Glygogenesis, glycogenolysis and uronic Acid pathway Bch226.pdf

## 2선 Work **BCH 226 (CARBOHYDRATE AND LIPID METABOLISM) FOR FHS&T** **TOPIC: GLYGOGENESIS, GLYCOGENOLYSIS AND URONIC ACID PATHWAY** **A LECTURE NOTE PREPARED BY CHARLES C. DIKE, PhD** ### Glycogenesis This is the conversion of glucose molecules to glycogen. This happens when there is excess blo...

## 2선 Work **BCH 226 (CARBOHYDRATE AND LIPID METABOLISM) FOR FHS&T** **TOPIC: GLYGOGENESIS, GLYCOGENOLYSIS AND URONIC ACID PATHWAY** **A LECTURE NOTE PREPARED BY CHARLES C. DIKE, PhD** ### Glycogenesis This is the conversion of glucose molecules to glycogen. This happens when there is excess blood glucose. Glycogen exists from precursors of glucose, derived from recently ingested carbohydrates or gluconeogenesis precursors, including lactate and alanine. Glycogen is a readily mobilized fuel store. It is a highly branched polymer of glucose residues. **Mechanism of Reaction** * Glucokinase catalyses the conversion of glucose to glucose-6-phosphate. * This is converted to glucose-1-phosphate by phosphoglucomutase. * Uridine phosphate glucose (UDP) is formed from glucose-1-phosphate and adenosine triphosphate (ATP). * Glycogen syntase catalyzes the transfer of glucose from UDP-glucose to the terminal hydroxyl residue of a growing chain. * If glycogen polymer did not exist, the cytosol osmolarity would be sufficiently high to cause water penetration into the cell through osmosis and result in cell lysis. Glycogen leads to higher solubility, mainly due to its formation of branches. On the other hand, most of the polymer formed by alpha 1-4 glycosidic bonds which get organized in chains, branching occurs every 4-8 glucose monomer via alpha 1-6 glycosidic bonds. The arrangement leads to the classical organization of glycogen called the beta-particle, with a selfglycosylating protein called glycogenin in the core. Glycogenolysis involves the disintegration of this polymer into glucose monomers through a specific enzyme that catalyses the breakdown of glycogen's branches and chains. **Glucose Glycokinase glucose-6-phosphate Phosphoglucomutase glucose-1-phosphate- ATP + urodine phosphate glicose (UDP)** **Fig.1: A schematic representation of glycogenesis pathway** * A simplified diagram of glycogenesis pathway with glucose converting to glycogen. **Fig. 2: Glycogenolysis pathway** * A simplified diagram showing breakdown of glycogen to glucose. **Mechanism of Reaction** * Glycogenolysis can occur via two pathways. * The first pathway revolves around cytosolic degradation via the synchronized action of glycogen phosphorylase and glycogen debranching enzyme, * The second pathway revolves around lysosomal degradation via the enzyme alpha-glucosidase. Corresponding to cytosolic degradation, glycogen phosphorylase, the rate-limiting enzyme of glycogenolysis, cleaves terminal glucose residue connected to a glycogen branch while substituting a phosphoryl group for the alpha 1-4 bond. Four resides before an alpha 1-6 bond, corresponding to a branch, glycogen debranching enzyme catalyzes the transfer of three of the four remaining glucose residues to the end of another glycogen chain, where they can again by degraded by glycogen phosphorylase. In other words, the breakage of alpha 1-4 glycosidic bonds present in linear chains is catalyzed by glycogen phosphorylase, and the addition of the phosphate group to position one results in the production of glucose-1-phosphate. The activity of glycogen phosphorylase is modulated allosterically and by phosphorylation. Glycogen production, conversely, inhibits glycogen degradation. Phosphoglucomutase is then in charge of converting glucose-1-phosphate to glucose-6-phosphate through an isomerization reaction that has no energy requirements. On the other hand, the debranching enzyme deals with alpha 1-6 bonds and transfers a branch to the end of the polymer so that glycogen phosphorylase can continue working with it. In most tissues, glucose-6-phosphate is internally utilized for glycolysis and energy production through conversion to pyruvate, acting as a critical metabolic intermediate for other pathways, including the TCA cycle, fatty acid synthesis, Cori cycle, and alanine cycle. Nevertheless, in gluconeogenic organs such as the liver, kidney, and intestines, glucose-6-phosphate needs to be dephosphorylated to glucose-with the aid of enzyme glucose-6-phosphatase- so that it can undergo transport from the ER to the interstitial space. Corresponding the lysosomal glycogen