CHO Metabolism PDF

CHO Metabolism Prof.Dr. Wajdy J. Majid MSc ,Ph.D. clinical biochemistry College of Medicine University of Thi-Qar  Learning objectives :  Describe the digestion and absorption of dietary CHO.  Explain glucose metabolism (glycolysis ).  Explain the reactions of citric acid...

CHO Metabolism Prof.Dr. Wajdy J. Majid MSc ,Ph.D. clinical biochemistry College of Medicine University of Thi-Qar  Learning objectives :  Describe the digestion and absorption of dietary CHO.  Explain glucose metabolism (glycolysis ).  Explain the reactions of citric acid cycle (importance).  Explain gluconeogenesis metabolism (importance).  Explain glycogen metabolism (importance).  Explain the metabolic importance of pentose phosphate pathway.  Study the metabolic disorders in CHO metabolism. Digestion of dietary CHO  The resultant glucose and other simple CHO are absorbed by S.I. and transport to the liver and other tissue , they are either converted to fatty acid , amino acid and glycogen, or else oxidized by catabolic pathways. The oxidation of glucose called glycolysis , glucose is oxidized either to pyruvate or l Lactate.  Under aerobic condition the dominant product in most tissue is pyruvate and the pathway is called aerobic glycolysis, when oxygen is depleted ex: during prolong vigorous exercise the dominant product of glycolysis in many tissue is lactate and the process is called anaerobic glycolysis.  Site of Glycolysis.Enzymes of glycolysis are present in the cytosol of most of the cells  Entry of the Glucose in to the Cells Entry of the Glucose in to the Cells.Glucose enters cells by facilitated transport.Liver Glucose enters liver cells by facilitated diffusion.1. It is an insulin-independent transport mechanism Extra hepatic tissues Glucose enters adipocytes, erythrocytes, brain and -2.skeletal muscle by facilitated transport involving carrier molecule The transport of glucose across the membranes of adipose tissue and skeletal muscle by carrier is dependent on insulin By GLUT4 GLUT3 present in the brain with( low Km = 1.6 mmol/L), this allows relatively constant rate of glucose uptake (independent of extracellular conc.) GLUT4 present in muscle and adipose tissue (Km= 5 mmol/L) which is “insuline regulatable” glucose GLUT2 present in (liver, kidney, intestine and pancreatic β-cell) with ( high Km = 7 - 20 mmol/L), this allows glucose equilibrium across the membrane Absorption of carbohydrates in the intestine (Na+, sodium ion; K+, potassium ion ;ATPase, adenosine triphosphatase)  Glycolysis  Learning objectives :  List the reactions and enzymes involved in glycolysis.  List the irreversible and regulated steps of glycolysis.  Discus the regulation of glycolysis.  Discus the outcome of glycolysis Reactions of Glycolysis  Initial reaction of glycolysis is catalyzed by hexokinase. It is widely distributed , It phosphorylates glucose at 6 carbon in presence of Mg2+ and ATP.  Hexokinase is an allosteric enzyme , the reaction catalyzed by this enzyme is irreversible under normal physiological conditions.  One ATP is used in this reaction to generate glucose-6-phosphate.  Liver contains glucokinase, which phosporylates only glucose , It is an inducible enzyme, Its Km for glucose is high compared to Km of hexokinase. Hence, it phosphorylates glucose only when blood glucose concentration is high.  Energetics of Glycolysis  Degradation of glucose to two molecules of pyruvate or lactate by sequence of enzyme catalyzed reactions constitutes the process of glycolysis. It is a catabolic pathway. If glucose is degraded to pyruvate then it is called as aerobic glycolysis. Usually it occurs in presence of oxygen. If glucose is degraded to lactate then it is anaerobic glycolysis , usually it occurs in the absence of oxygen. Generation and consumption of ATP in anaerobic and aerobic glycolysis is below : In aerobic glycolysis : 1. Number of ATPs generated by phosphoglycerate kinase 2 2. Number of ATPs generated by Pyruvate kinase 2 3. Number of ATPs generated by respiratory chain oxidation of 2 NADH produced in reaction 5 4. Number of ATPs consumed in reaction 1 and 3 –2 Net result = 7  In anaerobic glycolysis : 2 NADH produced in reaction 5 are used to convert pyruvate to lactate. Thus, oxidation of glucose to pyruvate (aerobic glycolysis) generates 7 ATP molecules whereas oxidation of glucose to lactate (anaerobic glycolysis) generates 2 ATP molecules. NADH is used for conversion of pyruvate to lactate. This results in regeneration of nicotinamide adenine dinucleotide (NAD+), which is required for continuation of glycolysis. up—hence, net yield is 2 ATPs.  