Glucose from module

• Carbohydrates


  Hydrates of carbon, organic compounds composed of carbon, hydrogen and oxygen, contain C=O and -OH functional groups

  
• Functions of carbohydrates


  • Provide a significant fraction of dietary calories and act as a storage form of energy
  • Serve as cell membrane components that mediate some forms of intercellular communication
  • Serve as a structural component of many organisms (RNA, DNA, cell walls, exoskeletons, cellulose)
• Classification of carbohydrates by size of base carbon chain

  • Trioses (3 carbons)
  • Tetroses (4 carbons)
  • Pentoses (5 carbons)
  • Hexoses (6 carbons)
  • Heptoses (7 carbons)
  • Nonoses (9 carbons)

  
• Aldose


  Has a terminal aldehyde group (O=CH-)
• Ketose


  Has a ketone group (O=C) in the middle linked to 2 other carbon atoms
• Stereoisomers (enantiomers)


  Compounds that are identical in composition and differ only in spatial configuration
• Types of carbohydrates by number of sugar units

  • Monosaccharides
  • Disaccharides
  • Oligosaccharides
  • Polysaccharides

  
• Monosaccharides


  Simple sugars that consist of a single polyhydroxy aldehyde or ketone unit, cannot be hydrolyzed to a simpler form, all are reducing agents

  
• Disaccharides


  Formed when 2 monosaccharides are joined by a glycosidic linkage, on hydrolysis will be split into 2 monosaccharides

  
• Common disaccharides


  • Maltose (glucose + glucose)
  • Lactose (glucose + galactose)
  • Sucrose (glucose + fructose)

  
• Reducing carbohydrates


  Contain a ketone or aldehyde group, can reduce other compounds
• Nonreducing carbohydrates


  Do not have an active ketone or aldehyde group, most common is sucrose

  
• Oligosaccharides


  Chaining of 2 to 10 sugar units, found in glycoproteins

  
• Polysaccharides

  Provide structural support, formed by the linkage of many monosaccharide units

  
• Glucose


  The primary source of energy for humans
• Nervous tissue


  • Totally depends on glucose from the extracellular fluid (ECF) for energy
  • Cannot concentrate or store carbohydrates
• Glucose concentration in the blood is maintained within a fairly narrow interval under diverse conditions (feeding, fasting or severe exercise)
• When the concentration of glucose in the nervous tissue falls below a certain level, the tissue loses its primary source of energy and becomes incapable of maintaining normal function
• Fate of Glucose


  1. Most ingested carbohydrates are polymers (starch, glycogen)

  2. Polymers are converted to disaccharides

  3. Disaccharides are converted to monosaccharides by enzymes

  4. Monosaccharides are absorbed by the gut and transported to the liver

  5. Glucose is the only carbohydrate directly used for energy or stored as glycogen

  6. Galactose and fructose must be converted first to glucose before they can be utilized as source of energy

  7. Once glucose enters cell, it is shunted into 1 of 3 metabolic pathways
• Energy production from glucose

  1. Conversion to CO2 and H2O through the Krebs (Citric acid cycle/ TCA cycle) and the mitochondrial electron transport chain (ETC) coupled to oxidative phosphorylation

  2. Oxidation of glucose to CO2 and H2O also occurs through the hexose monophosphate shunt (pentose phosphate pathway) or the Embden-Meyerhof pathway (Glycolytic pathway/Glycolysis) with production of NADPH
• Storage of glucose


  As glycogen in the liver or as triglyceride in adipose tissue
• Conversion of glucose


  To keto acids, amino acids or protein
• Liver


  • The major player in maintaining stable glucose concentrations
  • Stores glucose as glycogen (glycogenesis)
  • Degrades glycogen (glycogenolysis) depending on the body's needs
  • Creates glucose from non-sugar carbon substrates like pyruvate, lactate, and amino acids (gluconeogenesis) when glycogen becomes depleted
• Pathways in Glucose Metabolism

