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.