These figures show the four levels of protein structure: primary, secondary, tertiary, and quaternary structure. Which level of protein structure is characteristic of some, but not all, proteins
All proteins have the basic primary, secondary and tertiary structure. However the quartenary structure arises only when the protein has two or more polypeptide chains. These polypeptides are usually folded and interact with one another. The quartenary structure is important because some proteins require two or more polypeptides joined together before they become active.
