Glycogen Metabolism (8.1) - Complex Carbohydrates - PDF

8.1 Glycogen Metabolism The Structure and Function of Glycogen  Glycogen Glycogen = a polymeric storage form of glucose in animals that is found primarily in muscle and liver Glycogen breakdown in muscle delivers glucose needed for muscle contraction within...

8.1 Glycogen Metabolism The Structure and Function of Glycogen  Glycogen Glycogen = a polymeric storage form of glucose in animals that is found primarily in muscle and liver Glycogen breakdown in muscle delivers glucose needed for muscle contraction within seconds Glycogen stored in the liver provides a reservoir that maintains homeostasis of blood glucose  Glycogen Function Glycogen provides vertebrate animals with a ready source of glucose to supply the brain and skeletal muscles with energy. Although animals store about 100 times more energy as fat than as glycogen, they cannot metabolize fat into glucose. The highly branched polymeric structure of glycogen granules allows cells in the liver and muscle to make large numbers of glucose and glucose phosphate monomers available quickly without raising the osmolarity of the cytosol by storing them in monomeric form. Glycogen Granules Have Many Tiers of Branched Chains of D-Glucose Glycogen β-granules are cytosolic granules that vary in size, structure, and subcellular location – appear as electron-dense particles  β-Granules Cluster to Form α- Granules in the Liver α-granules are protein-rich granules composed of 20-40 clustered β-granules – release glucose slower than β-granules – visible in well-fed animals, but absent after a 24-hour fast – often associate with tubules of the smooth ER Structure of a Glycogen β-Granule Breakdown and Synthesis of Glycogen Glycogen Synthesis - Glycogenesis Glycogenesis is the synthesis of glycogen Glycogen synthesis requires a protein primer and an activated glucose precursor. Individual glucose molecules activated as sugar nucleotides are added to the nonreducing end of the growing linear chains in the outer tiers of the glycogen β-granules, and a branching enzyme adds branches periodically. Fates of Glucose Glycogenesis Pathway  Glycogen Synthesis Begins With Glucose 6-Phosphate sources of glucose 6-phosphate: – hexokinase isozymes derive glucose 6-phosphate from glucose – lactate taken up by the liver is converted to glucose 6-phosphate by gluconeogenesis Phosphoglucomutase = converts glucose 6-phosphate to glucose 1-phosphate UDP-Glucose Pyrophosphorylase Catalyzes A Key Step of Glycogen Biosynthesis Glycogen-Branching Enzyme Storage Location of Glycogen and Role  Glycogen Breakdown - Glycogenolysis Glycogenolysis is the breakdown of cellular glycogen to glucose 1-phosphate Advantages – Rapid energy source – Anaerobically generated – 3 ATP produced per glucose in contrast to non-glycogen glucose Disadvantages – Low ATP/Mass – Limited storage  Glycogen Breakdown - Glycogenolysis Monomers are released from glycogen granules by a phosphorolysis reaction that creates phosphorylated glucose molecules that can enter glycolysis to supply energy to the cell. Skeletal muscle cells especially require stores of glycogen to supply energy for bursts of activity. In the liver, the phosphate can be removed, allowing free glucose to be transported out of the cell to the blood for use in the brain and other tissues when dietary glucose is not sufficient. Glycogen Breakdown Is Catalyzed by Glycogen Phosphorylase Debranching Enzyme Glucose 1-Phosphate Can Enter Glycolysis or, in Liver, Replenish Blood Glucose phosphoglucomutase = catalyzes the reversible conversion of glucose 1-phosphate to glucose 6-phosphate  Fates of Glucose 6-Phosphate in skeletal muscle, glucose 6-phosphate enters glycolysis in liver, glucose 6-phosphatase converts glucose 6- phosphate to glucose in the ER for export to replenish blood glucose  The Sugar Nucleotide UDP-Glucose Donates Glucose for Glycogen Synthesis sugar nucleotides = compounds in which the anomeric carbon of a sugar is activated by attachment to a nucleotide through a phosphate ester linkage – involved in reactions where hexoses are transformed or polymerized Coordinated Regulation of Glycogen Breakdown and Synthesis  Glycogen Regulation Regulation of the balance between the formation of glycogen from excess glucose and the release of glucose from glycogen polymers when it is needed in metabolism is a critical function of cellular and organismal homeostasis. This balance, ultimately controlled by the hormones epinephrine, glucagon, and insulin, is achieved through allosteric regulation and phosphorylation of the synthetic and degradative enzymes. These enzymes and the regulatory proteins that act on them are integral parts of the glycogen granule. Glycogen