degradation, the primary enzyme involved in acid maltase. The hydrolysis of glycogen to glucose, catalyzed by acid alpha-glucosidase, has been hypothesized to serve a protective mechanism for the liver from high concentrations of glycogen. Of the total amount of glycogenolysis that happens in skeletal muscle, only 5% of glycogen degradation happens in lysosomes. For liver glycogenolysis, only 10% occurs in lysosomes. **Testing** * Visualization of glycogen molecules cannot be done through light microscopy but instead requires the implementation of electron microscopy. * Histological staining and implementation of light microscopy would only allow for visualization of conglomerates of glycogen particles. * Molecules of glycogen, per se, require electron microscopy. * Depending on the tissue sample gathered, glycogen has s been described as rosette-like beta particles or larger alpha particles. * Rosette-like beta particles are found on muscle, whereas the alpha particles, which are simply aggregates of beta particles, are found in the liver. * Beta particles correspond to the typical configuration of glycogen with average chain lengths of 13 residues consisting of inner chains with branch points and outer chains without branch points. * The method integrated for histological staining is Period Acid Schiff. * However, certain disadvantages of this method exist, including lack of specificity and general incompatibility with immunofluorescence techniques. * Therefore, a novel method for the detection of glycogen in cells is now available where a renewable, recombinant protein containing carbohydrate-binding module from starch-binding domain protein 1 (Stbd1) is subsequently employed to perform an enzyme-linked immunosorbent assay. **Concerning testing for glycogen storage diseases, current methods using DNA mutational analysis have eliminated the need to perform liver biopsies. This diagnostic test, for instance, applies to von Gierke disease and Cori disease. Diagnostic tests for Pompe disease include analysis of acid maltase activity in leukocytes or fibroblasts. A muscle biopsy portraying vacuolated myopathy with excessive lysosomal glycogen accumulation would also serve as a diagnostic test for Pompe disease.** **The rate of glycogenolysis is the difference between the rates of glucose production and absolute gluconeogenesis. Levels of gluconeogenesis are identified using techniques, including radioactive and stable isotopes. Quantification of glycogenolysis is also possible via nuclear magnetic resonance spectroscopy.** **Clinical Significance** * The importance of glycogenolysis is demonstrated through mutations in glycogen degradation leading to human genetic disorders and through the inability of skeletal muscle to cope with physical stress when glycogen scarcity exists. **Importance of Glycogen** **Liver** * As a meal containing carbohydrates or protein is eaten and digested, blood glucose levels rise, and the pancreas secretes insulin. * Blood glucose from the portal vein enters liver cells (hepatocytes). * Insulin acts on the hepatocytes to stimulate the action of several enzymes, including glycogen synthase. * Glucose molecules are added to the chains of glycogen as long as both insulin and glucose remain plentiful. * In this postprandial or "fed" state, the liver takes in more glucose from the blood than it releases. * After a meal has been digested and glucose levels begin to fall, insulin secretion is reduced, and glycogen synthesis stops. * When it is needed for energy, glycogen is broken down and converted again to glucose. * Glycogen phosphorylase is the primary enzyme of glycogen breakdown. * For the next 8-12 hours, glucose derived from liver glycogen is the primary source of blood glucose used by the rest of the body for fuel. * Glucagon, another hormone produced by the pancreas, in many respects serves as a countersignal to insulin. * In response to insulin levels being below normal (when blood levels of glucose begin to fall below the normal range), glucagon is secreted in increasing amounts and stimulates both glycogenolysis (the breakdown of glycogen) and gluconeogenesis (the production of glucose from other sources). **Muscle** * Muscle cell glycogen appears to function as an immediate reserve source of available glucose for muscle cells. * Other cells that contain small amounts use it locally, as well. * As muscle cells lack glucose-6-phosphatase, which is required to pass glucose into the blood, the glycogen they store