Glycogen Metabolism PDF

**Glycogen metabolism, disaccharide metabolism and the pentose phosphate pathway** **Learning Objectives** - Understand the importance of glycogen in maintaining blood glucose levels - Understand the pathways involved in glycogen synthesis and degradation - Differentiate between va...

**Glycogen metabolism, disaccharide metabolism and the pentose phosphate pathway** **Learning Objectives** - Understand the importance of glycogen in maintaining blood glucose levels - Understand the pathways involved in glycogen synthesis and degradation - Differentiate between various glycogen storage diseases - Understand the mechanisms of fructose and galactose metabolism - Understand the importance of enzymes involved in inborn errors of fructose and galactose metabolism - Understanding of the importance of the pentose-phosphate pathway and relevant enzymes and co-factors Key = any enzyme associated with clinical disease + understanding of how hormones regulate metabolism **Glycogen** - Major storage form of carbohydrate in animals - Branched polymer consisting of D-glucose units - α1-4-linked glucose with α1-6-linked side-branches (know difference between these) - Mainly stored in liver and in muscle - Glycogen in liver helps maintain euglycemia during fasting periods - Glycogen in muscle provides energy to muscle but does not supply glucose for circulation (only glycogen from liver does) **Glycogen --advantages to carbohydrate storage** - Energy cannot be yielded from fat in the same space of time as from glycogen - Some organs such as the brain preferentially burn glucose --liver glycogen can provide this during shorter stretches of fasting until depleted - RBCs can only utilize glucose as their energy source through anaerobic glycolysis **Glycogen structure** - UDP-glucose supplies the glucose moieties used during glycogen synthesis - New glucose molecules are attached at non-reducing ends of the molecule - At the reducing end one glycose molecule is attached to the protein glycogen in which primes glycogen synthesis - The reducing end (R) can be oxidised --typically at carbon 1 in the ring structure. Reducing end - Branching allows for quicker release of glycogen **Glycogen synthesis --regulated by glycogen synthase** Glucose-1-phosphate is used during glycogen synthesis (not 6 phosphate) UDP-glucose is used as a substrate by glycogen synthase Glycogen synthase is active in its dephosphorylated form and is positively regulated by insulin (hexokinase is glucokinase in the liver) **Glycogenin** - Linkage of the first few glucose units to form the minimal \"primer\" needed for glycogen synthase is catalyzed by Glycogenin, which attaches to the first glucose and catalyzes linkage of the first eight glucoses by alpha(1,4) bonds. - The primers which are attached to glycogenin are partially degraded glycogen molecules **Glycogen synthesis --formation of branches** - Highly branched molecule - Every 7-11 residues - Increases solubility - Increases number of non-reducing ends upon which glucose units can be added (and in turn removed) - Enzyme = glucosyl4:6 transferase (branching enzyme) --breaks α1,4 bond and creates an α1,6 bond **Glycogenolysis--breaking down of glycogen to yield glucose** - Glycogen phosphorylase key enzyme - Uses inorganic phosphate (Pi) to cleave α1,4 bonds, producing glucose-1-phosphate - Acts until there are 4 glucose units left till branch point - Four units remaining are removed by debranching enzyme--which has glucosyl4:4 transferase and α1,6 glucosidase activity. - Debranching enzyme is a single polypeptide with two catalytic sites, amylo-1,6-glucosidase and 4-alpha-glucanotransferase. **Glucose-6-phosphatase** - Glucose-6-phosphatase is used by both gluconeogenesis and glycogenolysis to release free glucose from glucose-6-phosphate (also common to both pathways) - Genetic deficiency of glucose-6-phosphatase observed in von Gierke's glycogen storage disease and results in profound hypoglycaemias both gluconeogenic and glycogenolytic pathways are affected. **Glucagon --Insulin Axis** - Glucagon and epinephrine are catabolic to release energy during periods of fasting/flight-fight response, so they act to break down glycogen to glucose --use cAMPas a second messenger and generally phosphorylate target enzymes - Insulin is anabolic and stimulates glycogenesis --insulin levels rise in the fed state and its role is to stimulate energy storage for periods of fasting **Hormonal Regulation of Glycogen Metabolism** Insulin turns on glycogen synthase and turns off glycogen phosphorylase to stimulate glycogen synthesis (via dephosphorylation) Glucagon turns off glycogen synthase and turns on glycogen phosphorylase to stimulate glycogen synthesis (via