carbohydrates metabolism-1.pptx

CARBOHYDRATE METABOLISM 1 DR RAFIAT AJALA-LAWAL LECTURER I, DEPARTMENT OF MEDICAL BIOCHEMISTRY, NILE UNIVERSITY OF NIGERIA. OUTLINE  INTRODUCTION  GLYCOLYSIS  GLUCONEOGENESIS  METABOLISM OF OTHER M...

CARBOHYDRATE METABOLISM 1 DR RAFIAT AJALA-LAWAL LECTURER I, DEPARTMENT OF MEDICAL BIOCHEMISTRY, NILE UNIVERSITY OF NIGERIA. OUTLINE  INTRODUCTION  GLYCOLYSIS  GLUCONEOGENESIS  METABOLISM OF OTHER MONOSACCHARIDES  Introduction.  Glucose occupies a central position in the metabolism of plants, animals, and many microorganisms.  It is relatively rich in potential energy, and thus a good fuel.  By storing glucose as a high molecular weight polymer such as glycogen, a cell can stockpile large quantities of hexose unit while maintaining a relatively low cytosolic osmolarity.  When energy demand increases, glucose can be released from these intracellular storage polymers and be used to produce ATP either aerobically or an-aerobically.  Metabolic fates of glucose It may be stored as polysaccharide or as sucrose. Oxidized to a three-carbon compound (pyruvate) via glycolysis to provide ATP and metabolic intermediates. Oxidized via the pentose phosphate (phosphogluconate) pathway to yield ribose 5-phosphate for nucleic acid synthesis and NADPH for reductive biosynthetic processes.  GLYCOLYSIS  Glycolysis( from greek, glykys= sweet, lysis=splitting).  Also known as Embeden-Myerhof-Panas pathway.  Occurs in the cytoplasm.  A molecule of glucose is degraded in a series of enzyme-catalysed reaction to yield Two molecules of the three-carbon compound, pyruvate some free energy released from glucose conserved in the form of ATP and NADH.  TYPES OF GLYCOLYSIS  There are 2 types of glycolysis Aerobic glycolysis;  it involves a sequence of 10 steps in which pyruvate is the end product which then gets converted to acetyl CoA( a major fuel for the citric acid cycle)by oxidative decarboxylation. It is called oxidative because oxygen is required to reoxidize the NADH formed during the oxidation of glyceraldehyde-3-phosphate.  GLYCOLYSIS Anaerobic glycolysis Alternatively, glucose can be converted to pyruvate which is reduced by NADH to form lactate. This conversion of glucose to lactate is called anaerobic glycolysis because it can occur without participation of oxygen. It allows the continued production of ATP in tissues that lack mitochondria e.g., Red blood cell or in cells deprived of sufficient oxygen.  PHASES OF GLYCOLYSIS  The conversion of glucose to pyruvate occurs in 2 stages.  the 1 st five reactions of glucose correspond to energy investment phase in which the phosphorylated form of intermediates are synthesized at the expense of ATP.  The subsequent reactions constitute an energy generation phase (payoff) in which a net of 2 molecules of ATP and NADH are formed by substrate level phosphorylation per glucose molecule metabolized while generating pyruvate Steps of glycolytic pathway  1. Phosphorylation Of Glucose  1 st step of glycolysis. Glucose is activated for subsequent reactions by its phosphorylation at C-6 to yield glucose-6- phosphate, with ATP as a phosphoryl donor.  Reaction is irreversible and is catalysed by hexokinase.  Hexokinase like many other kinases require mg 2+ for activity, because the true substrate of the enzyme is not ATP 4- but the MgATP 2- complex.  Glucose-6-phosphate is more reactive than glucose, and the addition of the phosphate also traps glucose inside the cell since glucose with a phosphate can’t readily cross the membrane 2. Conversion of glucose -6-phosphate to fructose-6-phosphate The enzyme, phosphohexose isomerase (phospho-glucose isomerase) catalyses the reversible isomerization of glucose 6-phosphate , an aldose, to fructose 6-phosphate, a ketose.  3. PHOSPHORYLATION OF FRUCTOSE 6- PHOSPHATE TO 1,6-BIPHOSPHATE.  Phosphofructokinase-1 (PFK-1) catalyses the transfer of a phosphoryl group from ATP to fructose 6-phosphate to yield 1,6- bisphosphate.  PFK-1 is a regulatory enzyme, and it is the major point of regulation in glycolysis. The activity of PFK-1 is increased whenever the cells ATP supply is depleted, or when the ATP breakdown products, ADP and AMP are in excess.  