For a protein composed of two or more polypeptide chains with independent tertiary structures, the spatial structure formed by combining polypeptide chains through secondary bonds is called the quarternary structure of the protein. Among them, each polypeptide chain unit with an independent tertiary structure is called a subunit. The quaternary structure refers to the subunits\' three-dimensional arrangement, interaction, and contact site layout

 

There is no covalent bond between the subunits, and the combination of the secondary bond between the subunits is looser than that of the secondary and tertiary structures. Therefore, under certain conditions, the protein of the quaternary structure can be separated into its constituent subunits, and the subbasin Body conformation remains unchanged.

 

In a protein, the subunit structures can be the same or different. For example, the coat protein of tobacco mottle virus is a multimer formed by 2200 identical subunits; normal human hemoglobin A is a tetramer formed by two α subunits and two β subunits; aspartic acid carbamoyl Transferases are composed of six regulatory subunits and six catalytic subunits. Some call the smallest unit with a set of different subunits protomers, such as a protomer in which a catalytic subunit is combined with a regulatory subunit to form an aspartate carbamoyltransferase.

 

Certain protein molecules can be further aggregated into polymers. The repeating unit in a polymer is called a monomer, and the polymer can be divided into dimers, trimers, oligomers, and polymers according to the number of monomers contained therein. And exist, such as insulin (insulin) can form dimers and hexamers in vivo.

 

The polymers we make generally have a large molecular weight and do not dissolve well, so we use n-hexane, or methylene chloride, and use a Soxhlet extractor to achieve polymer isolation.