In an enzyme-catalyzed reaction, the reactant is called a substrate. Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy required for the reaction to occur. The substrate is the specific substance upon which an enzyme acts. It binds to the enzyme’s active site, where the reaction takes place.
To understand this better, think of a lock and key. The enzyme is the lock, and the substrate is the key that fits into the lock. When the substrate binds to the enzyme, it forms an enzyme-substrate complex, which then leads to the transformation of the substrate into products.
Other options like cofactor, coenzyme, isozyme, and allostere refer to different components or concepts related to enzyme function and regulation, but they are not the primary reactant. For instance, cofactors are non-protein molecules that assist enzymes, while coenzymes are organic molecules that serve as cofactors. Thus, the correct answer to the question is a) substrate.