Proteins are made up of amino acid sequences that are folded in its nature conformation inside any living cell. The mechanism of the folding depends on the amino acid content of every protein structure. The Tertiary structure of the amino acids is affected by hydrogen bonding, disulfide bridges, electrostatic interaction and non polar interactions.
Insulin is a protein hormone secreted in the pancreas that basically functions as a regulatory molecule for carbohydrate metabolism. It exists in the its hexamer form (inactive form) but are converted into the monomer form (active form) upon utilization in the cells.
Intrinsically disordered proteins (IDP) are type of protein structure that lack stable tertiary structure. Being in that condition, IDP's are generally flexible which is beneficial in cellular functions. However, there are still ongoing studies regarding the full functions of IDP in cellular metabolism.
The insulin hexamer is relatively stable but as one monomer leaves the group, the attachment on the Zinc atom is released. The insulin monomer now is relatively unstable. There are possible reasons why some scientists are considering the insulin monomer to be an IDP. First, the disulfide bridge on chain A creates a strained structure. Second, the tails of both chains consist of water soluble amino acids thus allowing the insulin monomer to move freely in an aqueous environment.