University of Duhok Carbohydrate Metabolism Lecture Notes PDF

University of Duhok College of Medicine Metabolism module Session 3 Lecture 1+2 Carbohydrate metabolism part 2 Dr. HIVI M. MAHMOUD M.B.,Ch.B., M.Sc., PhD. (Clinical Biochemistry) 1 Objectives 1. 2. 3. 4. 5. 6. 7. Definition of the HMP pathway Major differences between EM and HMP pathway Reactio...

University of Duhok College of Medicine Metabolism module Session 3 Lecture 1+2 Carbohydrate metabolism part 2 Dr. HIVI M. MAHMOUD M.B.,Ch.B., M.Sc., PhD. (Clinical Biochemistry) 1 Objectives 1. 2. 3. 4. 5. 6. 7. Definition of the HMP pathway Major differences between EM and HMP pathway Reactions of HMP pathway Physiological significance of the HMP pathway Clinical significance of the HMP pathway, G6PD Role of TCA and how is regulated How and why glucose is formed from noncarbohydrate sources (Gluconeogenesis) 8. Galactose metabolism and biochemical and clinical basis of galactosaemia 9. Fructose metabolism. 2 3 4 LO 1 Definition of the shunt pathway It is also known as: pentose phosphate pathway or hexose monophosphate shunt instead of glucose going through glycolytic pathway, it is shunted through this pathway. Defined as an alternative pathway for oxidation of glucose About 10% of glucose per day are entering in HMP shunt pathway It occurs in the cytosol of the cell, 5 LO 1 The major purpose of this pathway are generation of: 1-reduced nicotinamide adenine dinucleotide phosphate (NADPH) production which used as biochemical reductant 2-pentose phosphate which used for nucleotide synthesis Three molecules of glucose -6-posphate enter the cycle, producing 3 molecules of CO2 and 3 molecules of 5 carbon residues which give 2 molecules of glucose 6 phosphate and 1 molecule of glyceraldehydes -3-phosphate 6 LO 2 Major differences between EM and HMP pathway EM pathway occurs in all tissues oxidation by dehydrogenation but NAD is hydrogen acceptor ATP is required and ATP is produced CO2 is never formed HMP pathway occurs in certain special tissues for special function oxidation by dehydrogenation but NADP is hydrogen acceptor ATP is required and ATP is not produced CO2 is produced 7  Reactions of Pentose Phosphate Pathway LO 3 Stage 1 oxidative reactions (irreversible) Three reactions: 1-dehydrogenation of glucose 6-phosphate 2-hydration of phosphogluconolactone 3-formation of ribulose 5-phosphate (second dehydrogenation) For each molecule of glucose 6-phosphate oxidization: a-ribulose 5-phosphate. b-CO2. c-two molecules of NADPH 8 LO 3 9 Stage 2 nonoxidative reactions (reversible) LO 3 These reversible reactions permit ribulose 5-phosphate (produced by the oxidative portion of the pathway) to be converted Either to ribose 5-phosphate for nucleotide synthesis or to fructose 6-phosphate (intermediates of glycolysis) or to Glyceraldehyde 3-phosphate (intermediates of glycolysis) 10 LO 3 11 Physiological significance of the HMP pathway LO 4 the oxidative reactions of the pentose phosphate pathway is operating in following organs: a-liver b-lactating mammary glands c-adipose (which are active in the biosynthesis of fatty acids) d-adrenal cortex (which is active in the NADPH-dependent synthesis of steroids) e-erythrocytes ( which require NADPH to keep glutathione reduced). f-testes and ovaries g-lens of eyes the nonoxidative reactions of the pentose phosphate pathway operating in 12 all cell types synthesizing nucleotides and nucleic acids. LO 4 1-generation of reducing equivalents NADPH enhance reductive biosynthesis of fatty acid cholesterol steroids 2-free radical scavenging NADPH enhance the regeneration of reduced glutathione reductase which has a role in removing the free radical (super oxide, hydrogen peroxide). 