Task: Question Answering
Input (English): Question: What is the chemical basis for the reversible inhibition of acetylcholinesterase by the alkaloid physostigmine and related compounds ?: 
 Options:  <1> Allosteric inhibition of the enzyme. <2> Covalent binding to a cofactor essential for the reaction. <3> Coordination of an Mg2 + cation in the active center of the enzyme. <4> Analogy with the transition state of the reaction catalyzed by the enzyme. <5> Formation of a carbamate (carbamoylation) of the serine residue in the active center of the enzyme.
Output (English): 5