Final answer:
Option A is NOT true of protein quaternary structure, as a single polypeptide cannot have quaternary structure, which involves multiple polypeptide subunits and various stabilizing interactions.
Step-by-step explanation:
'Which of the following is NOT true of protein quaternary structure?' is option A: A single polypeptide may have quaternary structure. This statement is false because quaternary structure refers to the arrangement and interaction of multiple polypeptide subunits.
Quaternary structure describes a protein that is composed of two or more polypeptides, known as subunits. These subunits may be identical or different, and the interactions between them, which can include hydrogen bonds, disulfide bridges, and other weak interactions, stabilize the overall structure of the protein.
Notable examples of proteins with quaternary structure include hemoglobin, which has four polypeptide subunits, and insulin, which also involves post-translational modifications resulting in disulfide linkages between its chains. The other statements B, C, D, and E, correctly describe aspects of quaternary structures, such as the stabilizing forces and the minimum presence of two carboxyl groups due to multiple polypeptides involved.