At low substrate concentration not all the active sites of the enzymes are filled as there are relatively few molecules of substrate.If the substrate concentration is increased, the rate of reaction increases as the substrate concentration is limiting.More active sites are filled and more enzyme-substrate complexes are formed so more product is formed per unit time.The rate reaches a maximum Vmax when the enzyme concentration becomes limiting. At this stage all the active sites are occupied and the excess substrate molecules cannot form enzyme-substrate complexes until the active sites become available.When the substrate is in excess, the maximum rate of reaction is determined by the amount of enzyme present.