UNIT 3 Proteins Proteins: Definition of Proteins Proteins are large biomolecules composed of amino acids linked by peptide bonds. They are essential macromolecules that perform a vast range of functions in biological systems, including enzymatic catalysis, structural support, transport, communication, and immune responses. I. Structure of Proteins Proteins have four levels of structural organization: 1. Primary Structure 2. Secondary Structure 3. Tertiary Structure 4. Quaternary Structure II. Classification of Proteins