Protein Structure Class: 4. Quaternary

quaternary structure of proteins

Proteins that have multiple polypeptide chains have a quaternary structure, which forms from the combination of multiple polypeptide subunits.
Quaternary structure is the spatial arrangement of and interaction between polypeptide units. These subunits work together to form a complex oligomeric protein.
Quaternary structure molecular interactions are the same as those of tertiary structure: hydrogen and ionic bonds, hydrophobic interactions, and disulfide bonds.
Whereas tertiary interactions exist between amino acids of a single polypeptide chain, quaternary interactions exist between different polypeptide chains.

quaternary structure oligomers

The dimer is the simplest quaternary structure; it has just two subunits.

– "homodimer" references the sameness of its subunits.

– "heterodimer" references the difference in its subunits.

Trimers have 3 subunits

We draw three interwoven strands to represent collagen, which is a homotrimeric protein.
Collagen polypeptides interweave to provide connective tissue with strength (just as a three-stranded rope is stronger than a single-stranded one).

Tetramers have 4 subunits

Alpha-2-beta-2 tetramer, which is a protein with four subunits, but with only two unique components.
Hemoglobin is an alpha-2-beta-2 tetramer; small changes in each of the subunits affects its oxygen carrying capacity.

Clinical correlation: Rhinovirus, which causes the common cold, has a viral coat of 240 subunits, with 60 copies of 4 different subunits.

The following four conformational changes can occur because of the multimeric nature of quaternary structure:

Changing the shape of one subunit results in a shift of the positions of all the other subunits to accommodate the change.
Changes in the shape of all of the subunits can cause a complete change in the structure of the protein (making it "open" or "closed" or "on" or "off").
The multiple sites for modifications, such as phosphorylation, allows for a wide range of functional states of the protein.
The ability for multiple molecules to bind all at once, or the same molecule in multiple positions, enhances the protein's functionality.