Structural basis for enzymatic terminal C-H bond functionalization of alkanes.

Alkane monooxygenase (AlkB) is a widely occurring integral membrane metalloenzyme that catalyzes the initial step in the functionalization of recalcitrant alkanes with high terminal selectivity. AlkB enables diverse microorganisms to use alkanes as their sole carbon and energy source. Here we present the 48.6-kDa cryo-electron microscopy structure of a natural fusion from Fontimonas thermophila between AlkB and its electron donor AlkG at 2.76 Å resolution. The AlkB portion contains six transmembrane helices with an alkane entry tunnel within its transmembrane domain. A dodecane substrate is oriented by hydrophobic tunnel-lining residues to present a terminal C-H bond toward a diiron active site. AlkG, an [Fe-4S] rubredoxin, docks via electrostatic interactions and sequentially transfers electrons to the diiron center. The archetypal structural complex presented reveals the basis for terminal C-H selectivity and functionalization within this broadly distributed evolutionary class of enzymes.

A self-supervised workflow for particle picking in cryo-EM.

High-resolution single-particle cryo-EM data analysis relies on accurate particle picking. To facilitate the particle picking process, a self-supervised workflow has been developed. This includes an iterative strategy, which uses a 2D class average to improve training particles, and a progressively improved convolutional neural network for particle picking. To automate the selection of particles, a threshold is defined (%/Res) using the ratio of percentage class distribution and resolution as a cutoff. This workflow has been tested using six publicly available data sets with different particle sizes and shapes, and can automatically pick particles with minimal user input. The picked particles support high-resolution reconstructions at 3.0 Šor better. This workflow is a step towards automated single-particle cryo-EM data analysis at the stage of particle picking. It may be used in conjunction with commonly used single-particle analysis packages such as Relion, cryoSPARC, cisTEM, SPHIRE and EMAN2.

Automatic processing of macromolecular crystallography X-ray diffraction data at the ESRF.

The development of automated high-intensity macromolecular crystallography (MX) beamlines at synchrotron facilities has resulted in a remarkable increase in sample throughput. Developments in X-ray detector technology now mean that complete X-ray diffraction datasets can be collected in less than one minute. Such high-speed collection, and the volumes of data that it produces, often make it difficult for even the most experienced users to cope with the deluge. However, the careful reduction of data during experimental sessions is often necessary for the success of a particular project or as an aid in decision making for subsequent experiments. Automated data reduction pipelines provide a fast and reliable alternative to user-initiated processing at the beamline. In order to provide such a pipeline for the MX user community of the European Synchrotron Radiation Facility (ESRF), a system for the rapid automatic processing of MX diffraction data from single and multiple positions on a single or multiple crystals has been developed. Standard integration and data analysis programs have been incorporated into the ESRF data collection, storage and computing environment, with the final results stored and displayed in an intuitive manner in the ISPyB (information system for protein crystallography beamlines) database, from which they are also available for download. In some cases, experimental phase information can be automatically determined from the processed data. Here, the system is described in detail.

ISPyB: an information management system for synchrotron macromolecular crystallography.

MOTIVATION: Individual research groups now analyze thousands of samples per year at synchrotron macromolecular crystallography (MX) resources. The efficient management of experimental data is thus essential if the best possible experiments are to be performed and the best possible data used in downstream processes in structure determination pipelines. Information System for Protein crystallography Beamlines (ISPyB), a Laboratory Information Management System (LIMS) with an underlying data model allowing for the integration of analyses down-stream of the data collection experiment was developed to facilitate such data management. RESULTS: ISPyB is now a multisite, generic LIMS for synchrotron-based MX experiments. Its initial functionality has been enhanced to include improved sample tracking and reporting of experimental protocols, the direct ranking of the diffraction characteristics of individual samples and the archiving of raw data and results from ancillary experiments and post-experiment data processing protocols. This latter feature paves the way for ISPyB to play a central role in future macromolecular structure solution pipelines and validates the application of the approach used in ISPyB to other experimental techniques, such as biological solution Small Angle X-ray Scattering and spectroscopy, which have similar sample tracking and data handling requirements.

Structural and functional insights into DR2231 protein, the MazG-like nucleoside triphosphate pyrophosphohydrolase from Deinococcus radiodurans.

