8.1.1 Metabolic Pathways

• Metabolic pathways involve a series of small steps, each step involves a chemical change
• The enzyme-catalysed reactions that make up metabolic pathways usually consist of chains or cycles:
  • Chain reactions are a linear sequence with a distinct beginning and end
    • Glycolysis, part of respiration, is an example of a reaction chain metabolic pathway
  • Cycles involve the end product starting the next cycle, these are less common than chain reactions
    • The Calvin cycle, part of photosynthesis, is an example of a cyclic metabolic pathway

A chain metabolic pathway has a distinct start and finish, whereas in a cycle the end product feeds back into the starting reactant

• Chemicals involved in metabolic pathways are called metabolites or intermediates
  • Some form new molecules within cells
  • Others breakdown molecules and involve an energy transfer

• Metabolic pathways are controlled by enzymes in a biochemical cascade of reactions
  • Virtually every metabolic reaction within living organisms is catalysed by an enzyme
  • Enzymes are therefore essential for life to exist
• Enzymes are biological catalysts
  • ‘Biological’ because they function in living systems
  • ‘Catalysts’ because they speed up the rate of chemical reactions without being used up or undergoing permanent change

The Enzyme-Substrate Complex

• The starting point of a metabolic pathway is a substrate which is converted to an end product
• The enzyme works by binding to the substrate at a special site on the enzyme called the active site
  • The active site of an enzyme has a specific shape to fit a specific substrate
• Substrates collide with the enzyme's active site and this must happen at the correct orientation and speed in order for a reaction to occur
• An enzyme-substrate complex is formed, temporarily, when the substrate binds to the active site
  • The substrate is said to be in a transitional state at this moment
• The product is formed and enzyme is released to take part in another reaction
• The reaction can be shortened to a simple equation

The simple equation can show how an enzyme reaction proceeds

The formation of the enzyme-substrate complex where the substrate is said to be in a transitional state, before forming the product(s)

Enzymes and the lowering of activation energy

• All chemical reactions, including metabolic pathways, are associated with energy changes
• Energy may either be released or absorbed during a reaction
  • If energy is released to the surroundings it is an exergonic reaction
  • If energy is absorbed from the surroundings it is an endergonic reaction
• For a reaction to proceed there must be enough activation energy
• Activation energy is the amount of energy needed by the substrate to become unstable enough for a reaction to occur and for new products to be formed
• Enzymes speed up chemical reactions because they reduce the stability of bonds in the substrate
• Enzymes lower the activation energy needed to catalyse a reaction
  • The energy released is unchanged but the activation energy required is lowered
  • The rate of reaction is therefore quicker