The structure of<unk> . The left side is a space-filling model of the<unk>  monomer, believed to be biologically active. Carbon is green, hydrogen white, oxygen red, and nitrogen blue. On the right side is a ribbon diagram of the<unk>  hexamer, believed to be the stored form. A monomer unit is highlighted with the A chain in blue and the B chain in cyan. Yellow denotes disulfide bonds, and magenta spheres are zinc ions. Contrary to an initial belief that hormones would be generally small chemical molecules, as the first peptide hormone known of its structure,<unk>  was found to be quite large. A single protein (monomer) of human<unk>  is composed of 51 amino acids, and has a molecular mass of 5808 Da. The molecular formula of human<unk>  is C257H383N65O77S6. It is a combination of two peptide chains (dimer) named an A-chain and a B-chain, which are linked together by two disulfide bonds. The A-chain is composed of 21 amino acids, while the B-chain consists of 30 residues. The linking (interchain) disulfide bonds are formed at cysteine residues between the positions A7-B7 and A20-B19. There is an additional (intrachain) disulfide bond within the A-chain between cysteine residues at positions A6 and A11. The A-chain exhibits two α-helical regions at A1-A8 and A12-A19 which are antiparallel; while the B chain has a central α -helix (covering residues B9-B19) flanked by the disulfide bond on either sides and two β-sheets (covering B7-B10 and B20-B23). The amino acid sequence of<unk>  is strongly conserved and varies only slightly between species. Bovine<unk>  differs from human in only three amino acid residues, and porcine<unk>  in one. Even<unk>  from some species of fish is similar enough to human to be clinically effective in humans.<unk>  in some invertebrates is quite similar in sequence to human<unk> , and has similar physiological effects. The strong homology seen in the<unk>  sequence of diverse species suggests that it has been conserved across much of animal evolutionary history. The C-peptide of proinsulin, however, differs much more among species; it is also a hormone, but a secondary one.<unk>  is produced and stored in the body as a hexamer (a unit of six<unk>  molecules), while the active form is the monomer. The hexamer is about 36000 Da in size. The six molecules are linked together as three dimeric units to form symmetrical molecule. An important feature is the presence of zinc atoms (Zn2+) on the axis of symmetry, which are surrounded by three water molecules and three histamine residues at position B10. The hexamer is an inactive form with long-term stability, which serves as a way to keep the highly reactive<unk>  protected, yet readily available. The hexamer-monomer conversion is one of the central aspects of<unk>  formulations for injection. The hexamer is far more stable than the monomer, which is desirable for practical reasons; however, the monomer is a much faster-reacting drug because diffusion rate is inversely related to particle size. A fast-reacting drug means<unk>  injections do not have to precede mealtimes by hours, which in turn gives people with diabetes more flexibility in their daily schedules.<unk>  can aggregate and form fibrillar interdigitated beta-sheets. This can cause injection amyloidosis, and prevents the storage of<unk>  for long periods.
Insulin
Regular insulin
Pulsatile insulin
Lente insulin
Proglucagon
Ultralente insulin
Glucuronide
Glucagon 