The outer surface of a protein's tertiary structure presents as ______ that can interact with ______.

• A. functional groups; other molecules
• B. charged residues; water molecules
• C. hydrophobic patches; lipid membranes
• D. disulfide bonds; neighboring polypeptides

Answer: (A)

Proteins present a variety of functional groups on their surface because exposed amino acid side chains define the exterior of the folded structure. These functional groups—polar, charged, and sometimes hydrophobic elements—are the sites that participate in interactions with other molecules. They can engage in hydrogen bonds or ionic interactions with water, ions, substrates, ligands, or other proteins, and in some cases with lipid membranes when hydrophobic patches come into contact with the membrane environment. The overarching idea is that the exterior is composed of these exposed functional groups, and it is these groups that mediate how the protein interacts with its surroundings.