X-ray crystallography and biological metal centers: Is seeing believing?

Metalloenzyme crystal structures have a major impact on our understanding of biological metal centers. They are often the starting point for mechanistic and computational studies and inspire synthetic modeling chemistry. The strengths and limitations of X-ray crystallography in determining properties of biological metal centers and their corresponding ligand spheres are explored through examples, including ribonucleotide reductase R2 and particulate methane monooxygenase. Protein crystal structures locate metal ions within a protein fold and reveal the identities and coordination geometries of amino acid ligands. Data collection strategies that exploit the anomalous scattering effect of metal ions can establish metal ion identity. The quality of crystallographic data, particularly the resolution, determines the level of detail that can be extracted from a protein crystal structure. Complementary spectroscopic techniques can provide crucial information regarding the redox state of the metal center as well as the presence, type, and protonation state of exogenous ligands. The final result of the crystallographic characterization of a metalloenzyme is a model based on crystallographic data, supported by information from biophysical and modeling studies, influenced by sample handling, and interpreted carefully by the crystallographer.