What type of protein structure involves interactions of amino acids on different subunits?

The quaternary structure of a protein is characterized by the assembly of multiple polypeptide chains, known as subunits, which interact to form a functional protein complex. In this level of structure, the interactions between the amino acids of the different subunits can include various types of bonding and forces, such as hydrogen bonds, ionic interactions, hydrophobic interactions, and disulfide bridges.

In contrast to primary (the linear sequence of amino acids), secondary (the local folding, such as alpha helices and beta sheets), and tertiary (the overall three-dimensional shape of a single polypeptide) structures, quaternary structure specifically refers to these higher-order assemblies. Therefore, understanding that the quaternary structure is the result of the combination of multiple subunits underscores its importance in the functionality of many proteins, such as enzymes or hemoglobin, which rely on this multimeric organization to perform their biological roles effectively.