CHAPTER 7

• Carbohydrate digestion


  1. Begins in the mouth

  2. Salivary enzyme "α-amylase" catalyzes the hydrolysis of a-glycosidic linkages of starch and glycogen to produce smaller polysaccharides and disaccharide-maltose

  3. Only a small amount of carbohydrate digestion occurs in the mouth because food is swallowed so quickly into the stomach

  4. In the stomach, very little carbohydrate is digested: No carbohydrate digestion enzymes present in the stomach, Salivary amylase gets inactivated because of stomach acidity

  5. The primary site for carbohydrate digestion is within the [small intestine]

  6. Pancreatic α-amylase breaks down polysaccharide chains into disaccharide maltose

  7. The final step in carbohydrate digestion occurs on the outer membranes of intestinal mucosal cells: Maltase - hydrolyses maltose to glucose, Sucrase - hydrolyses sucrose to glucose and fructose, Lactase - hydrolyses lactose to glucose and galactose
• Absorption of carbohydrates


  1. Glucose, galactose, and fructose are absorbed into the bloodstream through the intestinal wall

  2. Galactose and Fructose are converted to products of glucose metabolism in the liver
• Glycolysis


  Metabolic pathway by which glucose is converted into two molecules of pyruvate, chemical energy in the form of ATP is produced, and NADH-reduced coenzymes are produced
• Embden-Meyerhof pathway


  The name of the glycolysis pathway, after the German chemist Gustav Embden (1874-1933) and Otto Meyerhof (1884-1951), who discovered many of the details of the pathway in the early 1930s
• Pyruvate


  The carboxylate ion produced when pyruvic acid losses its acidic hydrogen atom
• Anaerobic pathway


  A pathway in which molecular oxygen is not a participant
• Aerobic pathway


  Pathways that require molecular oxygen
• 10 step process of glycolysis

  1. Step 1: Phosphorylation using ATP

  2. Step 2: Isomerization - Formation of Fructose 6-Phosphate

  3. Step 3: Phosphorylation using ATP - Formation of Fructose 1, 6-Bisphosphate

  4. Step 4: Formation of Two Triose Phosphate

  5. Step 5: Isomerization - Formation of Glyceraldehyde 3-Phosphate

  6. Step 6: Oxidation and Phosphorylation using Pi

  7. Step 7: Phosphorylation of ADP

  8. Step 8: Isomerization - Formation of 2-Phosphoglycerate

  9. Step 9: Formation of Phosphoenolpyruvate

  10. Step 10: Phosphorylation of ADP - Formation of Pyruvate
• Substrate-level phosphorylation


  A biochemical process whereby ATP is produced from ADP by hydrolysis of a high-energy compound
• Fate of pyruvate


  1. Oxidation to acetyl CoA under aerobic conditions

  2. Pyruvate is formed in the cytosol through glycolysis, crosses the mitochondrial membrane and mitochondrial matrix

  3. Overall reaction involves four separate steps and requires: NAD±, CoA-SH, FAD, and two other coenzymes. lipoic & thiamin pyrophosphate

  4. Citric Acid Cycle operates to change more NAD± to its reduced form, NADH

  5. NADH from glycolysis, from the conversion of pyruvate to Acetyl CoA and from the Citric Acid Cycle enters the electron transport chain directly or indirectly

  6. In the ETC, electrons from NADH are transferred to O2 and NADH is changed back to NAD±
• Fermentation processes


  1. Lactate fermentation - glucose or other six-carbon sugars are converted into cellular energy and the metabolite lactate

  2. Ethanol fermentation - sugars such as glucose, fructose, and sucrose are converted into ethanol and carbon dioxide by the action of microorganisms, primarily yeast and some bacteria
• Dihydroxyacetone phosphate-glycerol 3-phosphate shuttle


  A shuttle that allows NADH produced in the cytosol to participate in the electron transport chain in the mitochondria
• Table 7-2 shows ATP production for complete oxidation of glucose, with 30 ATP molecules per glucose. This is 15 times more efficient than lactate and ethanol processes.
• Glycogen


  A branched polymeric form of glucose; the storage form of carbohydrates in humans and animals. It is found primarily in muscle and liver tissue. In muscles, it is the source of glucose needed for glycolysis. In the liver, it is the source of glucose needed to maintain normal glucose levels in the blood.
• Glycogenesis


