Here's why:

* Proteins are large, complex molecules made up of chains of amino acids.

* Enzymes are biological catalysts, meaning they speed up chemical reactions in the body without being consumed in the process.

* The structure of an enzyme is crucial to its function. The specific shape of an enzyme allows it to bind to a particular molecule (its substrate) and facilitate a chemical reaction.

* The intricate three-dimensional structure of enzymes is determined by the sequence of amino acids that make them up.

While other macromolecules like carbohydrates, lipids, and nucleic acids play important roles in the body, enzymes are exclusively composed of proteins.