4. Cleavage of fructose 1,6-bisphosphate  The enzyme fructose 1,6-bisphosphatase aldolase, often called simply aldolase, catalyzes a reversible aldol condensation. Fructose 1,6-bisphosphatase yields two different triose phosphate; Glyceraldehyde 3-phosphate , an aldose. Dihydroxyacetone phosphate, a ketose.  5. Interconversion of the triose phosphate  Only one of the 2 triose phosphate formed by aldolase , glyceraldehyde -3-phosphate, can be directly degraded in the subsequent steps of glycolysis.  The other, dihydroxyacetone phosphate, is rapidly and reversibly converted to glyceraldehyde 3-phosphate by the 5 th enzyme of the sequence, triose phosphate isomerase.  The hexose molecule has been phosphorylated at C-1 and C-6 and then cleaved to form two molecules of glyceraldehyde 3-phosphate.  PAYOFF PHASE  In the second half of glycolysis, the three-carbon sugars formed in the first half of the process go through a series of additional transformations, ultimately turning into pyruvate.  In the process, four ATP molecules are produced, along with two molecules of NADH. 6. Oxidation of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate.  Oxidation of glyceraldehyde 3- phosphate to 1,3-bisphosphoglycerate, catalyzed by glyceraldehyde 3- phosphate dehydrogenase.  It is the 1 st of the two energy- conserving reactions of glycolysis that leads to the formation of ATP.  Remember that one molecule of glucose yields two molecules of glyceraldehyde 3-phosphate. Glycolysis  Both halves of the glucose molecule follow the same pathway in the second phase of glycolysis.  The conversion of two molecules of pyruvate is accompanied by the formation of four molecules of ATP from ADP.  However, the net yield of ATP per molecule of glucose degraded is only two, because two ATP were invested in the preparatory phase of glycolysis to phosphorylate the two ends of the hexose molecule. 7. Phosphoryl transfer of 1,3- bisphosphoglycerate to ADP to form 3- bisphosphoglycerate.  The enzyme phosphoglycerate kinase transfers the high energy phosphoryl group from the carboxyl group of 1,3- bisphosphoglycerate to ADP, forming ATP and 3-phosphoglycerate.  The formation of ATP by phosphoryl group transfer from a substrate such as 1,3-bisphosphoglycerate is referred to as substrate level phosphorylation  8. Conversion of 3-phosphoglycerate to 2- phosphoglycerate.  The enzyme phosphoglycerate mutase catalyzes a reversible shift of the phosphoryl group between C-2 and C-3 of glycerate; Mg 2+ is essential for this reaction.  the reaction occurs in 2 steps; A phosphoryl group initially attached to a His residue of the mutase is transferred to the hydroxyl group at C-2 of 3-phopshoglycerate, forming 2,3- bisphosphoglycerate(2,3-BPG). The phosphoryl group at C-3 of 2,3- bisphosphoglycerate is then transferred to the same histidine residue, producing 2-phosphoglycerate and regenerating the phosphorylated enzyme.  Importance of 2,-bisphosphoglycerate  Although in most cells 2,3-bisphosphoglycerate is present only in trace amounts, it is a major component of erythrocytes, where it regulates the affinity of haemoglobin for oxygen. 9. Dehydration of 2-phosphoglycerate to phosphoenolpyruvate  Enolase promotes reversible removal of a molecule of water from 2- phosphoglycerate to yield phosphoenolpyruvate (PEP). 10.Transfer of the phosphoryl group from phosphoenol pyruvate to ADP yielding pyruvate  The last step is the transfer of the phosphoryl group from phosphoenol pyruvate to ADP, catalyzed by pyruvate kinase, which requires k + and either Mg2+ or Mn 2+.  In this substrate-level phosphorylation, the product pyruvate first appears in its enol form, then tautomerizes rapidly and nonenzymatically to its keto form.  The overall reaction has a large, negative standard free energy , due in large part to the spontaneous conversion of the enol form to the keto form.  Illustration for step 10 FATES OF PYRUVATE  Fate of pyruvate 1. Oxidative decarboxylation of pyruvate-by-pyruvate dehydrogenase complex is an important pathway in tissues with a high oxidative capacity, such as cardiac muscle.  pyruvate dehydrogenase irreversibly converts pyruvate , the end product of glycolysis, into acetyl CoA, a major fuel for the TCA cycle and the building block for fatty acid synthesis.  