Your body contains tens of thousands of different proteins, each with a specific structure and function. The unique three-dimensional shape of each of these diverse proteins is based on several superimposed levels of structure.

Which of the following statements is an accurate description of proteins?
a. The primary structure of a protein is the order of amino acids in a polypeptide, as coded for in the DNA of a gene.
b. The 20 amino acids found in proteins differ in the composition of their R groups, which may be either polar or charged.
c. Secondary structures, which include the alpha helix or beta pleated sheet, are held together by interactions between R groups.
d. Tertiary structure is the overall shape of a polypeptide, which may be stabilized by hydrophobic interactions, hydrogen bonds, ionic bonds, and peptide bonds.
e. Quaternary structure is only found in proteins that have four identical polypeptide subunits.