Aerobic glycolysis generate substantially more ATP per mole of glucose oxidized than does anaerobic glycolysis.  The utility of anaerobic glycolysis to muscle cell when it needs large amount of energy during muscle contraction since that the rate of ATP production from anaerobic glycolysis is approximately 100 faster than from oxidative phosphorylation, during excersion muscle cells do not need to energize anabolic reaction pathway, the requirement is to generate the maximum amount of ATP for muscle contraction in the shortest time frame, this is why muscle cells derive almost all of ATP consumed during exertion from anaerobic glycolysis.  Regulation of glycolysis : Key enzyme Activated by (+) Inhibited by (–) Hexokinase Insulin, Glucagon, Glucokinase AMP*, cAMP†, ATP, Phosphofructokinase NAD+, NADH + H+, Pyruvate kinase CoASH Acetyl-CoA, Metabolic fates of pyruvate : - Pyruvate is the branch point molecule of glycolysis. - The fate of pyruvate depends on the oxidation state of the cell, - The fate of pyruvate during anaerobic glycolysis is reduction to lactate, or pyruvate enter the TCA in the form of acetyl-CoA by pyruvate dehydrogenase reaction with generation of NADH molecule to complete aerobic oxidation of pyruvate. Significance of Glycolysis It is metabolic pathway through which glucose generate cellular energy in form of ATP. Operates in both aerobic and anaerobic conditions Lactate generated in anaerobic glycolysis is used for gluconeogenesis in the liver Anaerobic glycolysis is a major pathway that provides energy for skeletal muscles during strenuous exercise Anaerobic oxidation of glucose (glycolysis) is the only source of energy for mature red blood cells (RBCs). The citric acid cycle ( catabolism of acetyl-CoA) : Learning objectives : Describe the reactions of the citric acid cycle. Describe the function of the citric acid cycle and identify the products produced. Describe the role of oxidative phosphorylation in energy metabolism.  The citric acid cycle (Krebs cycle, tricarboxylic acid cycle) is a series of reactions in the mitochondria that oxidize acetyl CoA to formation of ATP. Site : All cells with mitochondria (except RBC) Subcellular site : Mitochondria Starting material : Acetyl-coenzyme A + Oxaloacetate End product : CO2 + H2O + ATP  The TCA cycle is the final common pathway for aerobic oxidation of CHO , lipid and protein because glucose , fatty acid and most amino acid are metabolized to acetyl CoA or intermediate of the cycle.  Acetyl CoA react with oxaloacetate to form citrate by a series of dehydrogenation and decarboxylation , citrate is degraded , releasing reduced co-enzyme and CO2 and regenerating oxaloacetate. Generation of ATP in Citric Acid Cycle 1. Number of ATP generated by oxidation of 3 NADH 7.5 2. Number of ATP generated by oxidation of FADH2 1.5 3. Number of ATP generated from GTP 1 Total 10  Glycogen metabolism :  Learning objectives :  Describe composition and gylcosidic bonds in glycogen.  Describe the biochemical pathway for glycogen synthesis and glycogenolysis.  Discus the regulation of glycogen metabolism.  Discus the abnormality of glycogen metabolism. (glycogen storage disease)  Glycogen metabolism :  Glycogen is a highly branched polysaccharide composed of D- glucose unit joined to each other by glycosidic bond, the major linkage are α-1,4 glycosidic bond , at interval of about eight to ten units , there are branches in the chain involving α-1,6 linkage , each branch then continue with α-1,4 linkage.  The glycogen is the store of excess glucose to supply the tissue with an oxidizable energy source are found principally in the liver as glycogen.  The second major source of stored glucose is the glycogen of skeletal muscles. Muscle glycogen is not generally available to other tissue because muscle lack the enzyme glucose-6- phosphatase.  Glycogen is considered the principal storage form of glucose and found mainly in the liver and muscle , with kidney and intestine adding minor storage sites.  Stores of glycogen in the liver are consider the main buffer of blood glucose levels.(contain G6Pase)enzyme. GLYCOGENESIS Glycogenesis is the synthesis of glycogen from glucose. Site : glycogen it chiefly occurs in liver and skeletal muscle. In the muscle, about 245 gms of glycogen and in the liver about 72 gms of glycogen is stored under well fed condition. The rate of( glycogensis) may be increased by insulin which is secreted by β-cells of the pancreas in response to systemic hyperglycemia and stored as glycogen in the liver and also the excess glucose enter the muscle under influence of insulin and stored as glycogen.  