Lecture Notes on Carbohydrate Metabolism - MED 302 PDF

LECTURE NOTES ON : MED 302 Metabolism of Carbohydrates Ajilore B.S. (MBChB, PhD) Dept. of Medical Biochemistry, College of Health Sciences, Osun State University, Osogbo Overall Objectives:  By the end of this course, you should: 1) understand how carbohydrate metabolism normally respo...

LECTURE NOTES ON : MED 302 Metabolism of Carbohydrates Ajilore B.S. (MBChB, PhD) Dept. of Medical Biochemistry, College of Health Sciences, Osun State University, Osogbo 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 tisuue. - 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 REGULATION OF BLOOD GLUCOSE (Homeostasis)  Blood glucose level is maintained within physiological limits 60 to 100 mg% (“true” glucose) in fasting state and 100 to 140 mg % following ingestion of a carbohydrate containing meal, by a balance between two sets of factors: A. Rate of glucose entrance into the blood stream, and B. Rate of its removal from the blood stream. Rate of supply of glucose to blood: - Except for a possible minor contribution by the kidney, which probably does not occur under physiological conditions, the blood glucose may be derived directly from the following sources: i. By absorption from the intestine ii. Breakdown of glycogen of Liver (Hepatic glycogenolysis) iii. By gluconeogenesis in Liver, source being glucogenic amino acids, lactate and pyruvate, glycerol and propionyl-CoA iv. Glucose obtained from other carbohydrates, e.g. fructose, galactose, etc. Rate of removal of glucose from blood i. Oxidation of glucose by the tissues to supply energy ii. Glycogen formation from glucose in Liver (Hepatic glycogenesis) iii. Glycogen formation from glucose in muscles (Muscle glycogenesis) iv. Conversion of glucose to fats (lipogenesis) especially in adipose tissue v. Formation of ribose sugars from glucose required for nucleic acid synthesis. vi. Excretion of glucose in urine (glycosuria), when blood glucose level exceeds the renal threshold. Fasting (Post absorptive) State: - Approx. 12 to 14 hours after last meal. - There is practically no intestinal absorption. - It is the condition of a subject between 8 to 10 A.M, if he had his dinner previous evening about 8 P.M and had taken nothing thereafter. - Under such a situation only source of glucose is liver glycogen. - Muscle glycogen cannot provide blood glucose by glycogenolysis due to lack of the enzyme Glucose-6- phosphatase. Post-prandial State: - Condition following ingestion of food - Absorbed monosaccharides are utilised for oxidation to provide energy. - Remaining in excess is stored as glycogen in liver and muscles. - When load of glucose is very high renal mechanisms operate. - When blood glucose rises more than 160 to 180 mg% i.e. the renal threshold, glucose appears in urine (glycosuria). This is an abnormal state. - In normal intestinal absorption such situation does not occur. - It can take place with an IV load or disease processes AUTOREGULATION (Fundamental/ Central Regulatory Mechanisms)  Process of hepatic glycogenesis, glycogenolysis and tissue utilisation of glucose are sensitive to relatively slight deviation from the normal blood sugar concentration. 1. As blood sugar tends to increase ↑:  Glycogenesis is accelerated, and  Utilisation of glucose by tissues is increased, resulting to fall in blood glucose level. - The reverse occurs as the blood glucose level tends to fall↓. - Circulating blood glucose is approximately 80 mg % (4.4 mmol/L). - Balance between production and utilization of blood glucose depends on insulin in one hand, and hormones of adrenal cortex and anterior pituitary on the other hand. - Overall effect of Insulin is to lower the blood glucose level while adrenocortical/ and growth hormone to raise it.  Therefore, as blood sugar tends to rise, there is simultaneous increase in insulin secretion which leads to increase in ratio of insulin/glucocorticoids and GH.  This results in: i. Increased hepatic glycogenesis↑ ii. Decreased gluconeogenesis↓ iii. Decreased output of glucose from Liver↓ and iv. Increased utilisation of glucose↑.  