3.6 : Intrinsically Disordered Proteins

Proteins often have rigid secondary and tertiary structures that can be determined experimentally; however, many proteins have flexible structures without a fixed conformation.

These intrinsically disordered proteins, or IDPs, must change shape to perform their functions in an organism. 

Disordered sections of proteins contain many hydrophilic amino acids because their amino acid chain must be soluble in the cytoplasm.  

IDPs contain few hydrophobic amino acids when its entire chain is flexible; this is because, unlike compact protein structures, these extended structures do not have a protein core where the hydrophobic amino acids can cluster.

Unlike improperly or unfolded proteins, which are usually either refolded or degraded by the cell, IDPs may never fold into a fixed structure, or may only become ordered under specific cellular conditions.

When a structured arrangement of the amino acid chain forms in an IDP, this is called a disorder to order transition.  This can be triggered by a covalent modification or an interaction with another molecule that induces a new conformation.

Some IDPs have small flexible segments connecting rigid sections of protein. The segments tether the globular sections of proteins together while enabling them to either interact or act independently with other targets. 

Flexible segments can also act as molecular switches changing the function of a protein depending on its conformation.

IDPs’ flexible shape allows them to interact in unique ways with the surfaces of other proteins. These proteins can wrap around their binding partners or act as molecular glue, bringing various other proteins together.

Because of their flexibility, IDPs can have many different binding partners, and they may take different ordered conformations depending on their interactions. This allows a single protein to play several different roles in the cell.