Proinsulin is a precursor molecule to insulin, a hormone that regulates blood sugar levels. It is produced in the pancreas and consists of three parts: the A chain, the B chain, and the connecting C-peptide. When the body needs insulin, proinsulin is converted into insulin and C-peptide through a process called proteolytic cleavage. The conversion of proinsulin to insulin is crucial for maintaining proper glucose metabolism. Insulin helps cells absorb glucose from the bloodstream, lowering blood sugar levels. Abnormal levels of proinsulin can indicate issues with insulin production and may be associated with conditions like diabetes.