Final answer:
The claim that all proteins have a quaternary structure is false. Only some proteins, like hemoglobin, possess a quaternary structure characterized by the association of multiple polypeptide subunits, whereas others, such as insulin, do not.
Step-by-step explanation:
The statement that every protein has a quaternary structure is false. Not all proteins reach this level of structure. Proteins are complex molecules with a hierarchical structure that starts with the primary structure (the sequence of amino acids), progresses to the secondary structure (local folding into elements like α-helices and β-sheets), then to the tertiary structure (the overall three-dimensional conformation), and finally, for some proteins, to the quaternary structure. The quaternary structure is the interaction and association of multiple polypeptides or subunits within a protein complex.
An example of protein with a quaternary structure is hemoglobin, which consists of two alpha and two beta polypeptides. Proteins such as insulin begin as a single polypeptide that undergoes post-translational modifications resulting in an active form without quaternary structure. Silk is another protein that, despite being made of multiple polypeptide chains, relies on β-pleated sheets for its structure rather than a quaternary level of organization.
Not all proteins have a quaternary structure; this level of structure is present only in certain protein complexes where multiple polypeptide chains come together, interacting to form a functional unit.