[4] X-Ray Diffraction of Protein Crystals

The X-ray study of protein crystals is a difficult and highly specialized field. Eventually, when the structure of a few proteins has been unraveled, the results will be of vital interest to ensytiologists, since they should give information about the spatial arrangement of the “active site” of the enzyme. But meanwhile X-ray methods will have only a secondary interest for enzymologists. The equipment required for X-ray work is expensive and will normally not be available in an enzymological laboratory. This chapter has therefore been written on the assumption that the X-ray work will be carried out in collaboration with a professional crystallographer. It aims to show both the enzymologist and the X-ray crystallographer-who may not be familiar with the X-ray work on proteins*J-the kind of information that can usefully be obtained in a fairly short time. A knowledge of normal crystallographic terms and techniques3 is therefore assumed, but all the special techniques required are described in detail. The most useful information that can be obtained from an X-ray examination of protein crystals is a rather good value for the molecular weight. If crystals of a reasonable size are available, a figure correct to a few per cent can be obtained in a few days. This method has been neglected in the past, and enzymologists might well find it helpful. Occasionally it can be shown that two proteins from different sources have very similar structures, and more detailed studies may, in favorable cases, yield some information on the shape of the molecule.