CHAPTER 27:

• Glycolysis


  A series of 10 enzyme-catalyzed reactions by which glucose is oxidized to two molecules of pyruvate
• Glycolysis


  1. Phosphorylation of α-D-glucose

  2. Isomerization of α-D-glucose 6-phosphate to α-D-fructose 6-phosphate

  3. Phosphorylation of fructose 6-phosphate

  4. Cleavage of fructose 1,6-bisphosphate to two triose phosphates

  5. Isomerization of triose phosphates

  6. Oxidation of the –CHO group of D-glyceraldehyde 3-phosphate

  7. Transfer of a phosphate group from 1,3-bisphosphoglycerate to ADP

  8. Isomerization of 3-phosphoglycerate to 2-phosphoglycerate

  9. Dehydration of 2-phosphoglycerate

  10. Phosphate transfer to ADP and formation of ATP
• During glycolysis, there is net conversion of 2 ADP to 2 ATP
• Reactions of Pyruvate


  1. Pyruvate is most commonly metabolized in one of three ways, depending on the type of organism and the presence or absence of O2

  2. Pyruvate undergoes oxidative decarboxylation under aerobic conditions
• Pentose Phosphate Pathway


  Also called a shunt
• Glycerol enters glycolysis via dihydroxyacetone phosphate
• Fatty Acids and Energy


  • Fatty acids in triglycerides are the principal storage form of energy for most organisms
  • Hydrocarbon chains are a highly reduced form of carbon
  • The energy yield per gram of fatty acid oxidized is greater than that per gram of carbohydrate oxidized
• mitochondrion


  Organelle where oxidation of two C2 fragments in the citric acid cycle and oxidative phosphorylation common pathways occur, producing 10ATP for each C2 fragment
• Oxidation of two C2 fragments in the citric acid cycle and oxidative phosphorylation common pathways, producing 10ATP for each C2 fragment
• Total ATP produced is 30-32
• Catabolism of Glycerol


  Glycerol enters glycolysis via dihydroxyacetone phosphate
• Fatty Acids and Energy


  • Fatty acids in triglycerides are the principal storage form of energy for most organisms
  • Hydrocarbon chains are a highly reduced form of carbon
  • The energy yield per gram of fatty acid oxidized is greater than that per gram of carbohydrate oxidized
• β-Oxidation

  1. Activation of fatty acid to acyl CoA

  2. Oxidation of the α,β carbon-carbon single bond to a carbon-carbon double bond

  3. Hydration of the C=C double bond to give a 2° alcohol

  4. Oxidation of the 2°alcohol to a ketone

  5. Cleavage of the carbon chain by a molecule of CoA-SH
• The cycle of β-oxidation reactions is then repeated on the shortened fatty acyl chain and continues until the entire fatty acid chain is degraded to acetyl CoA
• β-Oxidation of unsaturated fatty acids proceeds in the same way, with an extra step that isomerizes the cis double bond to a trans double bond
• Energy Yield on β-Oxidation


  Yield of ATP per mole of stearic acid (C18)
• Ketone Bodies


  • Acetone, β-hydroxybutyrate, and acetoacetate
  • Formed principally in liver mitochondria
  • Can be used as a fuel in most tissues and organs
  • Formation occurs when the amount of acetyl CoA produced is excessive compared to the amount of oxaloacetate available to react with it and take it into the TCA
• Transamination


  • -NH2 groups move freely by transamination
  • Pyridoxal phosphate forms an imine (a C=N group) with the α-amino group of an amino acid
  • Rearrangement of the imine gives an isomeric imine
  • Hydrolysis of the isomeric imine gives an α-ketoacid and pyridoxamine. Pyridoxamine then transfers the α-NH2 group to another α-ketoacid
• Oxidative Deamination


  Nitrogen atoms to be excreted are collected in glutamate, which is oxidized to α-ketoglutarate and NH4+
• Urea Cycle


  A cyclic pathway that produces urea from CO2 and NH4+
• Amino Acid Catabolism


  • Glucogenic amino acids: Those whose carbon skeletons are degraded to pyruvate or oxaloacetate, both of which may then be converted to glucose by gluconeogenesis
  • Ketogenic amino acids: Those whose carbon skeletons are degraded to acetyl CoA or acetoacetyl CoA, both of which may then be converted to ketone bodies
• The breakdown of amino acid carbon skeletons follows two pathways: glucogenic and ketogenic
• Figure 27.13 A summary of catabolism showing the role of the common metabolic pathway