Glycogenolysis :  Degredation of stored glycogen ( glycogenolysis) occur by the action of glycogen phosphorylase , the phosphorylase is remove single glucose residue from α(1,4)-linkage within the glycogen molecules.  The product of this reaction is glucose -1-phosphate , which converted to G6P by phosphoglucomutase the conversion of G6P to glucose which occur in the liver , kidney and intestine by the action of enzyme glucose-6-phosphatase which does not occur in skeletal muscle as these cells because lack this enzyme.  Therefore any glucose released from glycogen stores of muscle will be oxidized in the glycolytic pathway. In the liver the action of G-6-phosphatase allow glycogenolysis to generate free glucose for maintaining blood glucose levels.  Glycogen phosphorylase can not remove glucose residue from the branch points (α-1,6 linkage) in glycogen. the activity of phosphorylase cease 4 glucose residue from the branch point.  The removal of these branch point glucose residue require the action of debranching enzyme ( also called glucan transferase) which contain 2 activities :glucotransferase and glucosidase , the transferase activity remove the terminal 3- glucose residue of one branch and attaches them to a free C-4 end of a second branch.  The glucose in α-(1,6)-linkage at the branch is then removed by action of glucosidase.  Hormonal Regulation of Glycogen Metabolism  Epinephrine and glucagon increases cAMP mediated phosphorylation, which in turn converts inactive glycogen phosphorylase B to active phosphorylase A , as a result glycogenolysis is enhanced. At the same time cAMP mediated phosphorylation converts active glycogen synthase A to inactive glycogen synthase B which results in decreased glycogenesis.  Insulin: decreases glycogenolysis by decreasing cAMP mediated phosphorylation. At the same time insulin favours dephosphorylation of glycogen synthase B , which results in formation of more glycogen synthase A and increased glycogenesis. Medical Importance of Hormonal Regulation of Glycogen Metabolism In between meals hypoglycemia induces glucagon production , Glucagon causes breakdown of glycogen in liver to maintain supply of glucose to brain and cardiac muscle. Epinephrine causes breakdown of glycogen in skeletal muscle to maintain fuel supply for muscle contraction. After a meal, hyperglycemia induces insulin secretion. Insulin causes inactivation of enzymes of glycogenolysis and activation of glycogen forming enzymes. As a result glycogenesis occurs in liver and muscle.  Glycogen Storage Diseases : These are group of inherited (genetic) diseases of glycogen metabolism. In these diseases, there is an abnormal accumulation of large amount of glycogen or its metabolites in the tissues due to deficiency or absence of enzymes of glycogen metabolism. Some of them are not serious mild disorders but few of them are fatal.  Disorder Enzyme defect Clinical features Von Gierke (type I) Glucose-6-phosphatase Hepatomegaly, fasting hypoglycemia, lactic acidosis, hyperuricemia, ketosis Pompe's disease (type II) 1,4-glucosidase Cardiomegaly and early death Cori's disease (type III) Debranching enzyme Hepatomegaly, hypoglycemia Anderson's disease (type IV) Branching enzyme Hepatomegaly,cirrhosis of liver, early death due to liver and heart failure McArdle's disease (type V) Muscle phosphorylase Muscle glycogen is high, exercise intolerance Hers' disease (type VI) Liver phosphorylase Hepatomegaly, hypoglycemia.  Gluconeogenesis Learning objectives :  Definition.  List the precursors for gluconeogenesis  Discus the regulation of gluconeogenesis with medical importance.  Gluconeogensis : It is the biosynthesis of new glucose ( not glucose from glycogen) , Gluconeogenesis is the formation of glucose from non- carbohydrate precursors. Site : Liver, kidneys, intestine Subcellular site: Mitochondria and cytosol Starting material: Lactate, pyruvate, glucogenic amino acids, glycerol, propionyl-CoA End product: Glucose Coenzymes: Guanosine triphosphate (GTP), NADH, NAD+ Energy requirement: To generate one molecule of glucose, 6 ATPs are required.  