  The metabolic pathway by which glycogen is synthesized from glucose 6-phosphate
• Dihydroxyacetone phosphate-glycerol 3-phosphate shuttle

  Produces more ATP than malate-aspartate shuttle
• Malate-aspartate shuttle


  Produces 2.5 ATP molecules
• The net overall reaction for glucose oxidation is the simple equation, which does not include substances like NADH, NAD+, and FADH2, as they cancel out
• Glycogen


  Branched polymeric form of glucose, storage form of carbohydrates in humans and animals
• Glycogen


  • Found primarily in muscle and liver tissue
  • In muscles, it is the source of glucose needed for glycolysis
  • In the liver, it is the source of glucose needed to maintain normal glucose levels in the blood
• Glycogenesis


  1. Isomerization: Formation of Glucose 1-phosphate

  2. Activation: Formation of UDP-glucose

  3. Linkage to Chain: Glucose Transfer to a Glycogen Chain
• Adding a single glucose unit to a growing glycogen chain requires the investment of two ATP molecules: one in the formation of glucose 6-phosphate and one in the regeneration of UTP
• Glycogenolysis


  1. Phosphorolysis: Formation of Glucose-1-phosphate

  2. Isomerization: Formation of Glucose 6-phosphate
• Gluconeogenesis


  Metabolic pathway that synthesizes glucose from non-carbohydrate materials
• Gluconeogenesis maintains normal blood glucose levels during times of inadequate dietary carbohydrate intake
• Non-carbohydrate starting materials for gluconeogenesis

  • Pyruvate
  • Lactate (from muscles and red blood cells)
  • Glycerol (from fat breakdown)
  • Certain amino acids (from protein breakdown)
• Gluconeogenesis


  • Starts with pyruvate, different from glycolysis which starts with glucose
  • Needs one extra step than glycolysis because converting pyruvate to phosphoenolpyruvate is a difficult step
  • This extra step uses oxaloacetate, which connects gluconeogenesis to the citric acid cycle
• Gluconeogenesis and glycolysis differ in terms of enzyme identity and/or ATP requirements


  • In steps 9 and 11, the reactant-product combinations match but different enzymes are needed and ATP is required in glycolysis but not in gluconeogenesis
  • In step 5, the same enzymes are operative in both directions but ATP is required in gluconeogenesis
  • In total, 6 nucleotide triphosphate molecules (4 ATP, 2 GTP) are hydrolyzed in synthesizing glucose from pyruvate in gluconeogenesis whereas there is a net production of 2 ATP in glycolysis
• Glycolysis


  Metabolic pathway by which glucose is converted into two molecules produce pyruvate, chemical energy in the form of ATP and NADH-reduced coenzymes
• Glycogenolysis


  Metabolic pathway by which a glucose 6-phosphate is produced from glycogen
• Glycogenesis


  The formation of glycogen from glucose
• Gluconeogenesis


  Metabolic pathway in which glucose and glycogen biosynthesis from noncarbohydrate sources
• Pentose Phosphate Pathway (PPP)

  • Critical metabolic pathway that occurs in the cytoplasm of cells
  • Source of ribose for nucleotides and NADPH for biosynthesis
  • Secondary pathway for glucose metabolism
• Tissues where PPP occurs

  • Liver (30% glucose metabolized)
  • Red blood cells (maintain oxidative capacity)
  • Muscles (doesn't utilize PPP so much)
• Functions of PPP


  Generate NADPH and pentoses (5-carbon sugars) for various cellular processes
• Pathways that require NADPH

  • Fatty Acid Synthesis (50%)
  • Oxidative Stress Homeostasis
  • Cytochrome P450 Enzymes
• Oxidative phase of PPP


  To make NADPH & Ribose 5-Phosphate
• Non-oxidative phase of PPP

  Carbon shuffling reactions, no carbon is removed
• Insulin


  Produced by the beta cells of the pancreas, function is to lower blood glucose levels
• Glucagon


  Polypeptide hormone produced in the pancreas by alpha cells, function is to increase blood glucose concentrations by speeding up the conversion of glycogen to glucose