Deinococcus radiodurans is among the very few bacterial species extremely resistant to ionizing radiation, UV light, oxidizing agents, and cycles of prolonged desiccation. The proteome of D. radiodurans reflects the evolutionary pressure exerted by chronic exposure to (nonradioactive) forms of DNA and protein damage. A clear example of this adaptation is the overrepresentation of protein families involved in the removal of non-canonical nucleoside triphosphates (NTPs) whose incorporation into nascent DNA would promote mutagenesis and DNA damage. The three-dimensional structure of the DR2231 protein has been solved at 1.80 Å resolution. This protein had been classified as an all-α-helical MazG-like protein. The present study confirms that it holds the basic structural module characteristic of the MazG superfamily; two helices form a rigid domain, and two helices form a mobile domain and connecting loops. Contrary to what is known of MazG proteins, DR2231 protein shows a functional affinity with dUTPases. Enzymatic and isothermal calorimetry assays have demonstrated high specificity toward dUTP but an inability to hydrolyze dTTP, a typical feature of dUTPases. Co-crystallization with the product of hydrolysis, dUMP, in the presence of magnesium or manganese cations, suggests similarities with the dUTP/dUDP hydrolysis mechanism reported for dimeric dUTPases. The genome of D. radiodurans encodes for all enzymes required for dTTP synthesis from dCMP, thus bypassing the need of a dUTPase. We postulate that DR2231 protein is not essential to D. radiodurans and rather performs "house-cleaning" functions within the framework of oxidative stress response. We further propose DR2231 protein as an evolutionary precursor of dimeric dUTPases.

Inducing phase changes in crystals of macromolecules: status and perspectives for controlled crystal dehydration.

The increase in the number of large multi-component complexes and membrane protein crystal structures determined over the last few years can be ascribed to a number of factors such as better protein expression and purification systems, the emergence of high-throughput crystallization techniques and the advent of 3rd generation synchrotron sources. However, many systems tend to produce crystals that can be extremely heterogeneous in their diffraction properties. This prevents, in many cases, the collection of diffraction data of sufficient quality to yield useful biological or phase information. Techniques that can increase the diffraction quality of macromolecular crystals can therefore be essential in the successful conclusion of these challenging projects. No technique is universal but encouraging results have been recently achieved by carrying out the controlled dehydration of crystals of biological macromolecules. A new device that delivers a stream of air with a precisely controlled relative humidity to the complicated sample environment found at modern synchrotron beamlines has been conceived at the EMBL Grenoble and developed by the EMBL and the ESRF as part of the SPINE2 complexes project, a European Commission funded protein structure initiative. The device, the HC1b, has been available for three years at the ESRF macromolecular crystallography beamlines and many systems have benefitted from on-line controlled dehydration. Here we describe a standard dehydration experiment, highlight some successful cases and discuss the different possible uses of the device.

Diffraction cartography: applying microbeams to macromolecular crystallography sample evaluation and data collection.

Crystals of biological macromolecules often exhibit considerable inter-crystal and intra-crystal variation in diffraction quality. This requires the evaluation of many samples prior to data collection, a practice that is already widespread in macromolecular crystallography. As structural biologists move towards tackling ever more ambitious projects, new automated methods of sample evaluation will become crucial to the success of many projects, as will the availability of synchrotron-based facilities optimized for high-throughput evaluation of the diffraction characteristics of samples. Here, two examples of the types of advanced sample evaluation that will be required are presented: searching within a sample-containing loop for microcrystals using an X-ray beam of 5 microm diameter and selecting the most ordered regions of relatively large crystals using X-ray beams of 5-50 microm in diameter. A graphical user interface developed to assist with these screening methods is also presented. For the case in which the diffraction quality of a relatively large crystal is probed using a microbeam, the usefulness and implications of mapping diffraction-quality heterogeneity (diffraction cartography) are discussed. The implementation of these techniques in the context of planned upgrades to the ESRF's structural biology beamlines is also presented.

MxCuBE: a synchrotron beamline control environment customized for macromolecular crystallography experiments.

The design and features of a beamline control software system for macromolecular crystallography (MX) experiments developed at the European Synchrotron Radiation Facility (ESRF) are described. This system, MxCuBE, allows users to easily and simply interact with beamline hardware components and provides automated routines for common tasks in the operation of a synchrotron beamline dedicated to experiments in MX. Additional functionality is provided through intuitive interfaces that enable the assessment of the diffraction characteristics of samples, experiment planning, automatic data collection and the on-line collection and analysis of X-ray emission spectra. The software can be run in a tandem client-server mode that allows for remote control and relevant experimental parameters and results are automatically logged in a relational database, ISPyB. MxCuBE is modular, flexible and extensible and is currently deployed on eight macromolecular crystallography beamlines at the ESRF. Additionally, the software is installed at MAX-lab beamline I911-3 and at BESSY beamline BL14.1.

