Among the compounds examined, the size of the substituents had little relation to reactivity or toxicity. The parabolic shape of the curve implies that toxicity resulting from cholinesterase inhibition does not increase indefinitely. There is an optimal reactivity because of the competition between hydrolysis and enzyme inactivation. For cholinesterase inhibition, reactivity of the substrate is the important determinant of toxicity as long as minimal structural requirements are met. The shape of the graph has more general implications for biochemistry.
Enzymes reduce the energy of activation by a specific amount.
Regardless of the substrate binding characteristics, the reaction will not occur if the energy of activation for a particular compound exceeds the catalytic capacity of the enzyme.