From Atomic Resolution to Molecular Giants: an Overview of Crystallographic Studies of Biological Macromolecules with Synchrotron Radiation

Protein crystals have huge unit cells ( ≅100 A) filled not only with ordered protein molecules but also in about 50% with liquid water. The phase problem in protein crystallography can be solved by molecular replacement (using a suitable model molecule), by isomorphous replacement (using heavy atom derivatives), or by multiwavelength anomalous diffraction (using resonant scattering of synchrotron-generated X-rays by anomalous atoms, such as Se). X-ray diffraction by protein crystals produces thousands of reflections but since the models are very complex (many thousands of atoms), paucity of data is a serious problem and stereochemical restraints are necessary. In consequence, the highest possible resolution (minimum d-spacing in Bragg's Equation) should always be the experimental goal. Protein structures determined by crystallography are deposited in protein data bank, which currently holds more than 62000 entries. Recent methodological advancements, stimulated by a wide-spread use of powerful synchrotron sources and by structural genomics, have resulted in rapid acceleration of the structure elucidation process, and in addition help to obtain a better data. Protein crystallography has produced atomic models of gigantic macromolecular assemblies, including the ribosome. It is also providing accurate targets for structure-guided development of drugs. (author)