Final answer:
Enzymes are not a separate class of macromolecules; they are a type of protein. The four major types of macromolecules are proteins, carbohydrates, lipids, and nucleic acids.
Step-by-step explanation:
The question asks which of the listed items is not one of the four major classes of macromolecules. The four major types of macromolecules found in living organisms are proteins, carbohydrates, lipids, and nucleic acids. Enzymes, while they are proteins, are not a separate class of macromolecules, but rather a type of protein. Therefore, the correct answer is a. enzymes.