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