The electrons from these oxidations are passed to O 2 through a chain of carriers in the mitochondrion to form H 2O.the energy from the electron- transfer reactions drives the synthesis of ATP in the mitochondrion. Fate of pyruvate 2. Reduction to lactate via lactic acid fermentation.  When vigorously contracting skeletal muscle must function under oxygen conditions (hypoxia), NADH cannot be re-oxidized to NAD+ (which is required as an electron acceptor for further oxidation of pyruvate)  Thus, pyruvate is reduced to lactate, accepting electrons from NADH and oxidizing it to NAD+.  Certain cells such as RBC convert glucose to lactate even under aerobic conditions. Fate of pyruvate  Fate of pyruvate  3. reduction of pyruvate to ethanol; this occurs in yeast and certain microorganisms, but not in humans. Energy yield from glycolysis  Despite the production of some ATP during glycolysis, the end products, pyruvate or lactate, still contain most of the energy originally contained in glucose. The TCA cycle is required to release the energy completely.  Anaerobic glycolysis: two molecules of ATP are generated for each molecule of glucose converted to two molecules of lactate. There is no net production or consumption of NADH. Anaerobic glycolysis, although releasing only a small fraction of the energy contained in the glucose molecule Energy yield from glycolysis  It is however valuable source of energy under several conditions including; I. When the oxygen supply is limited, as in muscle during intensive exercise. II. For tissues with few or no mitochondrial, such as the medulla of the kidney, mature erythrocytes, leukocytes, and cells of the lens, cornea, and testes.  Aerobic glycolysis: the direct formation and consumption of ATP is the same as in anaerobic glycolysis. That is, a net gain of two ATP per molecule of glucose.  Two molecules of NADH are also produced per molecule of glucose. Ongoing aerobic glycolysis requires the oxidation of most of this NADH by the electron transport chain, producing approximately three ATP for each NADH molecule entering the chain. Gluconeogenesis  Introduction.  In mammals, some tissues depend almost completely on glucose for their metabolic energy.  For the human brain and nervous tissue, as well as the erythrocytes, testes, renal medulla, and embryonic tissues, glucose from the blood is the sole or major fuel source.  The brain alone requires about 120g of glucose each day-more than half of all the glucose stored as glycogen in the liver and muscle.  introduction.  However, the supply of glucose from these stores is not sufficient; in between meals and during longer fasts, or after vigorous exercise, glycogen is depleted.  For these times, organisms need a method for synthesizing glucose from non carbohydrates precursors.  This is accomplished by a pathway called gluconeogenesis.(formation of new sugar) which converts pyruvate and three- and four-carbon compounds to glucose.  Introduction.  Gluconeogenesis is a ubiquotous multistep process in which pyruvate, or a related three-carbon compound (lactate, alanine) is converted to glucose.  Seven of the steps are catalysed by same enzymes used in glycolysis. These are the reversible reactions.  Three irreversible steps in the glycolytic pathway are bypassed by reactions catalysed by gluconeogenic enzymes.  Introduction.  The important precursors of glucose in animals are three- carbon compounds such as lactate, pyruvate , and glycerol as well as certain amino acids.  In mammals, gluconeogenesis takes place mainly in the liver and to a lesser extent in renal cortex.  The glucose produced passes into the blood to supply other tissues.  Lactate produced by anaerobic glycolysis in the skeletal muscles after vigorous exercise, returns to the liver and is converted to glucose, which moves back to the muscle and is converted to glycogen.  Introduction.  The important precursors of glucose in animals are three- carbon compounds such as lactate, pyruvate , and glycerol as well as certain amino acids.  In mammals, gluconeogenesis takes place mainly in the liver and to a lesser extent in renal cortex.  The glucose produced passes into the blood to supply other tissues.  Lactate produced by anaerobic glycolysis in the skeletal muscles after vigorous exercise, returns to the liver and is converted to glucose, which moves back to the muscle and is converted to glycogen.  