Three or more amino acids linked together in this way forms a polypeptide chain. A. Second Level of Protein Structure 1. Hydrogen bonds join the side groups of the amino acids in the primary chains. B. Third Level of Protein Structure 1. Interactions among R groups results in a complex three-dimensional shape.
Globular proteins have extensive tertiary structure.
C. Fourth Level of Protein Structure 1. Hemoglobin consists of four folded chains called globins, each with a heme group. Quaternary structure describes the complexing of two or more polypeptide chains. Keratin and collagen are examples of complex structural proteins. D. Glycoproteins and Lipoproteins 1.