Comparative assessment of crystallographic and cryo-EM models in the Protein Data Bank

With cryogenic electron microscopy (cryo-EM) on track to surpass X-ray crystallography as the preferred method for determining macromolecular structures, it is important to evaluate and compare the quality of structure models obtained by these methods. This allows us to assess whether the rapidly growing numbers (quantity) correlate with quality and to identify areas in which each method excels or falls short. Selected quality-related parameters were compared for 97 200 crystal structures and 30 139 cryo-EM structures released by the Protein Data Bank (PDB) between 2015 and 2025. Comparison of geometric and stereochemical parameters indicated that, despite significant differences in the resolution of the experimental data, these values were, in the vast majority of cases, close to the expected targets. Nevertheless, we found that crystal structures tend to exhibit more Ramachandran and rotamer outliers than cryo-EM structures, although they unexpectedly have lower clashscore values. Separately, we compared the quality of 612 crystal and 1817 cryo-EM structures in the PDB representing complete ribosomes or their subunits. For this subset of very large, well-defined macromolecules, we found that the quality of many cryo-EM models is higher than that of their crystal counterparts, and that the best cryo-EM structures were also determined at higher resolution. Overall, we conclude that the availability of both techniques has clearly resulted in major advances during the last decade and bodes very well for the future.