, lactase, maltase, isomaltase and sucrase. - Intestinal amylase hydrolyses terminal α-1→4, glycosidic linkage in polysaccharides and oligosaccharide molecules liberating free glucose molecule. - Lactase hydrolyses lactate to equimolar amounts of glucose and galactose. Specific Transporters For Glucose.  GluT 1 is present in RBC, brain, kidney, colon, retina and placenta. It is responsible for glucose uptake in most cells.  GluT 2 is found in the serosal surface of intestinal cells, liver, beta cells of pancreas. It is responsible for glucose uptake in liver and it is the glucose sensor in beta cells. It has low affinity for glucose.  GluT 3 is found in neurons and brain. It has high affinity for glucose. It is responsible for glucose uptake into the brain cells.  GluT 4 is found in skeletal muscle, heart muscle and adipose tissue. It is responsible for insulin mediated glucose uptake.  GluT 5 is found in small intestine, testis, sperms and kidney. It has poor ability to transport glucose but it serves as fructose transporter.  GluT 7 is found in liver ER. It involves in transport of glucose Absorption of Carbohydrates - Only monosaccharides are absorbed by the intestine. - Absorption rate is maximum for galactose, moderate for glucose and minimum for fructose. Mechanisms of Absorption  Simple/passive diffusion - This is dependent on sugar concentration gradients between the intestinal lumen, mucosal cells and blood plasma. - All the monosaccharides are probably absorbed to some extent by simple ‘passive’ diffusion.  Facilitated diffusion /“Active” Transport Mechanisms - Glucose and galactose are absorbed very rapidly and hence it has been suggested that they are absorbed actively and it requires energy. - Glucose, galactose and fructose are absorbed by facilitated diffusion which is mediated by specific carrier molecule (membrane proteins) present in the enterocyte membrane. Mechanism of glucose absorption I. Co-transport from lumen to intestinal cell - Process is mediated by sodium-dependent glucose transporter-1 (S GluT-1). - Glucose is co-transported with sodium. The sodium is later expelled by sodium pump. - This is involved in the treatment of diarrhea. - The oral rehydration fluid contains sodium and glucose. - Presence of glucose allows uptake of sodium to replenish sodium chloride lost due to dehydration. II. Uniport system - Intestinal cells release glucose into blood stream by carrier molecule called Glucose transporter 2 (GLUT 2). - This transporter is not dependent on sodium. It is a uniport, facilitated diffusion. - GluT2 is also involved in absorption of glucose from blood III. Glucose Transporter 4 - GluT4 is the major glucose transporter in skeletal muscle and adipose tissue. - It is under the control of insulin while other GluTs are not under insulin control. - Insulin induces GluT4 on the cell surface and thus increases glucose uptake. - In type 2 DM, membrane GluT4 is reduced leading to insulin resistance in muscle and fat cells. Pathways of carbohydrates metabolism  Glycolysis  Glycogenesis  Glycogenolysis  Citric acid cycle (Tricarbocylic acid cycle or Kreb’s cycle)  Gluconeogenesis (Neoglucogenesis)  Pentose phosphate pathway (Hexose monophosphate shunt) Glycolysis: Emden-Meyerhof Pathway - The sequence of enzyme-catalyzed reactions involved in the breakdown of one glucose molecule to two molecules of pyruvate or lactate Types of Glycolytic pathway 1. Aerobic glycolysis 2. Anaerobic glycolysis - Glucose is the body’s most readily available source of energy. - After digestive processes break polysaccharides into monosaccharides, including glucose, the monosaccharides are transported across the wall of the small intestine and into circulatory system, which transports them to the liver. - In the liver, hepatocytes either pass the glucose on through the circulatory system or store excess glucose as glycogen. - Cells in the body take up the circulating glucose in response to - Glycolysis can be divided into two phases: i. energy consuming (also called chemical priming) and ii. energy yielding. - During the energy consuming phase, two ATP molecules are required to start the reaction for each molecule of glucose. - However, the end of the reaction produces four ATPs, resulting in a net gain of two ATP energy molecules. - Glycolysis can be expressed as the following equation: Glucose + 2ATP + 2NAD+ + 4ADP + 2Pi → 2 Pyruvate + 4ATP + 2NADH + 2H - This equation states that glucose, in combination with A TP (the energy source), NAD (electron acceptor), and inorganic phosphate, breaks down into two pyruvate molecules, generating four A TP molecules— for a net yield of two ATP— and two energy containing NADH coenzymes GLYCOLYSIS Glucose ATP hexokinase ADP Glucose 6-phosphate phosphogluco- isomerase Fructose 6-phosphate ATP phosphofructokinase ADP Fructose 1,6-bisphosphate aldolase triose phosphate isomerase Dihydroxyacetone Glyceraldehyde phosphate 3-phosphate  Glyceraldehyde 3-phosphate glyceraldehyde NAD+ + Pi 3-phosphate dehydrogenase NADH + H+ 1,3-Bisphosphoglycerate ADP phosphoglycerate kinase ATP 3-Phosphoglycerate phosphoglyceromutase 2-Phosphoglycerate enolase H2O Phosphoenolpyruvate ADP Purpose- a metabolic pathway to convert one molecule of glucose into 2 molecules of pyruvate and produce 2 molecules each of NADH and ATP.  