Phosphorylase Is Regulated by Hormone-Stimulated Phosphorylation and by Allosteric Effectors skeletal glycogen phosphorylase has two forms: – glycogen phosphorylase a = catalytically active – glycogen phosphorylase b = much less active  Regulation of Muscle Glycogen Phosphorylase by Covalent Modification epinephrine (from vigorous muscle activity) and glucagon (in the liver) trigger phosphorylation of phosphorylase b, converting it to phosphorylase a  Elevated [cAMP] Initiates an Enzyme Cascade enzyme cascade = sequence of enzymatic reactions in which a catalyst activates a catalyst, which activates a catalyst rise in [cAMP] activates PKA, which phosphorylates phosphorylase b kinase phosphorylase b kinase = catalyzes the phosphorylation of glycogen phosphorylase b  Allosteric Control Mechanisms Ca2+ is a signal for muscle contraction – binds to and activates phosphorylase b kinase AMP accumulates in vigorously contracting muscle – binds to and activates phosphorylase to speed up glucose 1-phosphate release from glycogen ATP blocks the allosteric site, inactivating phosphorylase  Phosphoprotein Phosphatase 1 (PP1) phosphoprotein phosphatase 1 (PP1) = removes phosphoryl groups from phosphorylase a, converting it to the less active form, phosphorylase b activated by insulin inhibited by glucagon Liver Glycogen Phosphorylase a is a Glucose Sensor glucose binds to an allosteric site on phosphorylase a, making it more susceptible to dephosphorylation by PP1 Glycogen Synthase Also Is Subject to Multiple Levels of Regulation glycogen synthase has two forms: – glycogen synthase a = unphosphorylated and catalytically active – glycogen synthase b = phosphorylated and inactive unless glucose 6-phosphate is present glycogen synthase kinase 3 (GSK3) = catalyzes the phosphorylation of glycogen synthase a  Effects of GSK3 on Glycogen Synthase Activity insulin inactivates GSK3 and activates PP1 glucose 6- phosphate acts allosterically to make glycogen synthase b a better substrate for PP1 The Action of GSK3 is Hierarchical GSK3 cannot phosphorylate glycogen synthase until casein kinase II (CKII) has phosphorylated the glycogen synthase on a nearby residue (a priming event)  Allosteric and Hormonal Signals Coordinate Carbohydrate Metabolism Globally in the liver: – insulin activates glycogen synthase by inactivating GSK3 and activating PP1 – glucagon stimulates glycogen breakdown and gluconeogenesis while blocking glycolysis in muscle: – epinephrine provides ATP by stimulating glycogen breakdown and glycolysis Summary of Glycogen Regulation Fate of Glucose-1-Phosphate in Liver and Muscles 8.2 METABOLISM OF GALACTOSE FRUCTOSE & AMINO SUGARS  Lactose, present in milk & milk products. Principal dietary source of galactose. Lactase (β-galactosidase) of intestinal mucosal cells hydrolyses lactose to galactose and glucose. Galactose is also produced from lysosomal degradation of glycoproteins & glycolipids.  Galactose is metabolised almost exclusively by the liver and therefore galactose tolerance test is done to assess the functional capacity of the liver UDP-galactose is the active donor of galactose during synthetic reactions  Step: 1 Galactokinase reaction: Galactose is first phosphorylated by galactokinase to galactose -1- phosphate Step: 2 Galactose -1- phosphate uridyl transferase This is the rate limiting enzyme.  Galactose 1-phosphate reacts with UDP-glucose to form UDP- galactose & glucose 1- phosphate, in the presence of the enzyme Galactose 1- phosphate uridyl transferase  UDP-galactose is an active donor of galactose. UDP-galactose is essential for the formation of compounds like lactose, glycosaminoglycans, glycoproteins, cerebrosides & glycolipids. Step: 3 Epimerase reaction: UDP-galactose can be converted to UDP- glucose by UDP hexose 4-epimerase  Galactose is channeled to the metabolism of glucose. Galactose is not an essential nutrient since UDP-glucose can be converted to UDP — galactose by the enzyme UDP-hexose 4- epimerase and requires NAD+ Galactose is not essential in diet  Step: 4 Alternate pathway: The galactose 1-phosphate pyrophosphorylase in liver becomes active only after 4 or 5 years of life The enzyme will produce UDP- galactose directly which can be epimerized to UDP- glucose.  Disorders of galactose metabolism Classical galactosemia: Due to deficiency of enzyme galactose 1- phosphate uridyltransferase Rare congenital disease in infants Inherited as an autosomal recessive disorder  Salient features Due to the block in this enzyme, galactose 1-phosphate will accumulate in liver. This will inhibit galactokinase as well as glycogen phosphorylase It results in hypoglycemia. Galactose cannot be converted to glucose Increased galactose level increases insulin secretion, which lowers blood glucose level.  