is available solely for internal use and is not shared with other cells. * This is in contrast to liver cells, which, on demand, readily do break down their stored glycogen into glucose and send it through the blood stream as fuel for other organs. **Glycogen Storage Diseases** * Dysfunctions in glycogenolysis can lead to a variety of diseases, including glycogen storage diseases (GSD), lysosomal storage diseases, and Lafora progressive myoclonus epilepsy. * Disruptions in glycogenolysis frequently effectuate in dysfunction of organs, including the liver, skeletal muscle, brain, and kidney. * Depending on the affected enzyme in glycogenolysis, a particular spectrum of syndromes is possible. **1. von Gierke Disease** * A disruption in glycogenolysis can result in glycogen storage diseases such as von Gierke disease, the most common GSD. * Type I GSD effectuates due to a deficiency in glucose-6-phosphatase, responsible for dephosphorylating glucose-6-phosphate so that glucose can get transported outside the cell for the regulation of blood glucose levels and fuel usage in other tissues outside of the liver. * The impaired ability to generate glucose from glycogenolysis results in severe hypoglycemia, hyperuricemia, and increased levels of lactic acid and triglycerides. * Due to the deposition of fat, patients present with a rounded doll-like face. * Without treatment, failure to thrive, hepatomegaly, abnormal swelling, and delayed motor development has been evident in patients with this disease. * Long term complications can develop due to kidney glycogen accumulation leading to nephropathy, chronic kidney disease, and renal cancer. * The main form of treatment for von Gierke disease patients is to maintain normal glucose levels while avoiding hypoglycemia by having frequent feeds. **2. Pompe Disease (glycogen storage disease Type II).** * While degradation of glycogen by phosphorylase and debranching enzyme can happen in the cytosol, glycogen is also degraded via a lysosomal pathway, leading to a lysosomal storage disease called Pompe disease (glycogen storage disease Type II). * In Pompe disease, a mutation involving lysosomal alpha-glucosidase also called acid maltase-developed. * As a result, glycogen accumulates in the lysosome and its vesicles, leading to fatal outcomes, including cardiomyopathy and muscular hypotonia. * The precise pathway in which glycogen in transported to lysosomes is still unknown but is hypothesized to be through macroautophagy, in which engulfment of cargo within double-membrane vesicles called autophagosomes fuse with the lysosome. **3. Glycogen Storage Disease Type III** * Glycogen storage disease Type III also referred to as Cori disease, results due to a deficiency of glycogen debranching enzyme. * As a result, this disease manifests with an accumulation of abnormal glycogen since glycogenolysis halts when glycogen phosphorylase encounters a branching point. * The glycogen is then considered abnormal because it reflects very short outer chains. * With Cori disease, patients present with ketotic hypoglycemia and hepatomegaly. * In rare cases, it can lead to liver cirrhosis and hepatocellular carcinoma. **5. Glycogen storage disease Type V (McArdle disease)** * Glycogen storage disease Type V (McArdle disease) develops due to a deficiency in skeletal muscle glycogen phosphorylase. * In other words, with this disease, the liver is spared. * Patients demonstrate exercise intolerance, muscle weakness, cramping, and pain. * Creatine kinase levels become elevated, and myoglobinuria can be present. * Typical of this disease is a phenomenon called second wind, where patients can resume exercise when resting briefly. * Ingestion of sucrose before exercise can help alleviate symptoms since this becomes the source of energy during exercise before resorting to glycogen stores. **6. Type VI Glycogen Storage Disease.