phosphorylation) **Fate of muscle glycogen** Muscle does not contain glucose-6-phosphatase (why it doesn't release glucose into circulation) so the glucose-1-P produced by glycogenolysis provides energy via glycolysis for contraction **Glycogen Storage Diseases** - Glycogen break-down is impaired, primarily in the liver or muscle or both - In liver deficiency --enlarged liver (hepatomegaly), hypoglycaemia and lactic acidosis often observed - In muscle deficiency --exercise intolerance or cardiorespiratory failure can be observed (not hypoglycaemia -- muscle doesn't contain glucose 6 phosphate) - Most glycogen is broken down in the cytoplasm, but some is also broken down in lysosomes by α-glucosidase enzyme --mutation of which results in the fate glycogen storage disease Pompe disease. - Hypoglycaemias often accompanied by high ketone levels and lactic acidosis as the body tries to restore glucose and provide alternative fuel sources for the body Know difference between liver and muscle deficiency!! Be able to tell difference between glycogen storage disease and FA oxidation disease Body relies on fatty acids and ketones in hypoglycaemia FA = low ketones + hypoglycaemia Glycogen = high ketones + hypoglycaemia **Glycogen storage diseases** - Type I - GSDI - von Gierke disease --build-up of glycogen in body's cells. Hypoglycaemia and lactic acidosis can occur. Mutation in Glucose-6-phosphatase - Type III -GSDIII --Cori's Disease --debranching enzyme deficiency. Indistinguishable clinically from Type 1 --glycogen structure abnormal!! - Type V -GSDV --McArdle's --muscle glycogen phosphorylase absent = exercise intolerance. Least severe form of disease **Pompe Disease** - **Glycogen storage disease type II/lysosomal dysfunction** - Pompe disease is a rare (estimated at 1 in every 40,000 births), inherited and often fatal disorder that disables the heart and skeletal muscles. - Acid alpha-glucosidase (GAA) (acid maltase) mutation (also called α-1,4-glucosidase) - Classic form = infantile onset - Infants with this disorder typically experience muscle weakness (myopathy), poor muscle tone (hypotonia), an enlarged liver (hepatomegaly), and heart defects. - Lysosomal glycogen not necessary for maintaining blood glucose so hypoglycaemia not observed - Enzyme replacement therapy--increases lifespan Glycogen is stored in lysosomes -- lysosomes become severely impaired due to accumulation of polysaccharide **Cori-disease --Glycogen storage disease type III** - Autosomal recessive inherited disease - Mutation in debranching enzymes - Characterised by storage of structurally abnormal glycogen, termed limit dextrin (glycogen funny shape under microscope) - Poor muscle tone in children often first sign of this disease - Ketone/FA levels elevated as body switches to other fuel sources other than glycogen - Affects 1:100,000 births MCQ: A 3-month-old male is being evaluated for muscle hypotonia and feeding difficulties. Physical examination reveals hepatomegaly and severe cardiomegaly. Muscle biopsy shows polysaccharide accumulation within the lysosomes. Which of the following enzymes is deficient in this patient? A. Glucose-6-phosphate B. Glycogen phosphorylase C. Acid alpha-glucosidase D. Debrancher enzyme E. Galactokinase F. Pyruvate kinase **Fructose Transport** - Glucose and galactose use the sodium-glucose co-transporter system for absorption into the enterocyte (active transport --required energy) - Fructose uses the GLUT5 transporter for absorption into the enterocyte. - All three sugars, fructose, galactose and glucose enter the portal circulation where they are readily taken up by GLUT2 in the liver. The bulk of fructose is extracted by first pass in the liver. - Absorption of fructose does not induce an insulin response Dietary sources of fructose: - Sucrose, fruit and high-fructose corn syrup (HFCS) - Usually first exposed to fructose at around 6mths of age upon weaning which is when disorders of fructose metabolism usually present (Age is critical -- glucose and galactose are given to babies much earlier) - HFCS is found in multiple foods as a sweetener from yoghurts to fizzy drinks **Fructose metabolism -glycolysis** - In tissues other than liver fructose is phosphorylated by hexokinase to form fructose-6-phosphate which enters glycolysis. - The liver version of hexokinase (glucokinase) does not recognise fructose meaning metabolism is different in the liver. **Fructose Metabolism in the liver** - In the liver fructose can be converted to pyruvate or under fasting conditions glucose by feeding into the glycolytic/gluconeogenic pathway - Fructose phosphorylated on C1 by fructokinase to form fructose-1-P - This pathway bypasses the first major regulatory step of glycolysis -phosphofructokinase (PFK) step - Not reliant on insulin -- quicker metabolism **Disorders of fructose metabolism** Aldolase B deficiency: - (1/20000 births) (severe)--Infants healthy until they ingest fructose. Fructose-1-phosphate accumulates causing hypoglycaemia, nausea & vomiting, abdominal pain, sweating, tremors, confusion, lethargy, seizures, and coma. - Fructose-1-phosphate is a competitive inhibitor of phosphorylase (glycogen degradation) and thus accumulation linked to hypoglycaemia. - Prolonged ingestion may cause cirrhosis, mental deterioration, and proximal renal tubular acidosis with urinary loss of phosphate and glucose. - This deficiency causes benign elevation of blood and urine fructose levels (benign fructosuria). - The condition is asymptomatic and diagnosed accidentally when a non-glucose reducing substance is detected in urine. **Galactose metabolism** - Galactose found in milk/dairy products - Converted to intermediates of the glucose pathway - UDP galactose also used for synthesis of glycoproteins/glycolipids and proteoglycans Important enzymes: Glactokinase, GALT **Galactosemias** - Appearance of high concentrations of galactose in the blood after lactose intake may be due to either a galactokinase deficiency or a uridylyltransferase deficiency. - Classic galactosemia results from deficiency of galactose-1-phosphate uridyltransferase (GALT); this defect is the most common cause of galactosemia. The clinical features of this illness include vomiting, lethargy and failure to thrive soon after breastfeeding is begun. - Non-classical galactosemia results due to galactokinase deficiency --galactose accumulates and is reduced to galactitol, which causes cataracts. - Lack of uridylyltransferase is more serious as galactose-1-phosphate accumulates and interferes with glycogen synthesis and degradation. Accumulation of fructose-1-phosphate or galactose-1-phosphate more toxic to the cell than fructose or galactose and thus enzymes that impair their metabolism, resulting in accumulation of the monophosphate-sugar, presents with more severe disease. **Pentose Phosphate Pathway (the hexose monophosphate shunt**) - Important for DNA and RNA -- 5 carbon sugars - Main pathway for generation of NADPH and ribulose-5-phosphate - Occurs in the cytoplasm. - Branches from glycolysis at the level of glucose-6-phosphate - It is not a direct energy yielding pathway - NADPH produced is an important reduction reagent required for fatty acid synthesis as well as glutathione reduction - Reduction of glutathione protects against oxidative damage - Ribulose-5-phosphate provides ribose-5-phosphate which is used for the synthesis of nucleotides - Ribulose-5-phosphate can also be converted back to glycolytic intermediates (fructose-6-phosphate or glyceraldehyde 3-phosphate) for the generation of pyruvate **Pentose phosphate pathway** - **Oxidative pathway** - generates NADPH and ribulose-5-phosphate - Is an irreversible reaction - When NADPH levels are low the oxidative reactions of the pathway are used to generate ribose-5-phosphate for nucleotide synthesis **Oxidative Stage of Pentose Phosphate Pathway --glucose-6-dehydrogenase** - NADPH is synthesised by two dehydrogenase complexes at the 1st and 3^rd^ step of the pentose phosphate pathway - 6-phosphogluconate dehydrogenase is responsible for the decarboxylation of 6-phosphogluconate to yield the 5C sugar ribulose-5-phosphate - This pathway is irreversible **Pentose phosphate pathway and NADPH** - NADPH is a reducing agent - Tissues with active lipid synthesis (cholesterol, bile salts, steroid hormones, triglycerides) NADPH is required for the redox reactions - Liver also uses NADPH for detoxification of and excretion of drugs - NADPH is also used for the reduction of the important anti-oxidant glutathione **Pentose phosphate oxidative and non-oxidative pathways** - **Non-oxidative pathway** - Reversible - Pentose-phosphates produced can be used for generation of glycolytic intermediates - Similarly glycolytic intermediates can be used to generate ribose-5-phosphate for nucleotide synthesis via this pathway - When NADPH levels are high the non-oxidative pathway can be used to generate of ribose-5-phosphate **Glucose-6-phosphate dehydrogenase deficiency** - X-linked chromosomal disorder - Mainly affects red blood cells (RBC) - Haemolytic anaemia observed -usually in response to a trigger e.g. anti-malarial drug - Anti-malarial drugs usually undergo a redox reaction in the cell producing large quantities of reactive oxygen species - Insufficient production of NADPH in affected persons causes excessive oxidative damage to RBC and lysis occurs - 400 million people worldwide affected/ 200 known mutations - Natural protection against malaria