Sus scrofa (pig)
Insulin is one of the classical molecules of protein crystallography. Much of the work on this protein was done in the laboratory of Dorothy Crowfoot-Hodgkin who received the 1964 Nobel prize in chemistry for her determination of biochemically important molecules and has so far remained the only woman among structural chemists honored with a Nobel prize. Insulin is also one of the important hormones of the human body, produced by the pancreas, and insulin-related health problems, known as diabetes, are one of the most dominant health problems of the Western world; in third-world countries tuberculosis is much more of a problem. Insulin helps to let glucose enter into cells and if the body does not produce sufficient amounts of insulin (type I) or if the existing insulin cannot be properly used (type II) the net result is called diabetes. Diabetes can be treated with drugs and with additional supplies of insulin, but untreated diabetes can be fatal.
When insulin was first crystallized, reproducible growth of crystals had been a problem after "successful" purification. The reason was that insulin needed zinc for forming crystals and the zinc had been removed. Proinsulin is cleaved to form two separate chains. The zinc coordinates with a water and with the nitrogen of a histidine side-chain. Of course a metal ion needs more coordination partners than that. The remaining coordinating atoms are contributed from molecules related to the original one by the symmetry of the crystal. This explains why insulin crystals could not be obtained in the absence of zinc.
Protein Data Bank (PDB)
Baker, E.N. Blundell, T.L. Cutfield, J.F. Cutfield, S.M. Dodson, E.J. Dodson, G.G. Hodgkin, D.M. Hubbard, R.E. Isaacs, N.W. Reynolds, C.D. al., et; "The structure of 2Zn pig insulin crystals at 1.5 Angstroms resolution."; Philos.Trans.R.Soc.Lond. B Biol.Sci.; (1988) 319:369-456 PubMed:2905485.
author: Arno Paehler