Quaternary structure

Question 1

What is the definition of quaternary structure?

Answer 1

Quaternary structure refers to the organization and arrangement of subunits in a protein with multiple subunits. Each subunit is a separate polypeptide chain. A multisubunit protein is referred to as an oligomer (proteins with only one polypeptide chain are monomers). The subunits of a multisubunit protein may be identical or different.

(Reference: Chapter 4.8)

Question 2

How are subunits within an oligomeric protein held together?

Answer 2

The subunits within an oligomeric protein always have a defined stoichiometry and the arrangement of the subunits gives rise to a stable structure where subunits are usually held together by weak noncovalent interactions. Hydrophobic interactions are the principal forces involved although electrostatic forces may contribute to the proper alignment of the subunits.

(Reference: Chapter 4.8)

Question 3

Why is determination of the subunit composition of an oligomeric protein essential?

Answer 3

Determination of the subunit composition of an oligomeric protein is an essential step in the physical description of a protein. Typically, the molecular weight of the native oligomer is estimated by gel-filtration chromatography and then the molecular weight of each chain is determined by SDS-polyacrylamide gel electrophoresis (Section 3.6). For a protein having only one type of chain, the ratio of the two values provides the number of chains per oligomer.

(Reference: Chapter 4.8)

Question 4

What factors are related to the large proportion of proteins consisting of multiple subunits?

Answer 4

The fact that a large proportion of proteins consist of multiple subunits is probably related to several factors:
1. Oligomers are usually more stable than their dissociated subunits suggesting that quaternary structure prolongs the life of a protein in vivo.
2. The active sites of some oligomeric enzymes are formed by residues from adjacent polypeptide chains.

(Reference: Chapter 4.8)