If the substrate concentration and all other conditions are kept constant, changing the enzyme concentration changes the number of active sites available.With substrate in excess: increasing enzyme concentration means more active sites, so more enzyme-substrate complexes form per second and the rate increases. Because there is always plenty of substrate, the rate is directly proportional to enzyme concentration, and the graph of initial rate against enzyme concentration is a straight line through the origin. Doubling the enzyme concentration doubles the initial rate.With a limited amount of substrate: at high enzyme concentrations there are more active sites than substrate molecules can fill. Substrate concentration becomes the limiting factor, and the rate levels off; adding more enzyme has no further effect.In a reaction with a fixed amount of substrate, adding more enzyme makes the reaction finish sooner, but the total amount of product is unchanged, because it depends only on how much substrate there was. On a graph of product against time, the curves for different enzyme concentrations are steeper at the start but level off at the same height.
In cells, the amount of an enzyme is controlled by how much of it is made (gene expression), so this is one way cells control metabolism.Worked example: with excess substrate, a 1% enzyme solution gives an initial rate of 0.5 cm3 s−1. A 3% solution gives 3 × 0.5 = 1.5 cm3 s−1.
In cells, the amount of an enzyme is controlled by how much of it is made (gene expression), so this is one way cells control metabolism.Worked example: with excess substrate, a 1% enzyme solution gives an initial rate of 0.5 cm3 s−1. A 3% solution gives 3 × 0.5 = 1.5 cm3 s−1.