A polypeptide chain may take on a certain structure in and of itself because of the amino acid monomers it contains and their location within the chain. The chain may furthermore interact with other polypeptide chains to form larger proteins known as oligomeric proteins. In what follows, the levels of protein structure normally encountered will be highlighted:
- Primary structure—refers to the backbone of the polypeptide chain and to the sequence of the amino acids of which it is comprised.
- Secondary structure—refers to the shape (recurring arrangement in space in one dimension) of the individual polypeptide chain. In some cases, because of its primary structure, the chain may take on an extended or longitudinally coiled conformation.
- Tertiary structure—refers to how the polypeptide chain (the primary structure) is bent and folded in three-dimensional space in order to form the normal tightly folded and compact structure.
- Quaternary structures—refers to how, in larger proteins made up of two or more individual polypeptide chains, the individual polypeptide chains are arranged relative to each other. These large multipolypeptide proteins are called oligomeric proteins and the individual chains are called subunits. An example of such a protein is hemoglobin.