Energy must be pumped into molecule to reach transition state before lower energy state is achieved.
Enzymes lower the energy needed to get to transition state.
Activity is the rate of an enzyme reaction and can be measured experimentally as the rate of product formation or substrate disappearance. The reaction rate is called the velocity and denoted by Vo while the maximum rate of a reaction is denoted by Vmax. A unit of enzyme activity is equal to the amount of enzyme required to convert a certain amount of substrate to product per unit time. This point where velocity is no longer affected by an increase in substrate concentration is called saturation. The enzyme has become saturated with substrate at Vmax.