RELIABILITY OF X-RAY CRYSTALLOGRAPHIC STRUCTURES

The process of X-ray crystallographic structure determination requires growing crystals and visualizing these structures in electron density maps. This process introduces some limitations in the reliability of these structures. The resolution limit, crystallographic R-factor and atomic temperature factors provide important clues in assessing the confidence a researcher can have in the coordinates of any particular segment in the protein structure. The structure of subtilisin determined independently in a number of laboratories from crystals grown in different conditions provides a means to obtain an empirical estimate of error. In the case of subtilisin BPN', the structure of which has been determined at resolutions ranging from 1.8-1.6 Angstrom resolution with R-factors ranging from 0.18-0.14, there is very good agreement between structures determined from different crystal forms. This observation suggests that the individual models are fair representations of the structure of the enzyme in solution.