Explain the effect of temperature and pH on enzyme-catalysed reactions:

The rate of reaction typically increases with temperature, as heat provides kinetic energy and more substrate-enzyme collisions. however, going above a certain temperature will cause enzymes to denature and lose its shape. When this happens, the substrate can no longer fit into the active site and therefore there will be less enzyme-induced reactions, making the overall reaction slower.
The optimum pH depends on the enzyme, e.g. enzymes in the stomach will work best at lower pH, whilst intestinal enzymes work best at higher pH. Going much higher or lower than an enzyme's optimum pH will cause the reaction to slow down as it gets denatured, causing less enzyme-substrate reactions.