Insulin.

One of the best-known hormones, insulin is synthesized initially as preproinsulin. The mature insulin is a two-chain molecule with 21 and 30 residues. Thus, insulin consists of only 51 of the initial 110 amino acid residues of preproinsulin.
  Backbone model. The two chains are connected by two disulfide cross-links, and there is an additional -SS- bridge in the A chain (blue). Also observe the three short alpha-helices.
  Transparent view.
  Spacefilled model; chain colors.
  Cpk colors.