carbohydrate metabolism

• Carbohydrates


  Especially glucose, play major roles in cell metabolism

  
• Function of dietary carbohydrates

  To serve as a source of energy
• In a typical diet, 2/3 of daily energy needs are furnished by carbohydrates
• Carbohydrate digestion


  Disaccharides and polysaccharides are hydrolyzed to form monosaccharides, primarily glucose, fructose, and galactose

  
• Absorption of monosaccharides


  Glucose, fructose, and galactose are absorbed into the bloodstream through the lining of the small intestine and transported to the liver

  
• Conversion of fructose and galactose in the liver

  Fructose and galactose are rapidly converted to glucose or to compounds that are metabolized by the same pathway as glucose

  
• Blood sugar


  Glucose is the most plentiful monosaccharide in blood

  
• Normal blood sugar level in adults after a fast of 8-12 hours is 70-110 mg/100 mL

  
• Blood sugar level reaches a maximum of about 140-160 mg/100 mL about 1 hour after a carbohydrate-containing meal, and returns to normal after 2-2.5 hours

  
• Hypoglycemia


  Blood sugar levels are below the normal fasting level, leading to dizziness, fainting, convulsions, and shock

  
• Hyperglycemia


  Blood sugar levels are above the normal fasting level, causing glucose to be excreted in the urine

  
• Renal threshold


  The blood glucose level at which glucose starts to be excreted in the urine

  
• Liver


  • The key organ involved in regulating blood glucose levels
  • Removes glucose from the bloodstream and converts it to glycogens or triglycerides for storage when blood glucose levels are high
  • Converts stored glycogen to glucose and synthesizes new glucose from noncarbohydrate sources when blood glucose levels are low

  
• Glycolysis


  A series of ten reactions, with a net result of converting a glucose molecule into two molecules of pyruvate

  
• Glycolysis


  • All of the enzymes are found in cellular cytoplasm
  • There is a net gain of 2 moles of ATP for every mole of glucose that is converted to pyruvate

  
• Regulation of glycolysis


  Hexokinase, phosphofructokinase, and pyruvate kinase are the three key regulatory enzymes
• Hexokinase


  Catalyzes the conversion of glucose to glucose-6-phosphate and initiates the glycolysis pathway, inhibited by high concentration of glucose-6-phosphate

  
• Phosphofructokinase


  Catalyzes the irreversible conversion of fructose 6-phosphate to fructose 1,6-bisphosphate, inhibited by high concentrations of ATP and citrate, and activated by high concentrations of ADP and AMP

  
• Pyruvate kinase


  Catalyzes the conversion of 3-phosphoenolpyruvate to pyruvate, inhibited by high concentrations of ATP

  
• When ATP use decreases

  ATP concentration increases, binding to phosphofructokinase and pyruvate kinase, slowing down their activity and thus slowing the glycolysis pathway
• When ATP concentrations are low

  ADP and AMP concentrations are high, activating phosphofructokinase and accelerating the glycolysis pathway
• Fates of pyruvate


  1. Oxidation to acetyl CoA under aerobic conditions

  2. Reduction to lactate under anaerobic conditions

  3. Reduction to ethanol under anaerobic conditions for some prokaryotic organisms

  
• Oxidation of pyruvate to acetyl CoA

  Under aerobic conditions, pyruvate is oxidized in the mitochondria to form acetyl CoA, which can enter the citric acid cycle or be used for fatty acid biosynthesis

  
• Reduction of pyruvate to lactate

  Under anaerobic conditions, pyruvate is reduced to lactate to regenerate NAD+ for glycolysis, producing 2 ATP

  
• Reduction of pyruvate to ethanol

  Some organisms, including yeast, regenerate NAD+ under anaerobic conditions by alcoholic fermentation, decarboxylating pyruvate to acetaldehyde and then reducing acetaldehyde to ethanol

  
• Complete aerobic oxidation of glucose is 16 times more efficient than lactate fermentation or alcoholic fermentation, producing 32-36 ATP compared to 2 ATP

  
• The total energy available in glucose is 686 kcal/mol, and the efficiency of energy storage in ATP is 34%

  
• Alcoholic fermentation


  1. Decarboxylation (removing CO2) of pyruvate to produce acetaldehyde

  2. Acetaldehyde is then reduced by NADH to form ethanol (also regenerating NAD+ for glycolysis)

  
• The CO2 thus produced causes beer to foam and wine and champagnes to bubble
• Only 2 mol of ATP is produced per mole of glucose by lactate fermentation and alcoholic fermentation

  
• Complete aerobic oxidation of glucose is 16 times more efficient than either of these processes

  
• The total energy available in glucose is 686 kcal/mol

  
• The synthesis of 32 mol of ATP stores 234 kcal/mol
• Living cells can capture 34% of the released free energy and make it available to do biochemical work
• Automobile engines make available 20-30% of the energy actually released by burning gasoline
• Glycogenesis


  1. Excess glucose is converted into glycogen

  2. Glycogen is stored primarily in the liver and muscle tissue

  3. The energy is provided by the hydrolysis of uridine triphosphate (UTP; uracil + ribose + three phosphates)

  
• Glycogenolysis


  1. Breakdown of glycogen back into glucose

  2. Glycogen phosphorylase cleaves the α(1-4) linkages, releasing glucose 1-phosphate

  3. A debranching enzyme hydrolyzes the α(1-6) linkages

  4. Phosphoglucomutase isomerizes glucose 1-phosphate to glucose 6-phosphate

  5. Glucose 6-phosphatase hydrolyzes glucose 6-phosphate to free glucose (only in liver, kidney, and intestinal cells)

  
• Muscle cells lack glucose 6-phosphatase and cannot form free glucose from glycogen
• Liver glycogen is broken down all the way to form free glucose, which is released into the blood during muscular activity and between meals

  
• Gluconeogenesis


  1. Synthesizing glucose from noncarbohydrate materials

  2. When carbohydrate intake is low and glycogen stores are depleted, the carbon skeletons of lactate, glycerol (derived from the hydrolysis of fats), and certain amino acids are used to synthesize pyruvate, which is then converted to glucose