Effect of Substrate Concentration

When enzyme concentration and all other conditions are kept constant, changing the substrate concentration gives a characteristic curve.At low substrate concentrations, increasing substrate concentration increases the rate. There are more substrate molecules, so there are more successful collisions with active sites and more enzyme-substrate complexes form per second. Many active sites are empty, so substrate concentration is the limiting factor. In this region the rate is roughly directly proportional to substrate concentration.At higher substrate concentrations, the curve starts to level off, because more of the active sites are occupied at any moment.At high substrate concentrations, all the active sites are occupied. Enzyme-substrate complexes are forming as fast as the active sites become free. Adding more substrate has no effect on the rate, which has reached its maximum (V_max). Now enzyme concentration is the limiting factor: the rate could only be increased by adding more enzyme.Worked example: at low substrate concentration the rate is directly proportional to substrate concentration. If increasing the substrate concentration from 0.5 to 1.0 mmol dm⁻³ is in this region and the rate at 0.5 mmol dm⁻³ is 0.3 cm³ s⁻¹, doubling the substrate concentration doubles the rate to 0.6 cm³ s⁻¹.