Protein crystallography at subatomic resolution

During the last decade, a number of methodological and technical improvements in macromolecular crystallography (MC) led to the appearance of several protein structures at a level of resolution higher than 0.85 Angstrom. This leads to the possibility of analysing the structure at a very fine level of detail: one can refine multiple conformations, measure the deviations from the standard stereochemistry, visualize hydrogen atoms and determine the protonation states of catalytic residues of enzymes. In many cases, this information is directly related to the function of proteins. In this review, the methodological improvements are described, brief information about all the structures solved at resolution better than 0.85 Angstrom is given, with particular emphasis on the additional information given by subatomic resolution, and some examples are treated in detail. The availability of crystallographic data at this resolution opens wide perspectives for theoretical modelling. Among them the most promising are the approaches (mostly under development) to determine the reactivity directly from the diffraction data.