prostheticgroup

A prosthetic group is a non-polypeptide unit that is tightly bound to a protein and is essential for its activity. Unlike loosely bound cofactors, prosthetic groups remain associated with the apoprotein throughout catalysis and are integral to the holoenzyme.

Prosthetic groups can be inorganic metal ions, organometallic complexes, or organic molecules that are either covalently attached or non-covalently but tightly bound. Common examples include heme in hemoproteins, iron-sulfur clusters in some oxidoreductases, and flavin cofactors (FMN, FAD) bound in flavoproteins. Organic prosthetic groups include biotin in carboxylases, lipoic acid in pyruvate dehydrogenase, and retinal in certain visual pigments. Some enzymes also utilize covalently bound cofactors such as thymine pyrophosphate in transketolase and related enzymes; in many contexts TPP is described as a coenzyme, but it can function as a prosthetic group.

Functionally, prosthetic groups contribute catalytic power, redox properties, substrate binding, or structural stabilization that the polypeptide alone cannot provide. They often participate directly in the chemical transformation or electron transfer that defines the enzymatic reaction.

In terminology, the protein without its prosthetic group is called an apoenzyme, while the holoprotein refers to the protein with its prosthetic group. Prosthetic groups thus define a key aspect of an enzyme’s identity and mechanism.