Prosthetic groups are cofactors that are tightly or covalently bound to enzymes.

• A. Cofactors that are tightly bound to enzymes
• B. Cofactors loosely bound to enzymes
• C. Nonprotein cofactors that are bound
• D. Substrates

Answer: (A)

Prosthetic groups are cofactors that stay tightly attached to an enzyme, often bound covalently, and become part of the holoenzyme. This permanent or near-permanent association ensures the cofactor is ready at the active site whenever the enzyme works. That’s why describing prosthetic groups as cofactors that are tightly bound (often covalently) to enzymes is the best fit. In contrast, many other cofactors act more loosely and can diffuse in and out during catalysis, which is the realm of coenzymes. Substrates, on the other hand, are the molecules being transformed by the enzyme, not the tightly bound helpers. Prosthetic groups can be inorganic ions or small organic molecules like heme or biotin, illustrating the variety of forms these tightly bound cofactors can take.