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Insulin is a hormone which is responsible for carbohydrate metabolism and storage. The body senses the concentration of glucose in the blood and responds by secreting insulin. Insulin is produced by beta cells in the pancreas.

The two chains shown, A and B, are the two pieces that make up insulin. These chains are joined by two disulfide bonds. The location of these bonds are represented by the labeled cysteine molecules on chains A and B. This is the active form of insulin. This form binds to muscle or fat cells to signal them to take up glucose, or sugar from the blood for storage.

Proinsulin is the precursor of insulin. It is produced by the pancreas and then converted into insulin. It can bind to the insulin receptor and exhibits 5% to 10% of the metabolic activity of insulin. A high concentration of proinsulin in the blood can be a warning sign of islet cell tumors.