Vmax is not a threshold of substrate concentration for initiation an enzymatic reaction. Vmax is the maximum reaction rate that an enzyme can achieve when it is saturated with substrate. At this point, all of the enzyme's active sites are occupied with substrate molecules and the reaction is proceeding at its fastest possible rate. Increasing the substrate concentration beyond this point will not increase the reaction rate, because all of the enzyme's active sites are already saturated.

The threshold of substrate concentration for initiation an enzymatic reaction is known as the Michaelis constant, or Km. This is the concentration of substrate at which the reaction rate is half of the Vmax. At this point, half of the enzyme's active sites are occupied with substrate molecules and the reaction is proceeding at half of its maximum possible rate. Increasing the substrate concentration above the Km will increase the reaction rate, but it will not reach the Vmax until all of the enzyme's active sites are saturated.

The Km is an important parameter for understanding the kinetics of enzymatic reactions. It can be used to determine the affinity of an enzyme for its substrate, and it can also be used to calculate the rate of an enzymatic reaction at a given substrate concentration.