As a result of above, the blood glucose concentration tends to fall. 2. As blood sugar tends to decrease ↓: - A drop in blood glucose concentration below the normal resting level causes:  Decrease in secretion of insulin, ↓  Resulting to decrease in ratio of insulin/glucocorticoids and GH, ↓  Increased production of blood glucose mainly by gluconeogenesis ↑, and  If the blood glucose falls below to hypoglycaemic levels, additional emergency mechanisms come into play: a. Stimulation of secretion of catecholamines by  hypoglycaemia resulting in hepatic glycogenolysis and rise in blood glucose. - The increase in catecholamines may also b. Stimulate production of ACTH, hence of adrenocortical hormones  causing increased gluconeogenesis.  The blood glucose concentration in normal health regulates itself by the normal responsiveness of the pancreas to variation in blood glucose concentrations.This constitutes the “autoregulation” or central regulatory mechanism. HORMONAL / ENDOCRINE INFLUENCES ON CARBOHYDRATE METABOLISM - There are 2 categories of endocrine influences: A. Those which influence carbohydrate metabolism and exert a fundamental regulatory influence. Examples are insulin and hormones of adrenal cortex and anterior pituitary. B. Those which influence carbohydrate metabolism, but are not essential for its autoregulation under normal physiological conditions, e.g. hormones of adrenal medulla and hormones of thyroid gland. 1. Insulin  Administration of insulin is followed by a fall ↓ in blood glucose concentration to hypoglycaemic levels. 2. Adrenocortical hormones - The predominant glucocorticoids in man is cortisol. - Increases blood glucose level by gluconeogenesis 3. Anterior Pituitary Gland  Secretes hormones that tend to elevate the blood glucose level and therefore, antagonise the effect of insulin. These are growth hormone and ACTH (corticotropin) 4. Catecholamines  These are hormones produced by adrenal medulla: Epinephrine & Non epinephrine  Produce an increase in blood glucose level and also blood Lactic acid level↑.  It stimulates glycogen breakdown (glycogenolysis) in Liver as well as in muscle and is accompanied by a decrease in glycogen content. 5. Glucagon - In response to hypoglycaemia, α-cells produce glucagon which produces rapid glycogenolysis in Liver. - Glucagon cannot produce glycogenolysis in muscle as it lacks the receptor - Glucagon also enhances “gluconeogenesis” from amino acids, pyruvates and lactates. BLOOD SUGAR LEVEL AND ITS CLINICAL SIGNIFICANCE I. Normal values: The range for normal fasting or Postabsorptive blood glucose taken at least three hours after the last meal is 60 to 100 mg% II. Abnormal values - Hyperglycemia: increase above normal value - Hypoglycemia: decrease below normal value Causes of Hyperglycemia i. Diabetes ii. Hyperactivity of the thyroids (Hyperthyroidism), pituitary (Hyperpituitarism) and adrenal iii. Emotional stress iv. Disease of the pancreas: pancreatitis and carcinoma v. Sepsis vi. Brain diseases: meningitis, encephalitis, intracranial tumors and hemorrhage vii. Asphysia Causes of Hypoglycemia i. Overdose of insulin in treatment of DM ii. Insulin-secreting tumor (Insulinoma) of pancreas iii. Hypoactivity of thyroids (Hypothyroidism): e.g. in cretinism iv. Hypopituitarism: e.g. in Simmond’s disease v. Hypoadrenalism: e.g. in Addisson’s disease vi. Severe liver diseases vii. Deficincy of of glucagon production in children: spontaneous hypoglycemia viii. Severe exercise: due to depletion of liver glycogen ix. Glycogen storage diseases: e.g. Von Gierke’s disease x. Alcohol ingestion Glycosuria - Under ordinary dietary conditions, glucose is the only sugar present in the free state in blood plasma in demonstrable amounts. - Although normal urine contains virtually no sugar - Under certain conditions, glucose and other sugars may be excreted in the urine. This condition is called melituria (excretion of sugar in urine) - The terms glycosuria, fructosuria, galactosuria, lactosuria and pentosuria are applied specially to the urinary excretion of glucose, fructose, galactose, lactose, and pentose respectively. - Glucose is present in the glomerular filtrate in the same concentration as in the blood plasma. - Under normal conditions, it undergoes practically complete reabsorption by the renal tubular epithelial cells and is returned to the blood stream. - In normal subjects, a very small amount less than 0.5 gm of glucose may escape reabsorption by tubules and be excreted by urine. - But this amount is not detected by Benedict’s qualitative test. - Rate of glucose absorption is expressed as TmG (tubular maximum for glucose) which is 350 mg/mt. - When the blood levels of glucose are elevated, the glomerular filtrate may contain more glucose than can be reabsorbed, the excess passes in urine to produce “glycosuria”. - In normal individuals, glycosuria occurs when the venous blood glucose exceeds 170 to 180 mg/100 ml. This level of the venous blood glucose is termed as the renal threshold for glucose. Glycosuria is defined as the excretion of glucose in urine which is detectable by Benedict’s Qualitative test. DIABETES MELLITUS - DM is a chronic endocrine disorder characterized by elevated levels of glucose in the blood as a result of absolute or relative insulin deficiency. - In DM insulin may be insufficient or not properly utilized. CLINICAL TYPES AND CAUSES - These are two main groups: (a) Primary : constitute major group. - Exact cause is not known - metabolic defect is insufficient insulin which may be absolute or relative. (b) Secondary: constitute minor group - it can be secondary to some disease process. Primary  Two clinical types: i. “Juvenile”-onset diabetes: Now called as Type-I DM(Insulin dependent) IDDM. ii. “Maturity” onset diabetes: Type-II DM :NIDDM— (Non- Insulin Dependent). CAUSES: I. Hereditary II. Autoimmunity: Type I III. Infections IV. Obesity: Type II V. Diet VI. Insulin resistance: Type II Secondary - This forms a minor group. Diabetes is secondary to some other diseases: I. Pancreatic diabetes:  Pancreatitis  Haemochromatosis  Malignancy of Pancreas. II. Abnormal concentrations of insulin-antagonistic hormones:  Hyperthyroidism  Hypercorticism: like Cushing’s disease III. Iatrogenic: - In genetically susceptibles, DM may be precipitated by therapy like - corticosteroids, thiazide, diuretics. In experimental animals, DM is induced by chemicals like streptozotocin, alloxan CLINICAL FEATURES AND BIOCHEMICAL CORRELATIONS 1. Polyuria - Large amounts of glucose may be excreted in urine (may be 90 to 100 G/day in some cases). Loss of solute produces osmotic diuresis thus large volume of urine. 2. Loss of fluid leads to thirst and polydypsia. 3. Polyphagia: Eats more frequently. 4. Tissues including muscles received liberal supply of glucose but cannot use glucose due to absolute or relative deficiency of insulin/ or transport defect to cells. This causes weakness and tiredness. 5. As glucose cannot be used for fuel, fat is mobilised leading to increase FFA- in blood and liver. 6. Increased acetyl-CoA is diverted for cholesterol synthesis: Hypercholesterolaemia and atherosclerosis. 7. Increased ketone bodies leads to acidosis, which leads to hyperventilation (“air-hunger”). 8. If ketosis is severe, acetone will be breathed out, giving characteristic “fruity” smell in breath (due to acetone). 9. Along with above, there may be excessive breakdown of tissue proteins. Deaminated amino acids are catabolised to provide energy, which accounts for Loss of weight. 10. Due to ketosis, develops anorexia, nausea, and vomiting. Continued loss of water and electrolytes increases dehydration. 11. Ketoacidosisproduces increasing drowsiness, leading to diabetic coma in untreated cases METABOLIC CHANGES IN DIABETES MELLITUS 1. Hyperglycaemia: Occurs as a result of: i. Decreased and impaired transport and uptake of glucose into muscles and adipose tissues. ii. Repression of key glycolytic enzymes like Glucokinase, phosphofructokinase and pyruvate kinase takes place. iii. Derepression of key gluconeogenic enzymes like Pyruvate carboxylase, phosphoenol pyruvate carboxykinase, fructose biphosphatase and glucose-6-phosphatase occur, promoting gluconeogenesis in Liver. This further contributes to hyperglycaemia. iv. Elevated amino acid level in the blood particularly alanine provides fuel for gluconeogenesis in Liver. 2. Amino Acids Level i. Transport and uptake of amino acids in peripheral tissues is also depressed causing an elevated circulating level of amino acids, particularly alanine. Glucocorticoid activity predominate having catabolic action on peripheral tissue proteins, releasing more amino acids in blood. ii. Amino acids breakdown in Liver results in increased production of urea N ↑. 3. Protein synthesis: Protein synthesis is decreased in all tissues due to: iii. Decreased production of ATP↓ iv. Absolute or relative deficiency of Insulin. 4. Fat Metabolism v. Decrease extramitochondrial de Novo synthesis of FA and also TG synthesis due to decrease in acetyl-CoA from carbohydrates, ATP, NADPH and α-glycero-(p) in all tissues. vi. Stored lipids are hydrolysed by increased Lipolysis liberating free fatty acids (FFA)↑. Increased FFA interferes at several 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.