A triclinic crystal form of Escherichia coli 4-diphosphocytidyl-2C-methyl-D-erythritol kinase and reassessment of the quaternary structure.

4-Diphosphocytidyl-2C-methyl-D-erythritol kinase (IspE; EC 2.7.1.148) contributes to the 1-deoxy-D-xylulose 5-phosphate or mevalonate-independent biosynthetic pathway that produces the isomers isopentenyl diphosphate and dimethylallyl diphosphate. These five-carbon compounds are the fundamental building blocks for the biosynthesis of isoprenoids. The mevalonate-independent pathway does not occur in humans, but is present and has been shown to be essential in many dangerous pathogens, i.e. Plasmodium species, which cause malaria, and gram-negative bacteria. Thus, the enzymes involved in this pathway have attracted attention as potential drug targets. IspE produces 4-diphosphosphocytidyl-2C-methyl-D-erythritol 2-phosphate by ATP-dependent phosphorylation of 4-diphosphocytidyl-2C-methyl-D-erythritol. A triclinic crystal structure of the Escherichia coli IspE-ADP complex with two molecules in the asymmetric unit was determined at 2 A resolution and compared with a monoclinic crystal form of a ternary complex of E. coli IspE also with two molecules in the asymmetric unit. The molecular packing is different in the two forms. In the asymmetric unit of the triclinic crystal form the substrate-binding sites of IspE are occluded by structural elements of the partner, suggesting that the ;triclinic dimer' is an artefact of the crystal lattice. The surface area of interaction in the triclinic form is almost double that observed in the monoclinic form, implying that the dimeric assembly in the monoclinic form may also be an artifact of crystallization.

Biophysical characterization and mutational analysis of the antibiotic resistance protein NimA from Deinococcus radiodurans.

Metronidazole (MTZ) is an antibiotic commonly used to treat anaerobic bacterial infections in humans and animals. Antibiotic resistance toward this class of 5-nitroimidazole (5-Ni) drug derivatives has been related to the Nim genes thought to encode a reductase. Here we report the biophysical characteristics of the NimA protein from Deinococcus radiodurans (DrNimA) binding to MTZ and three other 5-Ni drugs. The interaction energies of the protein and antibiotic are studied by isothermal titration calorimetry (ITC) and with free energy and linear interaction energy (LIE) calculations, where the latter method revealed that the antibiotic binding is mainly of hydrophobic character. ITC measurements further found that one DrNimA dimer has two antibiotic binding sites which were not affected by mutation of the reactive His71. The observed association constants (K(a)) were in the range of 5.1-4910(4)M(-1) and the enthalpy release upon binding to DrNimA for the four drugs studied was relatively low (approximately -1 kJ/mol) but still measurable. The drug binding is mainly entropy driven and along with the hydrophobic drug binding site found by crystallography, this possibly explains the low observed enthalpy values. The effect of the His71 mutation and the presence of MTZ were studied by differential scanning calorimetry (DSC). Native DrNimA is a yellow colored protein where the interaction from His71 to the cofactor is thought to be responsible for the coloring. Mutations of His71 to Ala, Ser, Leu or Asp all gave transparent, colorless protein solutions, and the two mutant crystal structures of DrNimA-H71A and DrNimA-H71S presented revealed no cofactor binding.

High-resolution structure of the antibiotic resistance protein NimA from Deinococcus radiodurans.

Many anaerobic human pathogenic bacteria are treated using 5-nitroimidazole-based (5-Ni) antibiotics, a class of inactive prodrugs that contain a nitro group. The nitro group must be activated in an anaerobic one-electron reduction and is therefore dependent on the redox system in the target cells. Antibiotic resistance towards 5-Ni drugs is found to be related to the nim genes (nimA, nimB, nimC, nimD, nimE and nimF), which are proposed to encode a reductase that is responsible for converting the nitro group of the antibiotic into a nonbactericidal amine. A mechanism for the Nim enzyme has been proposed in which two-electron reduction of the nitro group leads to the generation of nontoxic derivatives and confers resistance against these antibiotics. The cofactor was found to be important in the mechanism and was found to be covalently linked to the reactive His71. In this paper, the 1.2 A atomic resolution crystal structure of the 5-nitroimidazole antibiotic resistance protein NimA from Deinococcus radiodurans (DrNimA) is presented. A planar cofactor is clearly visible and well defined in the electron-density map adjacent to His71, the identification of the cofactor and its properties are discussed.

