Acetylcholinesterase (AChE) is a protein that catalyzes the conversion of acetylcholine to acetate and choline. When the concentration of AChE in an aqueous solution is held constant, the rate of the reaction catalyzed by AChE increases with increasing concentrations of substrate. At low concentrations of acetylcholine, a small increase in the substrate concentration results in a large increase in the reaction rate. At high concentrations of acetylcholine, however, a large increase in the substrate concentration results in only a small increase in the reaction rate.

The kinetic of the enzyme is saturable

Explanation:

In this scenario, the information given implies that at low acetylcholine concentration, reaction rate is proportional to substrate concentration, while at high substrate concentrations, the rate is relatively insensitive to change in acetylcholine.

This kind of behaviour is characteristic of processes where the enzyme concentration is held constant.

For a reaction to take place, it's necessary that the enzyme interacts with the substrate, often by an specific site known as the active site. So, to a given concentration of enzyme, there's a specific amount of sites that participate in the reaction.

At low substrate concentration, most acetylcholine molecules bind the acetylcholinesterase (AChE) and start to react. That means, the addition of substrate, increases the rate.

But as the substrate concentration keeps increasing, most the sites start to get occupied, and the enzyme sites are now saturated.

As a result, even if we add more acetylcholine, the reaction rate won't increase, because all AChE are already occupied and working at their maximum rate.

In other words, the limitant to the reaction rate in this situation, where the enzyme concentration is held constant, is the total AChE concentration in the solution.