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Insulin-structure

Bovine serum insulin is a protein hormone made of two peptide chains, A (21 amino acids long) and B (30 amino acids long). In each chain, primary structure is indicated by three-letter abbreviations that represent the names of the amino acids in the order they are present. The amino acid cysteine (cys) has a sulfhydryl (SH) group as a side chain. Two sulfhydryl groups can react in the presence of oxygen to form a disulfide (S-S) bond. Two disulfide bonds connect the A and B chains together, and a third helps the A chain fold into the correct shape.

The A chain of insulin is 21 amino acids long. The sixth and eleventh amino acids in the A chain are cysteines and they are connected by a disulfide bond. The B chain of insulin is 30 amino acids long. The seventh amino acid in chain A and the seventh amino acid in chain B are cysteines and they are connected by a disulfide bond. The twentieth amino acid in chain A and the nineteenth amino acid in chain B are cysteines and they are connected by a disulfide bond.

Figure 1: The A and B chains of insulin are linked together by disulfide bonds. Note that all disulfide bonds are the same length, but are drawn different sizes for clarity.

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Insulin