3-erythrocyte membrane integrity NADPH is required by the red blood cells to keep the glutathione in reduced state which detoxify the free radical formed within the RBC 13 LO 4 4-prevention of methemoglobinemia NADPH is required to keep iron of Hb in reduced state (ferrous) and prevent accumulation of methemoglobinemia (cannot carry oxygen) 5-lens of eye NADPH is required for preserving the transparency of lens maintenance of lens protein 6-macrophage bacteria activity NADPH is required for production of superoxide anion radical by macrophage to kill bacteria. 14 Clinical significance of the HMP pathway LO 5 Glucose 6-phosphate dehydrogenase deficiency  Most common enzyme deficiency seen in clinical practice  Inherited (X-linked) disease  Hemolytic anemia which is caused by inability to detoxify oxidizing agents diminished G6PD activity impairs the ability of the cell to form the NADPH which is essential for the maintenance of the reduced glutathione pool, resulting in decrease in the cellular detoxification of free radicals and peroxides formed within the cell.  Although G6PD deficiency occurs in all cells of the affected individual, it is most severe in erythrocytes, where the pentose phosphate pathway provides the only means of generating NADPH hemolytic anemia which is characterized by ANEMIA JAUNDICE BLACK URINE 15 Precipitating factors in G6PD deficiency LO 5 Most individuals who have inherited one of the many G6PD mutations do not show clinical manifestations (depend on the amount of deficiency of enzyme activity) some patients with G6PD deficiency develop hemolytic anemia if they are: Treated with an oxidant drug. oAntibiotics (chloramphenicol) oAntimalarials (primaquine) oAntipyretics (acetanilid ) …….etc Ingesting fava beans (Favism). Severe infection. The generation of free radicals by inflammation in macrophages, which can diffuse into the red blood cells and cause oxidative damage. 16 LO 6 Citric Acid Cycle Tricarboxylic acid cycle Krebs cycle 17 LO 6  The aerobic processing of glucose starts with the complete oxidation of glucose derivatives to CO2 and H2O.  It occurs in mitochondria  The Citric acid cycle is the final common central pathway for the oxidation of fuel molecules –amino acids, fatty acids& CHO.  Most fuel molecules enter the cycle as Acetyl CoA.  Under aerobic conditions, the pyruvate generated from glucose is oxidatively decarboxylated to Acetyl CoA.  No known genetic defects that if present would be lethal. 18 LO 6 19 Biosynthetic roles of the citric acid cycle LO 6 20 LO 6 Regulation of TCA Cycle The TCA cycle is controlled by the regulation of several enzyme activities The main drive for the cycle is ATP/ADP:ratio and NADH/NAD+ ratio The most important of these regulated enzymes are :  Citrate synthase  Isocitrate dehydrogenase  Alpha ketoglutarate dehydrogenase complex 21 LO 6 22 LO 7 Central nervous system and other glucose-dependent tissues need glucose during fasting & starvation when the glucose is absent from diet. Initially this comes from the breakdown of liver glycogen (glycogenolysis). However, the amount of glucose stored as liver glycogen is only sufficient for 8-10 hours of fasting If fasting lasts more than 8-10 hours the body has to produce glucose by the process of GLUCONEOGENESIS. 23 • GLUCONEOGENSIS: • LO 7 The synthesis of glucose from noncabohydrate precursors is called GLUCONEOGENSIS.  Liver is the major site of GLUCONEOGENSIS.  Kidney: small percentage of GLUCONEOGENSIS .  Brain: very little. • A longer period of starvation, glucose must be formed from non-CHO sources such as lactate, amino acids& glycerol). MAIN GLUCONEOGENESIS SUBSTRATES:  lactate is formed by the active skeletal muscle when the rate of glycolysis exceeds the rate of oxidative metabolism,be reconverted to glucose by GLUCONEOGENSIS in the liver.  amino acids are derived from proteins in the diet &, during starvation, from the breakdown of proteins in the skeletal muscles.  Glycerol is produce from the hydrolysis of triglycerides. 