What defines a zymogen?

A zymogen is specifically defined as an inactive precursor of an enzyme. It is synthesized and stored in this inactive form to prevent premature enzyme activity that could potentially lead to damage to the tissues where it is produced, particularly in the case of digestive enzymes. Once the zymogen is activated, usually through specific cleavage of peptide bonds, it becomes an active enzyme capable of catalyzing biochemical reactions.

In the context of the other options, an activated enzyme is indeed involved in metabolism, but that describes the function of an enzyme rather than its inactive precursor. A protein that transports amino acids does not correspond to the definition of a zymogen, as it refers to a class of proteins with a different function related to nutrient transport. Similarly, a cofactor refers to non-protein molecules that assist enzymes in their catalytic activity and is distinct from the concept of a zymogen. Therefore, the precise definition and function of a zymogen solidify its identity as an inactive form of an enzyme.