Reactions unique to gluconeogenesis  Seven glycolytic reactions are reversible and are used in the synthesis of glucose from lactate or pyruvate. However, three of the reactions are irreversible and must be circumvented by four alternate reactions that energetically favour the synthesis of glucose.  These reactions, unique to gluconeogenesis, are described below; 1.Carboxylation of pyruvate  The 1st roadblock to overcome in the synthesis of glucose from pyruvate is the irreversible conversion in glycolysis of pyruvate to phosphoenol pyruvate (PEP).  Pyruvate is 1 st transported from the cytosol to the mitochondria or it is generated from alanine. Then pyruvate carboxylase, a mitochondrial enzyme that requires the co enzyme biotin, converts the pyruvate to oxaloacetate  Carboxylation of pyruvate  Oxaloacetate formed in the mitochondria must enter the cytosol where the other enzymes of gluconeogenesis are located however, OAA is unable to do that thus it must first be converted to malate by mitochondrial malate dehydrogenase which is then transported to the cytosol where it is re-oxidized to OAA by cytosolic malate dehydrogenase.  In the cytosol, the oxaloacetate becomes converted to phosphoenolpyruvate by phosphoenolpyruvate carboxykinase. The reaction is driven by hydrolysis of GTP. Dephosphorylation of fructose 1,6-bisphosphate PEP is then acted upon by the reaction of glycolysis running in the reverse direction until it becomes fructose 1,6-bisphosphate. The conversion of fructose 1,6-bisphosphate is the 2 nd bypass. because this reaction is highly exergonic and therefore irreversible in intact cells, the generation of fructose-6-phosphate from fructose1, 6- bisphophate is catalysed by a Mg2+ dependent fructose 1,6- bisphosphatase, which promotes the essentially irreversible hydrolysis of the C-1 phosphate Conversion of glucose 6-phosphate to glucose  This is the 3 rd bypass and the final reaction in gluconeogenesis.  Dephosphorylation of glucose 6-phosphate to glucose by glucose 6 phosphatase is more energy favourable and does not require synthesis of ATP as compared with reversal of the hexokinase reaction which would require phosphoryl group transfer from glucoser-6-phosphate to ADP, forming ATP.  It is simply hydrolysis of a phosphate ester. Conversion of glucose 6-phosphate to glucose  This mg 2+ -activated enzyme is found on the luminal side of the hepatocyte and renal cells.  The muscle and brain lack this enzyme and thus cannot carry out gluconeogenesis.  Glucose produced in the liver or kidney or ingested in the diet is delivered to the brain and muscle through the blood stream. CORI CYCLE  Cori cycle occurs in 5 steps: Lactate production Transport of lactate to the liver Gluconeogenesis Release of glucose in the blood Glucose uptake by muscles and other tissues.  Importance of lactic acid cycle: Prevent acidosis in muscles Increase exercise intensity Maintain glucose level and energy during stressful time. Glucose-Alanine cycle.  Of all the amino acids that can be converted to glycolytic intermediates, alanine is perhaps the most important.  When exercising muscle produces large quantities of pyruvate, some of these molecules are converted to alanine by a transamination reaction involving glutamate.  After it has been transported to the liver, alanine is reconverted to pyruvate and then to glucose Glycerol, a product of fat metabolism in adipose tissue, is transported to the liver where it is converted to glycerol-3-phosphate by glycerol kinase. Glycerol-3-phosphate is then oxidised to form DHAP by glycerol phosphate dehydrogenase when cytoplasm NAD+ concentration is relatively high. Regulation of glycolysis and gluconeogenesis Regulation of glycolysis.  Regulation of glycolysis is achieved : Hormonal regulation Allosteric regulation  Allosteric regulation  the rate at which the glycolytic pathway operates in a cell is primarily controlled by allosteric regulation of the enzymes that catalyse the three irreversible reactions: Hexokinase Phosphofructokinase1 PFK-1: of the three, this is the most carefully regulated enzyme Pyruvate kinase  Regulation of glycolysis  Hexokinase is inhibited by the reaction product, glucose 6-phosphate which accumulates when further metabolism of this hexose phosphate is reduced.  