Kvartäärirakenteen

Kvartäärirakenteen, often translated as Quaternary structure, refers to the arrangement of multiple independent polypeptide chains, or subunits, in a multi-subunit protein complex. This level of protein organization describes how these individual subunits associate and interact with each other to form a functional whole. Unlike primary, secondary, and tertiary structures, which describe the folding of a single polypeptide chain, quaternary structure deals with the interfaces and forces that hold separate polypeptide chains together.

The subunits in a quaternary structure can be identical or different. If they are identical, the protein is said to have homomeric quaternary structure. If the subunits are different, it is called heteromeric quaternary structure. The forces that stabilize quaternary structures are generally non-covalent, including hydrogen bonds, ionic bonds, hydrophobic interactions, and van der Waals forces. In some cases, disulfide bonds can also contribute to the stability of the quaternary arrangement, though this is less common than for tertiary structures.

Examples of proteins exhibiting quaternary structure are abundant and include hemoglobin, which is composed of four subunits, and enzymes like DNA polymerase, which can be made up of many subunits. The specific arrangement of these subunits is crucial for the protein's overall function, as it can create active sites, allosteric regulatory sites, or facilitate cooperative binding of ligands. The disruption of quaternary structure can lead to a loss of protein activity, highlighting its importance in biological processes.