Final answer:
The quaternary structure describes multiple sub-units in a protein where each sub-unit is a separate polypeptide. Hemoglobin is an example of a protein with quaternary structure consisting of four polypeptide chains. It is produced and stabilized by interactions maintaining the tertiary structure.
Step-by-step explanation:
The level of structure that describes multiple sub-units in which each sub-unit is a separate polypeptide is called the quaternary structure of a protein. When a protein contains more than one polypeptide chain, each chain is called a subunit. The arrangement of multiple subunits represents the quaternary structure.
For example, hemoglobin is a protein that has quaternary structure as it consists of four polypeptide chains or subunits. The quaternary structure is produced and stabilized by the same kinds of interactions that produce and maintain the tertiary structure.
A schematic representation of the four levels of protein structure including the quaternary structure is shown in Figure 19.5.7.