Galactose metabolism is impaired leading to increased galactose levels in circulation (galactosemia) & urine (galactosuria) Bilirubin uptake is less & bilirubin coniugation is reduced. Unconiugated bilirubin level is increased.  There is enlargement of liver, jaundice & severe mental retardation — due to accumilation of galactose & galactose 1- phosphate.  Development of cataracts Causes: Excess of galactose in lens is reduced to galactitol (dulcitol) by the enzyme aldose reductase Galactitol cannot escape from lens cells Osmotic effect of the sugar alcohol contributes to injury of lens proteins & development of cataracts.  Galactokinase deficiency The defect in the enzyme galactokinase. Results in galactosemia & galactosuria Dulcitol or galactitol is formed Absence of hepatic and renal complications. Development of cataracts very rare. Treatment: Removal of galactose & lactose from the diet.  Fructose metabolism Fructose is present in fruit juices & honey. Chief dietary source is sucrose. Sucrose is hydrolyzed in the intestine by the enzyme sucrase. Fructose is absorbed by facilitated transport and taken by portal blood to liver. It is mostly converted to glucose.  Biomedical Importance of Fructose Fructose is easily metabolized & a good source of energy Seminal fluid is rich in fructose & spermatozoa utilizes fructose for energy. In diabetics, fructose metabolism through sorbitol pathway may account for the development of cataract.  Fructose metabolism Fructose is phosphorylated to form fructose 6-phosphate, catalyzed by the enzyme hexokinase Affinity of the enzyme hexokinase for fructose is very low  Fructose is mostly phosphorylated by fructokinase to fructose-1-phosphate Fructokinase is present in liver, kidney, muscle and intestine. Hexokinase can also act on fructose to produce fructose 1- phosphate.  Fructose-1-phosphate is cleaved to glyceraldehyde & dihydroxy acetone phosphate (DHAP) by aldolase B Glyceraldehyde is phosphorylated by triokinase to glyceraldehyde 3- phosphate, along with DHAP enters glycolysis or gluconeogenesis.  SORBITOL / POLYL PATHWAY It involves the conversion of glucose to fructose via sorbitol Sorbitol pathway is higher in uncontrolled diabetes The enzyme aldose reductase reduces glucose to sorbitol in the presence of NADPH Sorbitol is then oxidized to fructose by Sorbitol dehydrogenase and NAD+ SORBITOL PATHWAY IN DIABETES MELLITUS In uncontrolled diabetes, large amounts of glucose enter the cells which are not dependent on insulin The cells with increased intracellular glucose levels in diabetes (lens, retina, nerve cells, kidney etc) possess high activity of aldose reductase and sufficient supply of NADPH.  This results in a rapid & efficient conversion of glucose to sorbitol The enzyme Sorbitol Dehydrogenase is either low in activity or absent in these cells. Sorbitol is not converted to fructose. Sorbitol cannot freely pass through the cell membrane and accumulate in the cells.  Sorbitol-due to its hydrophilic nature-causes osmotic effects leading to swelling of the cells. Pathological changes associated with diabetes are due to accumulation of sorbitol.  DEFECTS IN FRUCTOSE METABOLISM Essential fructosuria: Deficiency of the enzyme hepatic fructokinase. Fructose is not converted to fructose 1- phosphate. Excretion of fructose in urine. Treatment: Restriction of dietary fructose Urine gives positive benedicts & seliwanoff's test Hereditary Fructose Intolerance An autosomal recessive inborn error. Due to defect in the enzyme aldolase- B. Fructose 1-phosphate, cannot be metabolised Intracellular accumulation of fructose 1- phosphate will inhibit glycogen phosphorylase. Leads to accumulation of glycogen in liver & associated with hypoglycemia  Symptoms Vomiting, loss of appetite, hepatomegaly & jaundice. If liver damage progresses, death will occur. Fructose is excreted in urine. Restriction of dietary fructose.  AMINO SUGARS One or more hydroxyl groups of the monosaccharides are replaced by amino groups E.g.D-gIucosamine, D- galactosamine, mannoseamine, sialic acid They are present as constituents of GAG's, glycolipids & glycoproteins. Also found in some oligosaccharides & antibiotics.  The amino groups of amino sugars are sometimes acetylated e.g.N- acetyI D-glucosamine Fructose 6-phosphate is major precursor for glucosamine, N- acetylgalactosamine & NANA N-Acetyl neuramic acid (NAN) is derivative of N-Acetyl mannose & pyruvic acid 20% of glucose is utilized for the synthesis of amino sugars, which mostly occurs in the connective tissues. 