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Why does an enzyme reaction rate become nearly independent of substrate concentration at high substrate levels?

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dissociation of E–N — reaction mechanism Possible energy diagram for the dissociation of a covalent molecule, E–N, into its ions E⁺ and N⁻ (see text).

An enzyme reaction rate becomes nearly independent of substrate concentration at high levels because the enzyme reaches a state of saturation.12

According to the Michaelis-Menten kinetics model, an enzyme catalyzes a reaction by binding to a substrate at a specific region known as the active site. At low substrate concentrations, increasing the amount of substrate increases the reaction rate because more active sites are occupied.12

However, as the substrate concentration continues to rise, all available active sites on the enzyme molecules eventually become filled or "engaged". Once the enzyme is saturated, it is operating at its maximal velocity (V_M). At this point, the reaction rate is no longer limited by the availability of the substrate, but rather by the speed at which the active sites can convert the bound substrate into product.132