proinsulin
the protein after the removal of the signal sequence in the endoplasmic reticulum and formation of disulfide bonds
Proinsulin is a precursor molecule that is converted to insulin. It is initially synthesized as a single polypeptide chain in the beta cells of the pancreas and contains a signal peptide region at the beginning, followed by the B chain, a connecting peptide, and then the A chain.
After synthesis, proinsulin is transported to the Golgi apparatus where it is packaged into secretory vesicles. Within the vesicles, proinsulin undergoes enzymatic cleavage. The connecting peptide is removed and the molecule folds to form the mature insulin protein, consisting of two polypeptide chains, the A and B chains, held together by disulfide bonds.
The mature insulin is then released from the vesicles into the bloodstream in response to increased glucose concentration. Insulin regulates blood glucose levels by stimulating the uptake of glucose by the body’s cells and promoting its utilization or storage in tissues such as muscle, liver, and fat.
Abnormalities in proinsulin processing have been observed in some medical conditions such as type 2 diabetes, which can result in an increase in proinsulin levels in the circulation. The measurement of proinsulin levels is used in the diagnosis and monitoring of diabetes.
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