8.1 Complex Carbohydrates  Complex Carbohydrates Complex carbohydrates are carbohydrates composed of long chains of sugar molecules, known as polysaccharides Diet: rich in fiber, vitamins, and minerals found in whole foods such as grains, vegetables, and legumes and are an essential part of a healthy diet due to their slower digestion and sustained energy release.  Glycoprotein A glycoprotein is a protein molecule that has one or more carbohydrate (glycan) chains covalently attached to its amino acid side chains. The carbohydrate portion can be simple or complex. The primary structure of a glycoprotein is the protein, which is modified by the addition of carbohydrates (glycans). These glycans can be complex, branched chains composed of various sugars like mannose, glucose, galactose, fucose, N-acetylglucosamine, and sialic acid.  Glycoprotein 1. Glycosaminoglycans (GAGs) 2. Proteoglycans 3. Glycoproteins  Glycosaminoglycan (GAG) GAGs are long, unbranched polysaccharides consisting of repeating disaccharide units. These units usually contain an amino sugar (e.g., glucosamine or galactosamine) and a uronic acid (e.g., glucuronic acid or iduronic acid). Highly negatively charged due to sulfate and carboxyl groups, which allow them to attract water and ions. They provide structural support in the extracellular matrix, contribute to the hydration of tissues, and are involved in cell signaling, lubrication, and shock absorption. Examples: Hyaluronic acid, chondroitin sulfate, heparan sulfate, and keratan sulfate. Glycosaminoglycan (GAG) Glycosaminoglycan (GAG)  Proteoglycan Proteoglycans are large molecules consisting of a core protein to which one or more GAG chains are covalently attached. The core protein is heavily glycosylated with GAG chains, giving proteoglycans a "bottlebrush" appearance. The extensive GAG chains contribute to the molecule's high negative charge. They play critical roles in maintaining the structural integrity of tissues, influencing cell adhesion, migration, and signaling, and regulating the movement of molecules through the extracellular matrix. Examples: Aggrecan (found in cartilage), decorin, and syndecan. Amino Sugars Acidic Sugars Synthesis of GAG Degradatio n and Clinical Correlates  Mucopolysaccharidoses Hereditary diseases (1:25,000 births) caused by a deficiency of any one of the lysosomal hydrolases normally involved in the degradation of heparan sulfate and/or dermatan sulfate. They are progressive disorders characterized by accumulation of glycosaminoglycans in various tissues, causing a range of symptoms, such as skeletal and extracellular matrix, deformities, and mental retardation.  Glycoprotein A glycoprotein is a protein molecule that has one or more carbohydrate (glycan) chains covalently attached to its amino acid side chains. The primary structure of a glycoprotein is the protein, which is modified by the addition of glycans. These glycans can be complex, branched chains –N-linked: Attached to the nitrogen atom of asparagine residues, often featuring complex, branched structures. –O-linked: Attached to the oxygen atom of serine or threonine residues, typically simpler in structure. The carbohydrate portion varies in composition and branching, contributing to the diversity of glycoproteins.  Function Cell Recognition and Signaling: Glycans facilitate cell-cell interactions and communication by acting as recognition sites for receptors and other molecules. Immune Response: They play a critical role in immune functions, including the recognition of pathogens and modulation of immune cell activity. Protein Stability: The attached glycans help stabilize the protein structure, influencing its activity and lifespan in biological systems. Cell Adhesion: Glycoproteins are involved in binding cells to each other and to the extracellular matrix, which is vital for tissue formation and repair. Synthesis Transport and Clinical Correlates  Lysosome Degradation Degradation of glycoproteins is similar to that of the glycosaminoglycans. The lysosomal acid hydrolases are each generally specific for the removal of one component of the glycoprotein. They are primarily exoenzymes that remove their respective groups in sequence in the reverse order of their incorporation (“last on, first off”). If any one degradative enzyme is missing, degradation by the other exoenzymes cannot continue. Glycoprotein Storage Disease A group of very rare, autosomal recessive genetic diseases called the glycoprotein storage diseases (oligosaccharidoses), caused by a deficiency of any one of the degradative enzymes, results in accumulation of partially degraded structures in the lysosomes. For example, α-mannosidosis type 1 is a progressive, fatal deficiency of the enzyme, α-mannosidase. Presentation is similar to Hurler syndrome, and immune deficiency is also seen. Mannose-rich oligosaccharide fragments appear in the urine.