Raw diffraction data are our ground truth from which all subsequent workflows develop

Cryo-EM meets crystallography: A model-independent view of the heteronuclear Mn4Ca cluster structure of photosystem II

Photosynthesis converts solar energy to chemical energy by splitting water molecules and carbon dioxide to produce oxygen and carbohydrates with an efficiency that engineers working on solar energy device can only dream of. Photosystem II (PSII) is the enzyme that catalyzes the light-driven oxidation of water that occurs during photosynthesis. This oxygen-producing reaction occurs in the Mn4Ca cluster found inside the enzyme's oxygen-evolving center (OEC). Even though the structure and mechanism of action of the OEC have been intensively investigated for many decades, questions still remain about both. The Mn4Ca cluster stores the high-energy oxidizing equivalents required for water oxidation so that its own oxidation state depends on its chemical composition, i.e., the number of its oxygen ligands. The issue addressed here is the number of oxygen ligands associated with the Mn4Ca cluster after PSII has been exposed to two of the four flashes of light necessary for it to produce an oxygen molecule, PSII 2F. Comparisons of recently published cryo-EM maps and crystallographic OEC-omit maps described here reveal the OEC structure of PSII 2F contains only five internal oxygen ligands, and that the sixth O ligand identified earlier in PSII 2F crystal structures is an artifact caused by model bias. This finding should have a significant impact on our understanding of the mechanism of water oxidation that is catalyzed by PSII because it is incompatible with prevailing high valence paradigm (HVP) models for this process.

Managing macromolecular crystallographic data with a laboratory information management system

Protein crystallography is an established method to study the atomic structures of macromolecules and their complexes. A prerequisite for successful structure determination is diffraction-quality crystals, which may require extensive optimization of both the protein and the conditions, and hence projects can stretch over an extended period, with multiple users being involved. The workflow from crystallization and crystal treatment to deposition and publication is well defined, and therefore an electronic laboratory information management system (LIMS) is well suited to management of the data. Completion of the project requires key information on all the steps being available and this information should also be made available according to the FAIR principles. As crystallized samples are typically shipped between facilities, a key feature to be captured in the LIMS is the exchange of metadata between the crystallization facility of the home laboratory and, for example, synchrotron facilities. On completion, structures are deposited in the Protein Data Bank (PDB) and the LIMS can include the PDB code in its database, completing the chain of custody from crystallization to structure deposition and publication. A LIMS designed for macromolecular crystallography, IceBear, is available as a standalone installation and as a hosted service, and the implementation of key features for the capture of metadata in IceBear is discussed as an example.

Dependence of crystallographic atomic displacement parameters on temperature (25-150 K) for complexes of horse liver alcohol dehydrogenase

Enzymes catalyze reactions by binding and orienting substrates with dynamic interactions. Horse liver alcohol dehydrogenase catalyzes hydrogen transfer with quantum-mechanical tunneling that involves fast motions in the active site. The structures and B factors of ternary complexes of the enzyme with NAD+ and 2,3,4,5,6-pentafluorobenzyl alcohol or NAD+ and 2,2,2-trifluoroethanol were determined to 1.1-1.3 Å resolution below the `glassy transition' in order to extract information about the temperature-dependent harmonic motions, which are reflected in the crystallographic B factors. The refinement statistics and structures are essentially the same for each structure at all temperatures. The B factors were corrected for a small amount of radiation decay. The overall B factors for the complexes are similar (13-16 Å2) over the range 25-100 K, but increase somewhat at 150 K. Applying TLS refinement to remove the contribution of pseudo-rigid-body displacements of coenzyme binding and catalytic domains provided residual B factors of 7-10 Å2 for the overall complexes and of 5-10 Å2 for C4N of NAD+ and the methylene carbon of the alcohols. These residual B factors have a very small dependence on temperature and include local harmonic motions and apparently contributions from other sources. Structures at 100 K show complexes that are poised for hydrogen transfer, which involves atomic displacements of ∼0.3 Å and is compatible with the motions estimated from the residual B factors and molecular-dynamics simulations. At 298 K local conformational changes are also involved in catalysis, as enzymes with substitutions of amino acids in the substrate-binding site have similar positions of NAD+ and pentafluorobenzyl alcohol and similar residual B factors, but differ by tenfold in the rate constants for hydride transfer.

The four Rs and crystal structure analysis: reliability, reproducibility, replicability and reusability

Within science, of which crystallography is a key part, there are questions posed to all fields that challenge the trust in results. The US National Academies of Sciences, Engineering and Medicine published a thorough report in 2019 on the Reproducibility and Replicability of Science: replicability being where a totally new study attempts to confirm if a phenomenon can be seen independently of another study. Data reuse is a key term in the FAIR data accord [Wilkinson et al. (2016). Sci. Data, 3, 160018], where the acronym FAIR means findable, accessible, interoperable and reusable. In the social sciences, the acronym FACT (namely fairness, accuracy, confidentiality and transparency) has emerged, the idea being that data should be FACTual to ensure trust [van der Aalst et al. (2017). Bus. Inf. Syst. Eng. 59, 311-313]. A distinction also must be made between accuracy and precision; indeed, the authors' lectures at the European Crystallography School ECS6 independently emphasized the need for use of other methods as well as crystal structure analysis to establish accuracy in biological and chemical/material functional contexts. The efforts by disparate science communities to introduce new terms to ensure trust have merit for discussion in crystallographic teaching commissions and possible adoption by crystallographers too.