All carbohydrates to be catabolized must enter the glycolytic pathway.  Glycolysis is central in generating both energy and metabolic ENERGY YIELD PER GLUCOSE MOLECULE OXIDATION A. In Glycolysis in Presence of O2 (Aerobic Phase) B. In Glycolysis—in Absence of O2 (Anaerobic Phase) - In anaerobic phase per molecule of glucose oxidation 4 – 2 = 2 ATP will be produced. REGULATION OF GLYCOLYSIS - Regulation of glycolysis achieved by three types of mechanisms: I. Changes in the rate of enzyme synthesis, Induction/repression. II. Covalent modification by reversible phosphorylation. III. Allosteric modification. Hormone control  - Insulin increases rate of glycolysis by increasing concentration of glucokinase, PFK-1 and PK while glucagon, adrenalin and noradrenalin inhibits glycolysis.  Three irreversible kinase reactions primarily drive glycolysis forward.  hexokinase or glucokinase  phosphofructokinase  pyruvate kinase Three of these enzymes regulate glycolysis 1. HEXOKINASE Phosphorylation of glucose.  Inhibited by its product, glucose 6-phosphate, as a response to slowing of glycolysis Not GLUCOKINASE 2. PHOSPHOFRUCTOKINASE  major regulatory enzyme, rate limiting for glycolysis  an allosteric regulatory enzyme.  measures adequacy of energy levels. Inhibitors: ATP by decreasing fructose 6-phosphate binding and citrate  AMP and ADP reverse ATP inhibition And another activator Fructose 2,6 bisphosphate is a very important regulator, controlling the relative flux of carbon through glycolysis versus gluconeogenesis. - It also couples these pathways to 3. PYRUVATE KINASE PEP + ADP  pyruvate + ATP  An allosteric tetramer  inhibitors: ATP, PEP  activator: fructose 1,6-bisphosphate (“feed- forward”)  Phosphorylation (inactive form) and dephosphorylation (active form) under hormone control. -Insulin increases rate of glycolysis by increasing concentration of glucokinase, PFK-1 and PK  Also highly regulated at the level of  Pyruvate Alcohol Lactic Acid Fermentation Fermentation Aerobic Glycolysis Fate of Pyruvate  LACTIC ACID (CORI) CYCLE glucose glucose glucose glucose-6-P glucose-6-P glycogen glycogen ATP ATP NADH Blood NADH pyruvate pyruvate lactate lactate lactate Liver Muscle 26 GLYCOLYSIS IN RED BLOOD CELLS: The Peculiarities  RB cells are structurally and metabolically unique as compared to other cells.  Structural Peculiarities: Structurally mature erythrocytes do not possess nucleus nor cytoplasmic subcellular structures.  Metabolic peculiarities: Metabolically mature erythrocytes:  Entirely depends on glucose for its energy, i.e. glycolysis. More than > 90 per cent of total energy is met by glycolysis  Glucose is freely permeable to erythrocytes like liver cells.  Glucose oxidation always ends in formation of pyruvic acid and lactic acid, whether oxygen is available or not.  The enzyme pyruvate dehydrogenase complex is absent hence Pyruvic acid is not converted to RAPOPORT-LUEBERING SHUNT OR CYCLE This is the diversion of glycolytic pathway from 1, 3-BPG to produce 2,3- biphosphoglycerate (2,3-BPG. Biochemical Significance of Rapoport-luebering Shunt/ Cycle 1. Factors which waste energy are not present in RB Cells  Energy demanding endergonic reactions utilising ATP is not present in mature human red blood cells.  ATPase activity which controls ATP/ADP ratio is not active in mature RB Cells. - RB cells utilise more glucose than it requires to maintain cellular integrity, resulting in accumulation of ATP and ,3-BPG, causing cessation of glycolysis. - RLS or RLC provides a mechanism to dissipate the excess energy. (b) Role in Hb  Adult Hb-A1: 2,3-BPG concentration is high, affinity to oxygen is less and unloading/dissociation is more.  Hb-F: 2,3-BPG concentration is low, affinity to oxygen is more, and unloading/dissociation is less. Biochemical Significance of Rapoport-luebering Shunt/ Cycle 1. Factors which waste energy are not present in RB Cells  Energy demanding endergonic reactions utilising ATP is not present in mature human red blood cells.  ATPase activity which controls ATP/ADP ratio is not active in mature RB Cells. - RB cells utilise more glucose than it requires to maintain cellular integrity, resulting in accumulation of ATP and ,3-BPG, causing cessation of glycolysis. - RLS or RLC provides a mechanism to dissipate the excess energy. (b) Role in Hb  Adult Hb-A1: 2,3-BPG concentration is high, affinity to oxygen is less and unloading/dissociation is more.  Hb-F: 2,3-BPG concentration is low, affinity to oxygen is more, and unloading/dissociation is less. 3. Inherited enzyme deficiency: - The following hereditary defects in enzyme of red-cell glycolysis affect red cell BPG concentration i. hexokinase deficiency (rare) ii. pyruvate kinase deficiency (much more common) -In a patient with red cell hexokinase deficiency, there is a decrease in BPG concentration to about 2/3 of normal While in PK deficiency (pyruvate kinase) BPG is more than twice normal. As a result, affinity for oxygen of Hb is greater than normal in ‘hexokinase’ deficiency and less than normal in ‘pyruvate kinase’ deficiency.