Abstract
This chapter reviews the three-dimensional structure of immunoglobulins. The number of proteins that yield crystals, and the stability of the crystals, appear to be considerably greater when Fab or Fab' fragments of human IgG myeloma proteins, rather than the intact proteins, are used. Also, the smaller molecular size of either fragment facilitates detailed analysis by X-ray crystallography. Myeloma proteins used for the preparation of fragments are first fractionated by ion-exchange chromatography to remove most of the nonspecific IgG present. After digestion with papain, or by pepsin followed by reduction, the Fab or Fab' fragments are isolated by ion-exchange chromatography and gel filtration, and are crystallized in the presence of ammonium sulfate; the optimal pH varies for individual proteins. About one-half of the myeloma proteins investigated have yielded crystals of Fab or Fab'. A representation of the folding of the polypeptide chains within a domain is shown in the chapter. The folding patterns of the four individual domains are very similar, except for the presence of an extra segment in each V domain.
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