Examples of protein structures. Proteins can form a wide range of three-dimensional structures due to the variety of chemical properties of the 20 common amino acids. Shown here are (a) calmodulin, a Ca²⁺-binding protein; (b) Dicer, an enzyme that cleaves double-stranded RNA; and (c) hemoglobin, the oxygen carrier in red blood cells. See Section 4.3 and Figure 4-10 for an explanation of how the molecular structures of proteins are represented throughout this book.