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: