Drugs that inhibit enzymes typically bind to the active site of the enzyme where catalysis occurs. Binding of an inhibitor precludes binding of the normal reactants (substrates) for the enzyme, inhibiting its activity. In a new strategy, two small reactive molecules selected to bind independently in the active site can covalently react with each other to form a new drug with very high specificity and very high binding affinity (low Kd). Recently this has been used to synthesize noncovalent inhibitors of the enzyme acetylcholinesterase (Barry Sharpless Lab, Scripps Lab). The reactive groups chosen in the example below are azide and acetylene derivatives, which when held in close approximation in the binding site of the enzyme undergo a cycloaddition reaction to form a triazole.
The actual mechanism (not the simplified version show below) requires catalysis by cupper ions which forms a complex with the acteylide (deprotonated acetylene) effectively decreasing the pKa of the acetylene functional group, making it a better nucleophile. A dicopper intermediate is suggested in which the azide interacts with the second copper. Subsequen rearrangment lead to the triazole products.
Figure: Test tube and in situ synthesis of triazole inhibitors.