What is it called when a protein is made up of more than one folded amino acid chain?

When a protein is made up of more than one folded amino acid chain, it is referred to as quaternary structure. This level of protein organization involves multiple polypeptide chains, or subunits, that come together to form a functional protein complex. Each subunit can be a separate polypeptide that has its own tertiary structure, but their interaction creates the overall quaternary structure. This arrangement is crucial for the protein's functionality, as the specific interactions between the subunits can affect the protein's stability, activity, and ability to bind to other molecules.

The distinction between this and other structures of proteins is important. The primary structure refers to the linear sequence of amino acids in a single polypeptide chain. The secondary structure pertains to local folding patterns, such as alpha helices and beta sheets, formed through hydrogen bonding within the same polypeptide. The tertiary structure describes the three-dimensional shape of a single polypeptide chain resulting from complex interactions among the amino acids. Quaternary structure, therefore, signifies the assembly of multiple polypeptide chains into a single functional unit.