Carbohydrates Metabolism Lecture Notes PDF

# Carbohydrates Metabolism ## Department of Biochemistry Faculty of Pharmacy, Suez Canal University ## Outline: - Definition of metabolism - Digestion of carbohydrates - Glycolysis - Gluconeogenesis - Krebs cycle - Glycogen synthesis and degradation - Hexose mono phosphate shunt - Metabolism of...

# Carbohydrates Metabolism ## Department of Biochemistry Faculty of Pharmacy, Suez Canal University ## Outline: - Definition of metabolism - Digestion of carbohydrates - Glycolysis - Gluconeogenesis - Krebs cycle - Glycogen synthesis and degradation - Hexose mono phosphate shunt - Metabolism of mono and disaccharides ## Definition of Metabolism - The term metabolism is commonly used to refer specifically to the breakdown of food and its transformation into energy. - The biochemical processes (pathways) that occur within a living organism in order to maintain life. - Hundreds of enzymes are organized into these pathways. - Metabolism consists of: Anabolism and catabolism ## Digestion of Carbohydrates ### 1. Digestion of carbohydrates in the mouth - Dietary polysaccharides are of animal origin (glycogen) and plant origin (starch and cellulose), in addition to sucrose (glu, fruc) and lactose (glu, galac). - Salivary α-amylase hydrolyzes some α-1,4 bonds in starch and glycogen. - Salivary α-amylase cannot hydrolyze α-1,6 bonds in amylopectin and glycogen. - β-amylase is not found in humans, therefore humans cannot digest cellulose. - Digest in mouth consists of branched oligosaccharides and disaccharides. ### 2. Digestion of carbohydrates in the stomach - No carbohydrate digestion occurs in the stomach due to high acidity (low pH) that inactivate amylase. ### 3. Digestion of carbohydrates in the small intestine - Pancreas secretes HCO3- to neutralize stomach HCL. - Pancreatic α-amylase continues digestion (complete digestion of branched oligosaccharides but disaccharides are still as they are). ### 4. Final digestion of carbohydrates in the upper jejunum - Enzymes found in intestinal mucosal cells of luminal sides of brush border complete digestion of disaccharides. - Enzymes are disaccharidases and include: - Isomaltase: cleaves α(1,6) bond of isomaltose. - Maltase: cleaves maltose: 2 glucose. - Sucrase: cleaves sucrose: glucose + fructose. - Lactase: cleaves lactose: glucose + galactose. - All dietary carbohydrates are reached to the simplest form (monosaccharaides) ## Absorption of Monosaccharaides in the Small Intestine - Most dietary sugars are absorbed in small intestine. ### Mechanisms of absorption: 1. Sodium-independent facilitated diffusion transport system. 2. Sodium-monosaccharide co-transport system. ### 1. Sodium-independent facilitated diffusion transport system. - Not need Na, transport occurs with conc. gradient. - Mediated by 14 glucose protein transporters (GLUT 1-14) in cell membrane of different tissues. - Tissue specificity for glucose transporter - GLUT-2 in liver, kidney & B-cells of pancreas, transport glucose, to these cells in case of high blood glucose (fed state) and transport glucose from these cells to blood when blood glucose is low (fasting), with conc. gradient in two cases. - GLUT-2 in intestinal mucosa transports galactose, glucose and fructose. - GLUT-5 transports fructose in small intestine and testes ### 2. Sodium-monosaccharide co-transport system. - Energy-requiring carrier mediated GLUT. - Transports glucose against conc. gradient (active transport) coupled with a concurrent uptake of Na into the same cell. - In intestine, both mechanisms are required along with GLUT-2 and 5. ## Abnormal Degradation of Disaccharides - Deficiencies in disaccharidases result in disaccharides (undigested) reaching the large intestine. - Water is drawn into large intestine causing osmotic diarrhea. - Bacterial fermentation of carbohydrate produces two- and three-carbon compounds, CO₂ and H₂ gas. - Abdominal cramps, diarrhea, flatulence. ### Diagnosis of specific enzyme deficiency (disaccharidases): 1. Oral tolerance test (patterns of symptomatic response, appearance time, height of rise of blood sugar). 2. Measurement of H2 gas in breath. ### Causes of digestive enzymes deficiencies (disaccharidases): 1. Hereditary deficiencies. 2. Temporary acquired enzyme deficiency due to intestinal diseases or certain drugs alter intestinal enzymes. ## Lactose Intolerance - Genetic lactase deficiency mostly Africans and Asian. - Patients cannot consume milk. - Patients must consume yogurt, cheese for adequate calcium intake. - Patients can take lactase-pills prior to eating. ## Isomaltase-Sucrase Deficiency - Intolerance to ingested sucrose due to deficiency of isomaltase-sucrase enzymes. - Treatment needs withholding dietary sucrose and enzyme-replacement therapy. ## Glycolysis - GLYCO LYSIS - GLUCOSE BREAKDOWN ## Overview of glycolysis {The Embden-Meyerhof-Parnas Pathway} - Break down of glucose and producing small amount of energy in the form of ATP. - Occurs in cytosol (part of cytoplasm) of all cells. - Ten reactions - same in all cells - but differ in rates (depending on the requirements of each cell). - Substrate is glucose - Products are pyruvate, ATP and NADH. ## Glycolysis is divided into