Vmax

 V_max is the notation used to describe the maximum velocity at which a reaction can occur. Once V_max is reached a reaction cannot proceed any faster, regardless of how much extra substrate is added.  The relationship between the enzymes rate of reaction and the substrate is defined as the Km (Michaelas Constant) of the enzyme. There relationship is defined by the following equation:   v = Vmax / (1 + (Km/[S]))