Protein hormone INSULIN is a hormone produced by the pancreas that helps regulate the body's blood sugar levels. It is made up of two polypeptide chains, A and B, which are linked together by disulfide bonds. INSULIN helps to transport glucose from the bloodstream into cells, where it can be used for energy. It also helps to store glucose in the liver and muscles for later use. Without INSULIN, the body cannot properly use glucose, leading to high blood sugar levels and other health problems.