** * When glycogen phosphorylase is deficient in the liver, a different disease develops-Type VI GSD. * Hers disease exhibits normal creatine kinase and uric acid levels. * Patients present with growth retardation and liver enlargement. * Hyperlipidemia and ketotic hypoglycemia can be common. **7. Lafora** * In Lafora progressive myoclonus epilepsy, increased phosphorylation of glycogen is present in several tissues, leading to toxicity and cell death in neurons. * Symptoms include ataxia, seizures, myoclonus, and dementia. * The presence of more than normal phosphorylation levels in glycogen effectuates in longer chains and irregular branch points that render the polymer insoluble and degradation resistant. * As a result, patients with this condition have a conglomerate of inclusion bodies called Lafora bodies. **URONIC ACID PATHWAY** * Uronic acids are a class of sugar acids with both carbonyl and carboxylic acid functional groups. * They are sugars in which the terminal carbon's hydroxyl group has been oxidized to a carboxylic acid. * Oxidation of the terminal aldehyde instead yields an aldonic acid, while oxidation of both the terminal hydroxyl group and the aldehyde yields an aldaric acid. * The names of uronic acids are generally based on their parent sugars, for example, the uronic acid analog of glucose is glucuronic acid, meaning that it is synthesized from glucose units * Uronic acid pathway is an alternative pathway for glucose metabolism. * It catalyzes the conversion of glucose to glucuronic acid, ascorbic acid and pentoses. * It does not lead to the formation of of ATP. * The glucuronic acid pathway is a quantitatively minor route of glucose metabolism. * Like the pentose phosphate pathway, it provides biosynthetic precursors and inter-converts some less common sugars to ones that can be metabolized. * It takes place in the cytoplasm of the cell and precursors distributed to liver and adipose tissue. **Fig. 4:Uronic acid pathway** * A schematic representation of uronic acid pathway showing conversion of glucose to glucuronate. **Reaction Mechanism** * **STEP 1:** Glucose 6-phosphate is converted to Glucose1-phosphate via phosphoglucomutase. * **STEP 2:** Glucose 1-phosphate reacts with uridinetriphosphate (UTP) via UDP glucosepyrophosphorylase to form UDP glucose. * **STEP 3:** UDP glucose is oxidized at C6 by a 2-step process via an NAD+-dependent UDP glucosedehydrogenase to form UDP glucuronic acid. * **STEP 4:** UDP glucuronic acid is hydrolysed to form UDP and D-glucuronic acid. **UDP-glucuronate** * Source of glucuronate for reactions involving its incorporation into proteoglycans. * conjugated to nonpolar acceptor molecules such as steroid hormones, some drugs, bilirubin, or other foreign compounds in the liver for easier excretion via the bile. * **STEP 5:** Oxidation of D-glucuronic acid to L-gulonic acid via L-gulonic dehydrogenase in the presence of NADPH2. * **L-gulonate** * It is the direct precursor of ascorbate in those animals capable of synthesizing this vitamin, in an NADPH-dependent reaction. * In humans, ascorbic acid cannot be synthesized because of the absence of L-gulonolactone oxidase. * **STEP 6:** Oxidation of L-Udonic acid * L-gulonic acid may be oxidized to 3-keto-L-gulonicacid via β L-hydroxy acid dehydrogenase. * NADH is generated. * **STEP 7:** Decarboxylation of 3-Keto-L-Gulonic Acid * Followed by decarboxylation of 3-keto-L-gulonicacid to form L-xylulose,a ketopentose via β-L- gulonate decarboxylase; here, carbon 1 of 3-keto-L-gulonic acid is released as CO2. * **STEP 8:** Oxidation of L-Xylulose * L-xylulose is then reduced to xylitol via xylitoldehydrogenase (or xylulosereductase) * **STEP 9:** Reoxidation of Xylitol * **STEP 10:** Phosphorylation of D-Xylulose * D-xylulose is phosphorylated at carbon 5 to form D-xylulose 5-phosphat via xylulose kinase * Further metabolized via the HMP Shunt * Converted to intermediates of glycolysis for energy production **Regulation of Uronic Pathway** * Administration of drugs i.e. Chlorobutanol & Barbital significantly increases the uronic acid pathway. * Certain drugs are also found to enhance the synthesis of Ascorbic acid. **Significance of Uronic Pathway** * It is an alternative oxidative pathway for glucose. * It is concerned with the synthesis of glucuronic acid, pentoses & vitamin-ascorbic acid (except in primates & guinea pigs). * Major function is to produce D-Glucuronic acid which is required for: * Detoxification of foreign chemicals and synthesis of mucopolysaccharides. * Many wastes in the human body are excreted in the urine as their glucuronate salts. * Iduronic acid is a component of some structural complexes such as proteoglycans. **References** 1. Rodwell, V. W., Botham, K. M., Kennelly, P. J., Weil, P. A., & Bender, D. A. (2015). Harper's illustrated biochemistry (30th ed.). New York, N.Y.: McGraw-Hill Education LLC. 2. John W. Pelley, Edward F. Goljan (2011). Biochemistry. Third edition. Philadelphia: USA. 3. https://www.scribd.com/document/328465491/2-4-Biochemistry-Tca-Hmp-and-Uronic-Acid-Pathway 4. https://www.slideshare.net/sathi3366/presentation-29571791.