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Abstract: Structural biology plays a crucial role in the fight against COVID-19, permitting us to ‘see’ and understand SARS-CoV-2. However, the macromolecular structures of SARS-CoV-2 proteins that were solved with great speed and urgency can contain errors that may hinder drug design. The Coronavirus Structural Task Force has been working behind the scenes to evaluate and improve these structures, making the results freely available at https://insidecorona.net/.

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Abstract: High-resolution crystal structures of enzymes in relevant redox states have transformed our understanding of enzyme catalysis. Recent developments have demonstrated that X-rays can be used, via the generation of solvated electrons, to drive reactions in crystals at cryogenic temperatures (100 K) to generate `structural movies' of enzyme reactions. However, a serious limitation at these temperatures is that protein conformational motion can be significantly supressed. Here, the recently developed MSOX (multiple serial structures from one crystal) approach has been applied to nitrite-bound copper nitrite reductase at room temperature and at 190 K, close to the glass transition. During both series of multiple structures, nitrite was initially observed in a `top-hat' geometry, which was rapidly transformed to a `side-on' configuration before conversion to side-on NO, followed by dissociation of NO and substitution by water to reform the resting state. Density functional theory calculations indicate that the top-hat orientation corresponds to the oxidized type 2 copper site, while the side-on orientation is consistent with the reduced state. It is demonstrated that substrate-to-product conversion within the crystal occurs at a lower radiation dose at 190 K, allowing more of the enzyme catalytic cycle to be captured at high resolution than in the previous 100 K experiment. At room temperature the reaction was very rapid, but it remained possible to generate and characterize several structural states. These experiments open up the possibility of obtaining MSOX structural movies at multiple temperatures (MSOX-VT), providing an unparallelled level of structural information during catalysis for redox enzymes.

Open Access

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Abstract: The bacterial second messenger cyclic di-3′,5′-guanosine monophosphate (c-di-GMP) is a key regulator of bacterial motility and virulence. As high levels of c-di-GMP are associated with the biofilm lifestyle, c-di-GMP hydrolysing phosphodiesterases (PDEs) have been identified as key targets to aid development of novel strategies to treat chronic infection by exploiting biofilm dispersal. We have studied the EAL signature motif-containing phosphodiesterase domains from the Pseudomonas aeruginosa proteins PA3825 (PA3825EAL) and PA1727 (MucREAL). Different dimerisation interfaces allow us to identify interface independent principles of enzyme regulation. Unlike previously characterised two-metal binding EAL-phosphodiesterases, PA3825EAL in complex with pGpG provides a model for a third metal site. The third metal is positioned to stabilise the negative charge of the 5′-phosphate, and thus three metals could be required for catalysis in analogy to other nucleases. This newly uncovered variation in metal coordination may provide a further level of bacterial PDE regulation.

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Abstract: Since its first use to solve the structure of sodium chloride in 1915 X-ray crystallography has developed significantly to become the premier technique for obtaining 3D structural information of small molecules and macromolecules alike. As the technique continues to develop and focus its attention on weak diffraction from the likes of micro-crystals and poorly packed crystals of membrane proteins and large protein complexes; as well as ultra-high resolution data and weak anomalous signal from native atoms, data quality is becoming more and more important. Data quality is particularly important in the wake of long wavelength macromolecular crystallography (MX) for phasing using anomalous signal from native sulphur and phosphorous atoms in proteins and DNA. This thesis first investigated the use of a new sample handling technique using a humidity controlled stream to preserve macromolecular crystals while excess surrounding solvent is removed (Chapter 2). Following the successful development of this technique the effects of excess surrounding solvent on data quality was assessed when collecting at standard MX X-ray wavelengths (~ 1 Å) and longer X-ray wavelengths (~ 2 Å). Datasets were collected from large populations of control and test crystals at standard and longer wavelengths to allow robust statistical methods to be applied; a practice not widely adopted in method development studies in X-ray crystallography. This made it possible to assess the small differences in data quality in the presence and absence of excess surrounding solvent. The effects of surrounding solvent at longer wavelengths appear to be protein dependent with some proteins tested showing no significant difference and others a significant decrease in data quality at longer wavelengths (Chapter 3). Originally this project aimed to use the new long wavelength in-vacuum MX beamline, I23, at Diamond Light Source UK to carry out phasing experiments using native sulphurs for structure solution. However, the considerable complexity involved in developing in-vacuum MX meant these experiments could not be carried out during the time frame of this thesis. Chapters 4 and 5 outline the production of a novel cancer protein (cancerous inhibitor of protein phosphatase 2A) and two protein targets from the Achromobacter xylosoxidans (Ax) genome intended for sulphur single wavelength anomalous dispersion phasing experiments on I23. Of these proteins the structure of Ax-α/β hydrolase was solved by conventional methods, the structure of which is discussed in Chapter 5. Of the protein crystals used in long wavelength data quality experiments in Chapter 3 the molecular biology of PA3825-EAL, a biofilm regulating protein essential to the swarming ability of Pseudomonas aeruginosa, was investigated further. The crystal structure of PA3825-EAL was solved in the resting, substrate bound and product bound states to high resolution. Comparison of the crystal structures of monomeric and dimeric PA3825-EAL with the inactive dimeric structure of MucR-EAL suggests dimerisation via helix 8 plays a role in inhibition of EAL domains. Prior to this, dimerisation was thought to be an activating factor in EAL domains. The product bound state of PA3825-EAL showed the presence of a previously unreported third metal binding site which may form an essential component of the reaction mechanism of EAL domains. Inability of MucR-EAL to incorporate this third metal due to dimerisation may explain the lack of activity despite possessing the conserved catalytic residues necessary. The fast detector technology and improvements in automated data processing software that allowed diffraction data for large populations of crystals to be collected in Chapters 2 and 3 have also been applied to development of a serial data collection technique.