5.7.7 The Role of Coenzymes

Coenzyme A

• A coenzyme is a molecule that helps an enzyme carry out its function but is not used in the reaction itself
• Coenzyme A consists of a nucleoside (ribose and adenine) and a vitamin
• In the link reaction, CoA binds to the remainder of the pyruvate molecule (acetyl group 2C) to form acetyl CoA
• It then supplies the acetyl group to the Krebs cycle where it is used to continue aerobic respiration
• This is the stage that brings part of the carbohydrate (or lipid/amino acid) into the further stages of respiration and links the initial stage of respiration in the cytoplasm to the later stages in the mitochondria

NAD & FAD

• Coenzymes NAD and FAD play a critical role in aerobic respiration as hydrogen carriers
• When hydrogen atoms become available at different points during respiration NAD and FAD accept these hydrogen atoms
  • A hydrogen atom consists of a hydrogen ion and an electron

• When the coenzymes gain a hydrogen they are ‘reduced’
  • OIL RIG: Oxidation Is Loss, Reduction Is Gain

• They transfer the hydrogen atoms (hydrogen ions and electrons) from the different stages of respiration to the electron transport chain on the inner mitochondrial membrane, the site where hydrogens are removed from the coenzymes
• When the hydrogen atoms are removed the coenzymes are ‘oxidised’
• Hydrogen ions and electrons are important in the electron transport chain at the end of respiration as they play a role in the synthesis of ATP
  • Electrons from reduced NAD (NADH) and reduced FAD (FADH₂) are given to the electron transport chain
  • Hydrogen ions from reduced NAD (NADH) and reduced FAD (FADH₂) are released when the electrons are lost
  • The electron transport chain drives the movement of these hydrogen ions (protons) across the inner mitochondrial membrane into the intermembrane space, creating a proton gradient (there are more hydrogen ions in the intermembrane space)
  • Movement of hydrogen ions down the proton gradient, back into the mitochondrial matrix, gives the energy required for ATP synthesis

The reduction and oxidation of NAD and FAD

Sources of reduced NAD & FAD

• A certain amount of reduced NAD and FAD is produced during the aerobic respiration of a single glucose molecule
• Reduced NAD:
  • 2 x 1 = 2 from Glycolysis
  • 2 x 1 = 2 from the Link Reaction
  • 2 x 3 = 6 from the Krebs cycle

• Reduced FAD:
  • 2 x 1 = 2 from the Krebs cycle