Phosphorylation of fructose-6 phosphate by PFK1 is the most important control point and the rate limiting step of glycolysis.  PFK1 is controlled by: Regulation by energy levels within the cell: PFK1 is inhibited allosterically by elevated levels of ATP, citrate and conversely activated by high concentrations of AMP. Regulation of glycolysis Regulation by fructose 2,6-bisphosphate: fructose 2,6-bisphosphate is the most potent activator of PFK-1. it however also acts as an inhibitor of fructose 1,6 bisphosphatase (gluconeogenesis enzyme). The reciprocal actions of fructose 2,6- bisphosphate on glycolysis and gluconeogenesis ensure that both pathways are not fully active at the same time. Decreased levels of glucagon and elevated levels of insulin, such as during a well-fed state cause an increase in fructose 2,6- bisphosphate and thus in the rate of glycolysis in the liver and vice versa.  Regulation of glycolysis  Fed forward regulation: increased fructose 1,6-bisphosphate activates pyruvate kinase.  Covalent modulation of pyruvate kinase: Phosphorylation by a protein kinase leads to inactivation of pyruvate kinase in the liver When blood glucose levels are low, elevated glucagon increases the intracellular level of cAMP, which causes the phosphorylation and inactivation of pyruvate kinase. Therefore, phosphoenolpyruvate is unable to continue in glycolysis, but instead enters the gluconeogenesis pathway. This, in part, explains the observed inhibition of hepatic glycolysis and stimulation of gluconeogenesis by glucagon. Dephosphorylation of pyruvate kinase by a phosphoprotein phosphatase results in reactivation of the enzyme  Hormonal regulation of glycolysis  These effects can result in ten-fold to twenty-fold increases in enzyme activity that typically occur over hours to days  Regular consumption of meals rich in carbohydrate or administration of insulin initiates an increase in the amount of glucokinase, phosphofructokinase and pyruvate kinase  These changes reflect an increase in gene transcription, resulting in increased enzyme synthesis. High activity of these three enzymes favors the conversion of glucose to pyruvate, a characteristic of the well-fed state.  Conversely, gene transcription and synthesis of glucokinase, phosphofructokinase, and pyruvate kinase are decreased when plasma glucagon is high and insulin is low, for example, as seen in fasting or diabetes.  Regulation of gluconeogenesis  The regulation of gluconeogenesis is determined primarily by the circulating level of glucagon, and by the availability of gluconeogenic substrates. A. Glucagon stimulates gluconeogenesis by the following mechanisms; I. Changes in the allosteric effectors; glucagon lowers the level of fructose 2,6-bisphosphate, resulting in the activation of fructose 1,6-bisphosphatase and inhibition of fructokinase. II. Covalent modification of enzyme activity; glucagon, via an elevation in the cAMP, level and cAMP-dependent protein kinase activity, stimulates the conversion of pyruvate kinase to its inactive (phosphorylated) form. This decreases the conversion of PEP to pyruvate, which has the effect of diverting PEP to the synthesis of glucose. III. Induction of enzyme synthesis: glucagon increases the transcription of the PEP carboxykinase gene, thereby increasing the availability of this enzyme’s activity as the level of the substrates increases during fasting.  Regulation of gluconeogenesis  Substrate availability; the availability of gluconeogenic precursors, particularly glucogenic amino acids, significantly influences the rate of hepatic glucose synthesis. Decreased levels of insulin favour mobilization of amino acids from muscle protein and provide the carbon skeleton for gluconeogenesis.  Regulation of gluconeogenesis  Allosteric inhibition by AMP; fructose 1,6- bisphosphatase is inhibited by AMP, a compound that activates phosphofructokinase.  Regulation of gluconeogenesis  Allosteric activity by acetyl CoA; allosteric activation of hepatic pyruvate carboxylase by acetyl CoA occurs during fasting. As a result of excessive lipolysis in adipose tissue, the liver is flooded with fatty acids. The rate of formation of acetyl CoA by β- oxidation of these fatty acids exceeds the liver capacity to oxidize it to CO2 and H20. As a result, acetyl CoA accumulates and leads to activation of pyruvate carboxylase