General Index


Quaternary Structure of Proteins




Definition

A protein has Quaternary Structure when it is formed by more than one polypeptide chain. In this case, the individual polypeptides are called Subunits or Monomers. Thus we define as Dimers proteins having two subunits, Trimers three, Tetramers four, and so on. Quaternary structure is very frequent in proteins, and specially in intracellular proteins. Very often queaternary structure is related to regulatory properties of proteins, and in particular those showing allosteric behavior. That is the case of Hemoglobin, the normal oxygen transporter in blood.

The protein Concanavallin A is an example of quaternary structure:

This protein is composed of four identical subunits (Homotetramer). The different subunits can be seen by the command "color chain":



Forces maintaining quaternary structure

In general, quaternary structure is maintained by the same forces as the tertiary structure.

Among the weak interactions, we have in the first place Hydrophobic Contacts beween the surfaces of the subunits. Also, Hydrogen Bonds are important. In some cases, Salt Bridges maintain quaternary structure, what can be illustrated in the case of Hemoglobin, in which its four subunits are linked by this type of forces.

We can also find Covalent bonds maintaining quaternary structure. An important case is that of Immunoglobulins, whose different chains are linked by disulfide bridges.



Examples

Quaternary structure present various degrees of complexity.An example of Homodimer is the Glycogen Phosphorylase:

It is a big protein, with two identical subunits (Homodimer):



Another example is that of G-Proteins, that participate in many processes of Signal Transduction:

G-proteins are formed by three different subunits (Heterotrimer). Alpha-subunit:



An example of high complexity is the enzyme Aspartate transcarbamylase:

This enzyme is formed by twelve subunits, six catalytic:


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