• 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
  • They speed up reactions by?reducing the?activation energy?of reactions

• Enzymes are?globular proteins?with?complex tertiary structures
  • Some are formed from a single polypeptide, whilst others are made up of two or more polypeptides and therefore have a?quaternary structure
• 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
• All enzymes are?proteins?that are produced via the process of protein synthesis?inside cells
• Some enzymes?remain?inside?cells, whilst others are?secreted?to work?outside?of cells
• Enzymes can be?intracellular?or?extracellular?referring to whether they are active inside or outside the cell respectively
  • Intracellular?enzymes are?produced?and?function?inside the cell
  • Extracellular?enzymes are?secreted?by cells and catalyse reactions?outside?cells (eg. digestive enzymes in the gut)

Intracellular and Extracellular Enzymes Table

Enzymes and the lowering of activation energy

• All chemical reactions are associated with?energy changes
• For a reaction to proceed there must be enough?activation energy
• Activation energy is the?amount of energy needed?by the substrate to become just?unstable?enough for a?reaction to occur?and for?products to be formed
  • Enzymes speed up chemical reactions because they reduce the?stability?of?bonds?in the reactants
  • The?destabilisation of bonds?in the substrate makes it?more reactive
• Rather than lowering the overall energy change of the reaction, enzymes work by?providing an?alternative energy pathway?with a?lower activation energy
• Without enzymes,?extremely high temperatures?or?pressures?would be needed to reach the activation energy for many biological reactions
  • Enzymes avoid the need for these?extreme conditions?(that would otherwise?kill cells)

The activation energy of a chemical reaction is lowered by the presence of a catalyst (i.e. an enzyme)