Affinity label

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An affinity label is a molecule that is similar in structure to a particular substrate for a specific enzyme. It is considered to be a class of enzyme inactivators. These molecules covalently modify active site residues in order to elucidate the structure of the active site. The inhibitor binds covalently to the enzyme so that the substrate can no longer bind, causing a permanent and irreversible change.

These types of compounds are highly reactive and usually alkylate nucleophilic amino acids present in the enzyme. They are often indiscriminate in their action, therefore their usefulness is limited as they are non-selective and therefore inherently toxic.