Substrate for gluconeogensis : lactate : is a predominant source for glucose synthesis by gluconeogensis. during anaerobic glycolysis in skeletal muscle , pyruvate is reduced to lactate by lactate dehydrogenase (LDH) this reduction serves two critical function during anaerobic glycolysis first , in the direction of lactate formation the LDH reaction require NADH and yield NAD which then available for use by glyceraldehydes -3- phosphate dehydrogenase reaction of glycolysis.secondly , the lactate produced by the LDH reaction is released to blood stream and transport to the liver where it is converted to glucose , the glucose then returned to the blood for use by muscle as an energy source and to replenish glycogen stores. This cycle is termed the Cori cycle. pyruvate : pyruvate generated in muscle and other peripheral tissue can transaminated to alanine which is returned to the liver for gluconeogensis. Within the liver alanine is converted back to pyruvate and used as substrate for gluconeogensis (if that is hepatic requirement ), or oxidized in TCA cycle. Amino acids : All 20 of the amino acid except leucine and lysine can degraded to TCA cycle intermediate. This allow the amino acid to be converted to those in oxaloacetate and then into pyruvate , the pyruvate can be utilized by gluconeogensis. When glycogen store are depleted in muscle during exertion and in liver during fasting , so catabolism of muscle protein to amino acid contribute the major source of carbon for maintenance of blood glucose levels. glycerol : In the liver, glycerol is converted to dihydroxyacetone phosphate which enters pathway of gluconeogenesis , the glycerol backbone of lipid can be used for gluconeogensis, this require phosphorylation to glycerol 3-phosphate by glycerol kinase and dehydrogenation to (DHAP) by glyceraldehydes-3- phosphate dehydrogenase (G3PDH). so the triacylglycerol which stored in adipose tissue can be used its glycerol as substrate for gluconeogensis. propionate : Oxidation of fatty acid with an odd number of carbon atoms and oxidation of some amino acid generate as the terminal oxidation product (propionyl-CoA) ,propionyl –CoA is converted to the TCA intermediate , (succinyl-CoA) this conversion is carried out by the ATP-requiring enzyme, propionyl-CoA carboxylase. Medical and Biological Importance of gluconeogenesis : 1. Gluconeogenesis meets the glucose requirement of body when carbohydrate is in short supply i.e., during fasting and starvation. 2.Tissues like brain, erythrocytes and testis are completely depend on glucose for energy and hence decrease in glucose supply cause brain dysfunction. Body glycogen can meet glucose requirement for only 24 hours so, beyond that period gluconeogenesis ensures glucose supply to these organs. 3. Gluconeogenesis clears metabolic products of other tissues from blood , for example, lactate produced by erythrocytes, skeletal muscle, glycerol produced by breakdown of adipose tissue TG and a. a. produced by muscle protein breakdown. 4. Gluconeogenesis converts excess of dietary glucogenic amino acids into glucose. 5. Gluconeogenesis is impaired in alcoholics. Regulation of Gluconeogenesis Enzymes of gluconeogenesis are subjected to allosteric regulation and hormone regulation. Pyruvte carboxylase and fructose-1, 6-bisphosphatase regulates gluconeogenesis. Allosteric regulation Pyruvate carboxylase is an allosteric enzyme. Acetyl-CoA is its activator. When glucose is in short supply fatty acid oxidation generates acetyl-CoA this in turn activates gluconeogenesis. Fructose-1, 6- bisphosphatase is another allosteric enzyme. AMP is its allosteric inhibitor. So when there is energy crisis gluconeogenesis is inhibited. Hormonal regulation Insulin decreases the synthesis of key enzymes of gluconeogenesis thus inhibit gluconeogenesis. Regulation of blood glucose level : Because of the demands of the brain for oxidizable glucose, that the human body regulate the level of glucose circulating in the blood , this level maintained in the range of 5 mm/L. Nearly all CHO ingested in the diet are converted to glucose following transport to the liver , catabolism of dietary or cellular protein can be utilized for glucose synthesis via gluconeogensis, additionally other tissue besides the liver that incompletely oxidize glucose ( predominantly skeletal muscle and erythrocyte) provide lactate that can be converted to glucose via gluconeogensis. Maintenance of blood glucose homeostasis is very important for survival of human organism. The hormones concerned with glucose homeostasis are : Insulin : Is the most important hormone controlling the plasma glucose concentration , it is secreted by β-cell of pancreas , these cells produce proinsulin, which consists of the 51-amino-acid polypeptide insulin and a linking peptide ( C-peptide) , then released into plasma mainly in response to rising plasma glucose level. Insulin bind to specific receptors on the surface of insulin- sensitive cells of adipose tissue and muscles, the most important effect is stimulation of glucose entry into these cells with resultant decrease in plasma level , also insulin promote glycogen synthesis in the liver and in the muscle, also it stimulate fat synthesis in adipose tissue and protein synthesis in the muscle, but it inhibit gluconeogensis , lipolysis and proteolysis. The normal response to hyperglycemia there for depend on : 1- adequate insulin secretions. 