Structure of Deinococcus radiodurans tunicamycin-resistance protein (TmrD), a phosphotransferase.

The open-reading frame (ORF) DR_1419 in the Deinococcus radiodurans genome is annotated as a representative of the wide family of tunicamycin-resistance proteins as identified in a range of bacterial genomes. The D. radiodurans ORF DR_1419 was cloned and expressed; the protein TmrD was crystallized and its X-ray crystal structure was determined to 1.95 A resolution. The structure was determined using single-wavelength anomalous diffraction with selenomethionine-derivatized protein. The refined structure is the first to be reported for a member of the tunicamycin-resistance family. It reveals strong structural similarity to the family of nucleoside monophosphate kinases and to the chloramphenicol phosphotransferase of Streptomyces venezuelae, suggesting that the mode of action is possibly by phosphorylation of tunicamycin.

The crystal structure of DR2241 from Deinococcus radiodurans at 1.9 A resolution reveals a multi-domain protein with structural similarity to chelatases but also with two additional novel domains.

A unique family of proteins have been identified in the Deinococcus genus with an N-terminal cobalamin (vitamin B(12)) chelatase domain denoted CbiX and an additional unique C-terminal domain with unknown function. Here we report the first crystal structure from this new family of proteins with the structure of Deinococcus radiodurans protein DR2241. The structure reveals a multi-domain protein where domains A (residues 1-132) has the same fold as the small CbiX (CbiX(S)), domains A and B (residues 1-272) follow the chelatase super-family fold and the two additional unique domains C and D have no structural homologues. Domain D harbours the sequence motifs CxxC and CxxxC, in which DR2241 gives the first evidence that these motifs bind a [4Fe-4S] iron-sulphur cluster. In solution there are indications of multimeric forms, and in the crystallographic asymmetric unit a tetramer is found where domains C and D are involved in stabilising the tetrameric assembly.

Structures of Staphylococcus aureus D-tagatose-6-phosphate kinase implicate domain motions in specificity and mechanism.

High resolution structures of Staphylococcus aureus d-tagatose-6-phosphate kinase (LacC) in two crystal forms are herein reported. The structures define LacC in apoform, in binary complexes with ADP or the co-factor analogue AMP-PNP, and in a ternary complex with AMP-PNP and D-tagatose-6-phosphate. The tertiary structure of the LacC monomer, which is closely related to other members of the pfkB subfamily of carbohydrate kinases, is composed of a large alpha/beta core domain and a smaller, largely beta "lid." Four extended polypeptide segments connect these two domains. Dimerization of LacC occurs via interactions between lid domains, which come together to form a beta-clasp structure. Residues from both subunits contribute to substrate binding. LacC adopts a closed structure required for phosphoryl transfer only when both substrate and co-factor are bound. A reaction mechanism similar to that used by other phosphoryl transferases is proposed, although unusually, when both substrate and co-factor are bound to the enzyme two Mg(2+) ions are observed in the active site. A new motif of amino acid sequence conservation common to the pfkB subfamily of carbohydrate kinases is identified.

Structural basis of the nickel response in Helicobacter pylori: crystal structures of HpNikR in Apo and nickel-bound states.

The survival of Helicobacter pylori in the human stomach critically relies on the availability and use of nickel, an absolute cofactor of the important virulence determinant urease. Nickel-responsive gene regulation is mediated by HpNikR, a protein belonging to the ribbon-helix-helix family of transcriptional regulators. Unlike its homologues, HpNikR acts as both a repressor and an activator within an acid adaptation cascade. We report the crystal structure of the full-length HpNikR in a nickel-free conformation and two nickel-bound structures obtained in different conditions: Ni1-HpNikR and Ni2-HpNikR. Apo-HpNikR shows the same global fold as its bacterial homologues although with an unusual closed trans-conformation and asymmetrical quaternary arrangement. The structure of Ni1-HpNikR in the presence of nickel has two different sides, one showing nickel binding similar to that of known NikRs and the other reflecting an intermediate state. The structure of Ni2-HpNikR obtained using a shorter exposure to nickel provides another snapshot of the nickel incorporation. Altogether, the three structures have allowed us to determine the route for nickel within HpNikR and reveal the cooperativity between the tetramerization domain and the DNA-binding domain. Experiments using point mutations of HpnikR residues involved in nickel internalisation confirm that these residues are critical for HpNikR functions in vivo.

