The results of an investigation in which different amounts of catalase were added to the same amount of hydrogen peroxide. You can see that the shape of all five curves is similar. In each case, the reaction begins very quickly (steep curve) and then gradually slows down (curve levels off). Because the amounts of hydrogen peroxide are the same in all five reactions, the total amount of oxygen eventually produced will be the same so, if the investigation goes on long enough, all the curves will meet.

To compare the rates of these five reactions, in order to look at the effect of enzyme concentration on reaction rate, it is fairest to look at the rate right at the beginning of the reaction. This is because, once the reaction is under way, the amount of substrate in each reaction begins to vary, as substrate is converted to product at different rates in each of the five reactions. It is only at the very beginning of the reaction that we can be sure that differences in reaction rate are caused only by differences in enzyme concentration. To work out this initial rate for each enzyme concentration, we can calculate the slope of the curie 30 seconds after the beginning of the reaction, as explained earlier Ideally, we should do this for an even earlier stage of the reaction, but in practice this is impossible. We can then plot a second graph, figure 3. 5 b, showing this initial rate of reaction against enzyme concentration.

This graph shows that the initial rate of reaction increases linearly. In these conditions, reaction rate is directly proportional to the enzyme concentration. This is just what common sense says should happen. The more enzymes present, the more active sites will be available for the substrate to slot into. As long as there is plenty of substrate available, the initial rate of a reaction increases linearly with enzyme.