Explain how changes in temperature and pH affect the rate of an enzyme-catalysed reaction. Give appropriate diagrams to illustrate your answer. [6]

At the optimum temperature for a given enzyme (specific to each enzyme dependent on the composition of the protein), catalytic activity is at a maximum and hence the reaction proceeds at maximum rate. Both enzyme and substrate have relatively high kinetic energy such that the frequency of successful collisions between active site and enzyme (required for the formation of an enzyme-substrate complex) is at a maximum. At temperatures below the optimum, the kinetic energy of enzyme and substrate molecules is reduced; thus, the frequency of successful collisions (and so the reaction rate) is reduced. At temperatures above the optimum, atoms in the enzyme have sufficient kinetic energy such that hydrogen bonds, ionic bonds (salt bridges) and covalent bonds (disulphide bonds) stabilising the tertiary structure are broken; the 3D structure of the active site is distorted as the polypeptide begins to unfold, resulting in fewer successful collisions between enzyme and substrate, thus a reduced reaction rate.Each enzyme also has a unique optimum pH, again, dependent on the composition of the protein. At pH values which differ from this optimum, the ionisation states of amino acids side chains of the polypeptide can change, disrupting the electrostatic interactions between these which constitute ionic bonds, thus, disrupting the tertiary structure of the enzyme, leading to denaturation of the active site. Hence, as pH tends away from the optimum value, the frequency of successful collisions between enzyme and substrate is reduced, and so, the rate of reaction is reduced.