Enzymes as Catalysts
- 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)
Examiner Tip
Don't forget that enzymes are proteins and so anything that could denature a protein, rendering it non-operational (extremes of heat, temperature, pH etc.) would also denature an enzyme.
