Despite the impressive developments in NMR spectroscopy in recent years, which now allow the secondary and tertiary structures of small polypeptides (< 10000 daltons) to be determined in solution, X-ray crystallography will probably remain the only method capable of providing detailed structural information (namely, three-dimensional atomic coordinates) on larger, globular proteins. Whereas application of NMR techniques to the determination of protein structure is limited by the requirements that the protein be available in large amounts, be sufficiently soluble, and exhibit good stability, the use of protein crystallography primarily depends on the ability to crystallize the macromolecule. Once suitable crystals of the protein have been obtained, the elucidation of its structure can be accomplished within a reasonable length of time, provided that its amino acid sequence is known.
New techniques for the collection of diffraction data (area detectors) and for the interpretation of electron density (high-resolution vector graphics) have not only decreased the time required for protein structure determination, but, even more importantly, have improved the accuracy of the structures obtained. Crystallographic refinement has progressed to a point where the methods available, while still requiring considerable patience, often afford. atomic coordinates with error limits of only 0.1 to 0.2 A. Such highly refined protein structures are, in turn, essential for modeling the interaction of these proteins with small molecules, such as substrates and inhibitors, or for planning site-directed mutagenesis experiments. These applications, in particular, have made protein crystallography an interesting and potentially important tool for the pharmaceutical industry.
The rapid development of this field makes periodic reviews of protein crystallographic methods all the more ur-