Quaternary structure or Quaternary level of protein structural organization, is the structure that results of the assembly of several polypeptide to make an unique functional protein, stabilized through several noncovalent interactions between the R side chain of amino acids from different peptide chains.
The non covalent interactions that maintain this structure are the same non covalent interactions that maintain the tertiary structure: Hydrogen bonds, Ionic interactions, Hydrophobic attractions and Van der Waals Forces.
Based on the definition we gave above, not all proteins show a quaternary level of organization. For having a quaternary structure:
a) The protein should be formed by more than one peptide chain.
b) These chains can not be attached by covalent bonds among them.
Some examples for clarifying the concept:
Myoglobin is formed by a single peptide chain and a hem group. Since Myoglobin is formed by just one peptide chain, it does not show quaternary structure.
Insulin, for example, is formed by two peptide chains, but since these two chains are linked by disulfide linkage, insulin does not qualify as a protein with quaternary structure.
Hemoglobin is formed by four peptide chains (and four Hem groups) that are forming a unique functional protein. These peptide chains are associated through non covalent bonds between their lateral chains: Hemoglobin is the typical example of a protein with quaternary structure.
The quaternary structure of a protein is intimately related to the feature of Allostery or Allosterism
It’s the property of some proteins to change its conformation and activity (it changes from a native conformation to a different native conformation), when interact specifically with some ligands.
In the case of enzymes, the ligand binds at a site different to the active site, that is called allosteric site. This change in conformation results in a change in the activity of the enzyme.
Characteristics of the Allosteric proteins
• All reach the quaternary level
• They have two different but native structures
• Each structure has a different functionality
• The two native structures are in equilibrium
• The equilibrium between the two structures is displaced when the protein interact with specific ligands
• The interaction with the ligand takes place in specific sites of the protein known as allosteric sites
Hemoglobin, mentioned above, is a typical example of a protein with quaternary structure that shows allosterism.
Hemoglobin has two different native structures:
- Oxyhemoglobin (Relaxed structure).
- Deoxyhemoglobin (Tense or Taught structure).
These different native structures have different functionality;
- The Relaxed (R ) structure has high O2 affinity.
- The taught or tense (T) structure has low O2 affinity.
These two forms are in equilibrium:
R ß——à T
Presence or absence of oxygen changes the conformation: when Hemoglobin binds to Oxygen, the equilibrium between these two kinds of structures is displaced to the R form; when Oxygen is released, the equilibrium is displaced to the T form.
These two animations reflect the change of Hemoglobin structure as a result of Oxygen binding:
Allosterism is also an important physiological mechanism of regulation of enzyme activity.
For more information about these topics, visit the following sites:
Berg, J.M.; Tymoczko, J.L. Stryer, L.: Biochemistry, 5t. Edition:
The Medical Biochemistry page