1.4

• Knowing about several key intermediate compounds in metabolic reactions makes it easier to understand the yet-to-come details of metabolic processes
• Knowing about a particular type of bond present in certain phosphate-containing metabolic intermediates makes the details of metabolic processes easier to understand
• High-energy compound


  A compound that has a greater free energy of hydrolysis than that of a typical compound
• High-energy compounds


  • They contain one or more very reactive bonds, often called strained bonds
  • The energy required to break these strained bonds during hydrolysis is less than that generally required to break a chemical bond
  • The balance between the energy needed to break bonds in the reactants and that released by bond formation in the products is such that more than the typical amount of free energy is released during the hydrolysis reaction
• Bond strain


  Greater-than-normal electron–electron repulsive forces at specific locations within a molecule
• Causes of bond strain

  • Highly electronegative atoms and/or highly charged atoms occurring together in a molecule
• The parent molecule for phosphate groups is phosphoric acid, H3PO4, a weak triprotic inorganic acid
• The dominant form of phosphoric acid at cellular pH is the HPO4^2- ion
• Diphosphate and triphosphate ions can also exist in cellular fluids
• Reduction of FAD


  Two hydrogen ions and two electrons are acquired by the FAD to produce FADH2
• Oxidation of FADH2


  Two hydrogen ions and two electrons are released by the FADH2 to produce FAD
• Loss of one phosphoryl group by ATP as Pi

  Produces ADP
• Loss of two phosphoryl groups by ATP as PPi

  Produces AMP
• Release of the acetyl group from acetyl CoA


  Produces coenzyme A (H–S–CoA)
• Free energy is the amount of energy released by a chemical reaction that is actually available for further use at a given temperature and pressure
• The energy released in a chemical reaction is divisible into two parts: one part lost as heat and not available for further use, and the other part, the free energy, available for further use
• Commonly encountered phosphate-containing compounds

  • Phosphoenolpyruvate
  • 1,3-bisphosphoglycerate
  • Acetyl phosphate
  • Creatine phosphate
  • Arginine phosphate
  • ATP
  • ADP
  • AMP
  • PPi
  • Glucose 1-phosphate
  • Fructose 6-phosphate
  • Glucose 6-phosphate
  • Glycerol 3-phosphate
• The more negative the free energy of hydrolysis, the greater the extent of bond strain in the molecules