How is a protein's quaternary structure formed?

A protein's quaternary structure is formed by the assembly of multiple polypeptide chains into a single functional unit.

The quaternary structure of a protein is the highest level of protein structure and is essentially the final three-dimensional shape that a protein assumes. This structure is formed when two or more polypeptide chains, also known as subunits, come together and interact. These subunits can be identical or different, and their assembly is guided by the same types of interactions that shape the secondary and tertiary structures of proteins - hydrogen bonding, ionic interactions, hydrophobic interactions, and disulfide bridges.

The quaternary structure is crucial for the protein's function. For instance, the haemoglobin molecule, which is responsible for oxygen transport in the blood, has a quaternary structure consisting of four polypeptide chains. Each of these chains can bind to an oxygen molecule, allowing haemoglobin to carry four oxygen molecules at once. This multi-subunit structure is what enables haemoglobin to perform its function effectively.

The assembly of the subunits into the quaternary structure is not a random process. It is dictated by the specific sequence of amino acids in each polypeptide chain, which determines how the chain folds and how it interacts with other chains. The process is also influenced by the cellular environment, including factors such as pH and temperature.

In some cases, the quaternary structure can be altered or disrupted, leading to a loss of protein function. This can occur due to changes in the cellular environment or mutations in the gene encoding the protein. For example, in the disease sickle cell anaemia, a single mutation in the haemoglobin gene leads to a change in the protein's quaternary structure, causing the haemoglobin molecules to stick together and form fibres. This disrupts the normal shape of red blood cells and impairs their ability to transport oxygen.

In conclusion, the quaternary structure of a protein is a complex and dynamic entity, shaped by a multitude of factors and crucial for the protein's function. Understanding this structure is key to understanding how proteins work and how they can be affected by genetic mutations and environmental changes.