Explain how proteins act as enzymes, including control by feedback inhibition in allosteric enzymes [9 marks]

The following answers this past exam question. Test your understanding by defining all the CAPITALISED words. Enzymes are GLOBULAR PROTEINS, which contain an active site to which the substrate binds. The shape of the substrate and the active site changes as the substrate binds. This is called INDUCED FIT MODEL. When bound to the enzyme, the bonds in the substrate weaken and the ACTIVATION ENERGY is lowered. You can show this in a sketch of the energy levels of a reaction – hint: the free energy of the substrate and the products will be the same, but the peak (which represents the activation energy) will be different. END PRODUCT inhibition is a type of NEGATIVE FEEDBACK, whereby an end product binds to an ALLOSTERIC SITE on the enzyme, away from the active site. This causes a change in shape of the enzyme’s active site. As a result, the substrate no longer sits into the active site and the reaction can no longer be catalysed. This allosteric inhibition is NON-COMPETITIVE – it can bind both the free enzyme or the enzyme bound to the substrate. The higher the concentration of the end-product, the lower the enzyme activity. Since the enzyme which is inhibited usually catalyses the first or an early reaction in the pathway, the whole pathway will be inhibited. This prevents the build-up of intermediates.