two phases: - First phase (energy investment phase) - Second phase (energy generation phase) ## Reactions of glycolysis ### 1. Phosphorylation of glucose: - First step in glycolysis. - (hexokinase, found in all cells or Glucokinase, in the liver & ẞ-cells of pancreas). - Energy investment phase. ### 2. Isomerization of Glu-6-p: - by phosphoglucose isomerase, (Reversible step). ### 3. Phosphorylation of Fru-6-P: - by phosphofructokinase-1 (PFK-1). - Irreversible step (rate limiting step). - Energy investment phase. ### 4. Cleavage of Fru-1,6-bisP: By aldolase A - Reversible step. ### 5. Isomerization of dihydroxy-acetonephosphate: - By triose phosphate isomerase, reversible step. - The result is 2 molecule of glyceraldehyde-3-phosphate. ## Energy investment phase ## Reactions of glycolysis ### 6. Oxidation of glyceraldhyde-3-phosphate: - To 1,3-bisphosphoglycerate (1,3 BPG) by glyceraldehyde- 3-phosphate dehydrogenase. - Revisable step. - NADH + H is produced. ### 7. Formation of ATP from 1,3 BPG and ADP: - 1,3 BPG has high energy phosphate group used in synthesis of ATP in a reaction catalyzed by phosphoglycerate kinase, reversible step. - In this reaction 2 ATP is formed and 3-PG. - Substrate-level phosphorylation. - Energy generation phase. ### 8. Shift of the phosphate group from C-3 to C-2: - By mutase to form 2-PG. - Reversible reaction. ### 9. Dehydration of 2 PG: - By enolase to form phosphoenolpyruvate (PEP) which is high energy phosphate group. - Reversible step. ### 10. Formation of pyruvate: - PEP → Pyr (end product of aerobic glycolysis) by Pyruvate kinase. - Irreversible step (rate limiting step). - Substrate-level phosphorylation. - Energy generation phase. ## Energy generation phase ## Reactions of glycolysis ### 11.Reduction of Pyruvate to lactate: - By lactate dehydrogenase, it is the end product of anaerobic glycolysis. - Reversible step. ## Regulation of glycolysis ### 1. Hexokinase, Glucokinase | | Hexokinase | Glucokinase | |---|---|---| | **Function** | Add PO4 group on all hexose sugars (broad specificity) | Add PO4 group on glucose only | | **Location** | Found in most tissues | Found in liver & β-cells of pancreas | | **Km & Vmax** | Low Km & Vmax, Not induced by insulin | High Km &Vmax, Require high concentration of glucose, Activity induced by insulin (well fed state) | | **Inhibition** | Inhibited by Glu-6-P (product) | Inhibited by Fru-6-P and NOT by Glu-6-P | ### 2. PFK-1 - It is regulated by: - Energy level: Inhibited by ATP and activated by AMP. - Compounds: Inhibited by citrate and activated by Fru 2,6 bisp (potent activator even in the presence of high energy level). ### PFK-1: Effect of Fru 2,6 bisP on PFK-1 - High insulin/glucagon ratio causes decreased cAMP and reduced levels of active protein kinase A. - Decreased protein kinase A activity favors dephosphorylation of PFK-2/FBP-2 complex. - Elevated concentration of fructose 2,6-bisphosphate activates PFK-1, which leads to an increased rate of glycolysis. - Dephosphorylated PFK-2 is active, whereas FBP-2 is inactive; this favors formation of fructose 2,6-bisphosphate. ### 3. Pyruvate kinase - It is regulated by: - Energy level: Inhibited by ATP and activated by AMP. - Compounds: Inhibited by acetyl CoA and activated by Fru 1,6 bisp (feed forward regulation). - Covalent modification: inhibit the enzyme as it will be present in the inactive form (phosphorylated form). ### Pyruvate kinase Effect of covalent modification: - Glucagon activates protein kinase A which causes phosphorylation of Pyruvate kinase, inhibiting it. ### Hormonal Regulation of Glycolysis - From all of the previous figures - Insulin activates glycolysis (well fed state) and - Glucagon inhibits glycolysis (fasting state) by affecting the synthesis of the regulatory key enzymes. ## The Fate of NADH and Pyruvate, Aerobic or Anaerobic: - NADH is energy - two possible fates: - If O₂ is available, NADH is re-oxidized in the electron transport pathway, making ATP in oxidative phosphorylation. - In anaerobic conditions, NADH is re-oxidized by lactate dehydrogenase (LDH). - Pyruvate is also energy - two possible fates: - Aerobic: citric acid cycle. - Anaerobic: LDH makes lactate, major fate of pyruvate in RBCs, lens, cornea of the eye, kidney medulla, testes & leukocytes (tissues with no or low mitochondria) and in exercising skeletal muscle (O₂ is limited). ## Energy yield from Glycolysis: - Aerobic: 8 ATP (2 NADH+2 ATP) - Anaerobic: 2 ATP ## Pyruvate Kinase Deficiency - Mature erythrocytes lack mitochondria & depend on glycolysis for energy production. ATP is required to maintain the flexible shape of RBCs which allow them to squeeze through narrow capillaries. - Deficiencies in pyruvate kinase decrease in glycolysis and ATP production - RBCs lysis hemolytic anemia. ## Lactic Acidosis - Elevated conc. of lactate in plasma. - Occur in: collapsed circulation in case of myocardial infarction, pulmonary embolism & shock. - Failure to bring O₂ to tissues impaired oxidative phosphorylation and decreased in ATP synthesis. - So NADH will be utilized by LDH for production of lactate. - Lactic acid accumulates, diffused to the plasma, decrease pH leading to lactic acidosis ## Thanks for Your Attention