A protein complex is a collection of several proteins . It is only in this complex that many proteins can perform their cellular function. Depending on the context, a protein complex is also called a quaternary structure, in analogy to the names primary , secondary and tertiary structure for the spatial levels of observation of a protein.
A protein complex can either be an aggregation of different proteins or an association of two or more polypeptide chains that have arisen from one and the same polypeptide chain, the precursor protein (see: insulin ). The individual proteins are often linked to one another by hydrogen bridges and salt bridges, but also by covalent bonds. The individual subunits of such a complex are called protomers . Some protomers can also function as independent proteins, but many only achieve their functionality in complexes.
The immunoglobulins ( antibodies ), in which two identical heavy and two identical light proteins are linked via a total of four disulfide bridges to form a functional antibody, can serve as an example of complexes made up of several proteins .
Similarly, hemoglobin forms the hemoglobin heterotetramer in which there are two identical α-subunits and two identical β-subunits.
An example of a protein complex composed of 13 different subunits is cytochrome c oxidase .
Many protein domains mediate very specific interactions between proteins and thus form the basis for the formation of protein complexes. Examples are the PDZ domains , Ankyrin repeats and many other domains.
Often, specific scaffold proteins with such protein domains also form the basis of a protein complex in which they act as a bridge between different proteins.
Many biochemical methods can be used to detect protein complexes, especially in the field of proteomics.