  1. Glycolysis

  2. Gluconeogenesis

  3. Glycogenolysis

  4. Glycogenesis

  5. Lipogenesis

  6. Lipolysis
• Glycolysis


  Metabolism of glucose molecule (or other hexoses) to pyruvate or lactate for production of energy
• Gluconeogenesis


  Formation of glucose-6 phosphate (G6P) from noncarbohydrate sources
• Glycogenolysis


  Breakdown of glycogen to glucose for use as energy
• Glycogenesis


  Conversion of glucose to glycogen for storage
• Lipogenesis


  Conversion of carbohydrates to fatty acids
• Lipolysis


  Decomposition of fat
• TCA cycle


  The final common pathway in the oxidation of fuel molecules that produce acetyl CoA
• Oxidative phosphorylation


  Generates large amounts of ATP as electrons flow from NADH and FADH2 to oxygen
• Regulation of Carbohydrate Metabolism

  • The liver, pancreas, and other endocrine glands control blood glucose concentrations within a narrow range
• Insulin


  • Anabolic protein hormone produced by b-cells of the islets of Langerhans in the pancreas
  • First substance to be measured by radioimmunoassay (RIA) and the first compound produced by recombinant DNA technology for clinical use
  • Target organs: liver, skeletal muscle and adipose tissue
  • Actions: Stimulates glucose uptake, promotes conversion of glucose to glycogen or fat, inhibits glucose production by the liver, stimulates protein synthesis, inhibits protein breakdown
  • Normal glucose disposal depends on: Ability of the pancreas to secrete insulin, ability of insulin to promote uptake of glucose into peripheral tissue, ability of insulin to suppress hepatic glucose production
  • Only hormone that decreases glucose levels and can be referred to as a hypoglycemic agent
• Counter-regulatory hormones


  • Action: Increases glucose (which is opposite the action of insulin)
  • Catabolic in nature
  • Increase hepatic glucose production initially by enhancing the breakdown of glycogen to glucose (glycogenolysis), and later by stimulating the synthesis of glucose (gluconeogenesis)
• Glucagon


  • A 29-amino-acid polypeptide secreted by the a-cells of the pancreas
  • Primary hormone responsible for increasing glucose levels
  • Major target organ: liver
  • Stimulates production of glucose in the liver by glycogenolysis and gluconeogenesis and enhances ketogenesis in the liver
  • Minor target organ: adipose tissue where the hormone increases lipolysis
  • Long-standing diabetes impairs the glucagon response to hypoglycemia, resulting in increased incidence of hypoglycemic episodes
  • Stress, exercise, and amino acids induce glucagon release
  • Can be referred to as a hyperglycemic agent
• Epinephrine


  • A catecholamine secreted by the adrenal medulla
  • Stimulates glucose production (via gluconeogenesis and glycogenolysis) and decreases glucose use, thereby increasing blood glucose concentrations
  • Stimulates glucagon secretion and inhibits insulin secretion by the pancreas
  • Has a key role in glucose counter-regulation when glucagon secretion is impaired (e.g., in type 1 diabetes)
  • Physical or emotional stress increases epinephrine production, releasing glucose for energy
  • Pheochromocytomas (tumors of the adrenal medulla), secrete excess epinephrine or norepinephrine and produce moderate hyperglycemia
• Growth Hormone


  • Polypeptide secreted by the anterior pituitary gland
  • Stimulates gluconeogenesis, lipolysis, and antagonizes insulin-stimulated glucose uptake
• Cortisol


  • Secreted by the adrenal cortex in response to ACTH
  • A glucocorticoid which stimulates gluconeogenesis
  • Increases breakdown of protein and fat
  • Increased cortisol in Cushing syndrome results in hyperglycemia
  • Decreased cortisol in Addison's disease results in hypoglycemia
• Thyroxine


  • Secreted by the thyroid gland
  • Not directly involved in glucose homeostasis but stimulates glycogenolysis and increase rates of gastric emptying and intestinal glucose absorption
  • May produce glucose intolerance in thyrotoxic individuals