Why does the reaction rate of an enzyme-catalyzed reaction plateau at higher reactant (substrate) concentrations?

Hi! 

The reaction rate of an enzyme-catalyzed reaction plateaus at higher reactant (substrate) concentration because there aren't any active sites to bind more substrate.

The mechanism of an enzyme-catalyzed reaction is the following (E=Enzyme, S=Substrate, ES= Enzyme-Substrate Complex, and P=Product):

E + S ↔ ES → E + P

The key step of this mechanism is the formation of the Enzyme-Substrate Complex. 

The Enzyme has a limited amount of active sites where the substrate can bind to form the Enzyme-Substrate Complex. When the substrate concentration is too high, there aren't any more available active sites and an increase in substrate concentration doesn't change the reaction rate. This causes the plateau in the graph.

Have a nice day!