2- Adequate insulin receptors. 3- Normal intracellular response to receptors binding of insulin ( post receptors events). Glucagon : Glucagon is a single-chain polypeptide , synthesized by α-cell of pancreas and it secretion stimulated by hypoglycemia and fasting , glucagons stimulate hepatic glycogenolysis by activating glycogen phosphorylase and stimulate gluconeogensis. Growth hormone : Released from anterior pituitary gland and act to increase blood glucose by inhibiting uptake of glucose by extrahepatic tissue specially in muscle, and increase lipolysis , but it increase synthesis of muscle protein , it is secretion stimulated by hypoglycemia, stress and sleep. Glucocorticoid : It act to increase blood glucose level by inhibiting glucose uptake in the muscles , cortisol the major glucocorticoid released from adrenal cortex, is secreted in response to the increase in circulating ACTH , glucocorticoid increase gluconeogensis , increase protein breakdown in the muscles and increase lipolysis in adipose tissue. It is secretion stimulated by hypoglycemia and stress. Adrenalin : It is secreted from adrenal medulla it stimulate production of glucose by activating glycogenolysis in the liver and muscle by activating phosporylase enzyme in response to stressful stimuli , adrenalin also stimulate lipolysis.  Pentose phosphate pathway : Learning objectives :  Definition and functions of PPP.  Outline the two majors phases of PPP  Discus the medical importance.  Pentose phosphate pathway : The pentose phosphate pathway is primarily an anabolic pathway that utilize the 6 carbons of glucose to generate 5 carbon sugars and reducing equivalents. Primary function of this pathway are : 1-To generate reducing equivalents in the form of NADPH for reductive biosynthesis reaction within the cells. 2-To provide the cell with ribose-5-phosphate (R5P) for the synthesis of nucleotides and nucleic acids. 3-It can operate to metabolize dietary pentose sugars derived from the digestion of nucleic acids as well as to rearrange the carbon skeletons of dietary CHO into glycolytic and gluconeogenic intermediates. Site : Enzymes of this pathway are present in cytosol of liver, adipose tissue, erythrocytes, adrenal cortex, thyroid, testis, ovaries and lactating mammary gland. In the skeletal muscle the pathway is less active. The reaction of F.A and steroid biosynthesis utilize large amount of NADPH , erythrocyte utilize the reaction of PPP to generate large amount of NADPH. Reactions of hexose monophosphate shunt : The reaction of PPP take place in the cytoplasm , the PPP has both oxidative and non oxidative arm.  The oxidation steps , utilizing glucose-6-phosphate (G6P) as the substrate occur at the beginning of pathway and the reaction that generate NADPH. the reaction catalyzed by glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase generate one mole of NADPH each for every mole of glucose-6-phosphate that enter the PPP.  The non oxidative reactions of PPP are primarily designed to generate R-5-P.  Also the important reaction of PPP are to convert dietary 5 carbon sugars into both (fructose-6-phosphate) and (glceraldehyde-3-phosphate) which can then be utilized by pathway of glycolysis. Medical Importance of Pentose Phosphate Pathway: Glucose-6-Phosphate Dehydrogenase Deficiency ( G6PDD ) : The PPP supplies the RBC with NADPH to maintain the reduced state of glutathione. In some individual carry defective gene which less active glucose-6-phosphate dehydrogenase and becomes inactive in presence of certain drugs. So, the affected individuals are normal until they are exposed to those drugs. Glucose-6-phosphate dehydrogenese deficiency occurs when drugs like aspirin, antibiotics : ciprofloxacin , nitofurantoin , sulphonamide also anti-malarial drug and sulfonamide are administered to these individuals. Since NADPH production is blocked in these individuals due to the deficiency of G6PD the susceptibility of RBC to hemolysis is increased. Therefore, the affected individuals develop hemolytic anemia on exposure to these drugs. Consumption of fava beans also causes G6PD deficiency in the susceptible individuals. Favism is the name given to this type of G6PD deficiency.