24 . LO 7 25 Regulation of gluconeogenesis LO 7  Gluconeogenesis occurs as part of the response to stress situations (e.g. fasting, starvation, prolonged exercise) and is largely under hormonal control.  The major control sites are PEPCK and Fructose 1,6-bisphosphatase.  The activity of PEPCK is increased by glucagon and cortisol and decreased by insulin.  The activity of Fructose 1,6- bisphosphatase is also increased by glucagon and decreased by insulin.  Thus, the insulin/glucagon ratio plays a major role in determining the rate of gluconeogenesis.  In the absence of adequate levels of biologically effective insulin, such as occurs in DIABETES, increased rates of gluconeogenesis contribute significantly to the hyperglycaemia. 26 Regulation of gluconeogenesis LO 7 1-GLUCAGON HORMONE stimulates gluconeogenesis by three mechanisms. a. Changes in allosteric effectors: Glucagon lowers the level of fructose 2,6-bisphosphate, resulting in activation of fructose 1,6-bisphosphatase and inhibition of phosphofructokinase-1, thus favoring gluconeogenesis over glycolysis. b. Covalent modification of enzyme activity c. Induction of enzyme synthesis: Glucagon increases the transcription of the gene for PEP-carboxykinase. 27 LO 7 2-substrate availability: The availability of gluconeogenic precursors, particularly glucogenic amino acids, significantly influences the rate of hepatic glucose synthesis. 3-inhibition by AMP: Fructose 1,6-bisphosphatase is inhibited by AMP—a compound that activates PFK. 4-activation by acetyl CoA: Activation of hepatic pyruvate carboxylase by acetyl CoA occurs during fasting. As a result of increased lipolysis in adipose tissue, the liver is flooded with fatty acids. 28 Galactose metabolism LO 8  Dietary lactose is hydrolysed by the digestive enzyme lactase to release glucose and galactose that are absorbed into the blood stream.  Galactose is metabolised largely in the liver (some in kidney and gastrointestinal tract)  The epimerase reaction is reversible enabling galactose to be synthesized from glucose via UDP-glucose.  This is important during lactation when breast tissue is synthesizing large amounts of lactose for milk production.  There are a number of clinical conditions that affect galactose metabolism including Lactose Intolerance and Galactosaemia. 29 LO 8 30 Galactosaemia LO 8 Unablity to utilize galactose obtained from the diet because of a lack of the kinase or transferase enzyme. The absence of the kinase is relatively rare (Duarte type) and is characterized by accumulation of galactose in tissues. The absence of the transferase is more common (classical type) and more serious as both galactose and galactose 1-phosphate accumulate in tissues. Accumulation of galactose in tissues leads to its reduction to galactitol (aldehyde group reduced to alcohol group) by the activity of the enzyme aldose reductase: 31 Aldose reductase LO 8  This reaction depletes some tissues of NADPH.  In the eye the lens structure is damaged, (cross linking of lens proteins by S-S- bond formation) causing cataracts. In addition, there may be nonenzymatic glycosylation of the lens proteins because of the high concentration of galactose and this may contribute to the cataract formation.  The accumulation of galactose and galactitol in the eye may lead to raised intra-ocular pressure (glaucoma) which if untreated may cause blindness.  Accumulation of galactose 1-phosphate in tissues causes damage to the liver, kidney and brain and may be related to the sequestration of Pi making it unavailable for ATP synthesis. 32 Fructose metabolism LO 9  Dietary sucrose is hydrolysed by the digestive enzyme sucrase to release glucose and fructose. These are absorbed into the blood stream.  Fructose is metabolized largely in the liver by soluble enzymes that catalyze its conversion to glyceraldehyde 3-phosphate (intermediate of glycolysis). 33 THANK YOU 34