Illustrated Glossary of Organic Chemistry

Transition state analog: A drug that binds to and therefore inhibits an enzyme because the drug resembles the transition state of a reaction normally catalyzed by the enzyme.

Transition state normally bound by enzyme		Transition state inhibitor

The enzyme methylthioadenosine nucleosidase catalyzes the ionization of the bond between the adenosine nucleobase and the ribofuranose moieties of molecules such as 5'-methylthioadenosine. The transition state for this ionization is shown. The enzyme catalyzes this process because it binds to, and stabilizes, this transition state. The transition state inhibitor drug resembles this transition state, and binds to the enzyme. It binds so strongly that the enzyme is inhibited from binding with its natural substrate, and the ionization reaction is halted.