What is the main structural feature of enzymes?

Enzymes are primarily composed of proteins, which serve as catalysts in biochemical reactions. The protein structure of enzymes is critical because it determines their shape, functionality, and the specific substrate interactions they can have. The three-dimensional structure formed by the folding of the polypeptide chain gives enzymes their active sites, where substrates bind and undergo chemical transformations.

While holoenzymes and multimeric structures can describe certain types of enzymes—such as those that require additional components (cofactors or coenzymes) to be active, or those that consist of multiple subunits—the fundamental characteristic that identifies enzymes as biological catalysts is their protein composition. Polypeptide chains are the building blocks of proteins, but the correct answer emphasizes that enzymes themselves are functional proteins, highlighting their importance in biological processes. Understanding that enzymes are proteins allows one to appreciate the relationship between structure and function in biological systems.