Here's why:

* Saturation: When the enzyme has all its active sites occupied by substrate molecules, it reaches saturation. This means the enzyme is working at its maximum capacity.

* Maximal Velocity (Vmax): Vmax represents the highest rate of reaction that can be achieved by an enzyme at a given concentration. Increasing substrate concentration beyond this point will not increase the reaction rate because the enzyme is already fully saturated.

Let me know if you'd like to learn more about enzyme kinetics!