Is radiation damage dependent on the dose rate used during macromolecular crystallography data collection?

This paper focuses on the radiation-damage effects when applying the same total X-ray dose to protein crystals at different dose rates. These experiments have been performed on both a selenomethionated protein and on bovine trypsin using dose rates that span nearly two orders of magnitude. The results show no clear dose-rate effect on the global indicators of radiation damage, but a small measurable dose-rate effect could be found when studying specific radiation damage. It is hypothesized that this observed dose-rate effect relates to differences in the steady-state free-radical concentration.

Structural basis of 5-nitroimidazole antibiotic resistance: the crystal structure of NimA from Deinococcus radiodurans.

5-Nitroimidazole-based antibiotics are compounds extensively used for treating infections in humans and animals caused by several important pathogens. They are administered as prodrugs, and their activation depends upon an anaerobic 1-electron reduction of the nitro group by a reduction pathway in the cells. Bacterial resistance toward these drugs is thought to be caused by decreased drug uptake and/or an altered reduction efficiency. One class of resistant strains, identified in Bacteroides, has been shown to carry Nim genes (NimA, -B, -C, -D, and -E), which encode for reductases that convert the nitro group on the antibiotic into a non-bactericidal amine. In this paper, we have described the crystal structure of NimA from Deinococcus radiodurans (drNimA) at 1.6 A resolution. We have shown that drNimA is a homodimer in which each monomer adopts a beta-barrel fold. We have identified the catalytically important His-71 along with the cofactor pyruvate and antibiotic binding sites, all of which are found at the monomer-monomer interface. We have reported three additional crystal structures of drNimA, one in which the antibiotic metronidazole is bound to the protein, one with pyruvate covalently bound to His-71, and one with lactate covalently bound to His-71. Based on these structures, a reaction mechanism has been proposed in which the 2-electron reduction of the antibiotic prevents accumulation of the toxic nitro radical. This mechanism suggests that Nim proteins form a new class of reductases, conferring resistance against 5-nitroimidazole-based antibiotics.

Biosynthesis of isoprenoids: crystal structure of 4-diphosphocytidyl-2C-methyl-D-erythritol kinase.

4-Diphosphocytidyl-2C-methyl-d-erythritol kinase, an essential enzyme in the nonmevalonate pathway of isopentenyl diphosphate and dimethylallyl diphosphate biosynthesis, catalyzes the single ATP-dependent phosphorylation stage affording 4-diphosphocytidyl-2C-methyl-d-erythritol-2-phosphate. The 2-A resolution crystal structure of the Escherichia coli enzyme in a ternary complex with substrate and a nonhydrolyzable ATP analogue reveals the molecular determinants of specificity and catalysis. The enzyme subunit displays the alpha/beta fold characteristic of the galactose kinase/homoserine kinase/mevalonate kinase/phosphomevalonate kinase superfamily, arranged into cofactor and substrate-binding domains with the catalytic center positioned in a deep cleft between domains. Comparisons with related members of this superfamily indicate that the core regions of each domain are conserved, whereas there are significant differences in the substrate-binding pockets. The nonmevalonate pathway is essential in many microbial pathogens and distinct from the mevalonate pathway used by mammals. The high degree of sequence conservation of the enzyme across bacterial species suggests similarities in structure, specificity, and mechanism. Our model therefore provides an accurate template to facilitate the structure-based design of broad-spectrum antimicrobial agents.

Tryparedoxins from Crithidia fasciculata and Trypanosoma brucei: photoreduction of the redox disulfide using synchrotron radiation and evidence for a conformational switch implicated in function.

Tryparedoxin (TryX) is a member of the thioredoxin (TrX) fold family involved in the regulation of oxidative stress in parasitic trypanosomatids. Like TrX, TryX carries a characteristic Trp-Cys-Xaa-Xaa-Cys motif, which positions a redox-active disulfide underneath a tryptophan lid. We report the structure of a Crithidia fasciculata tryparedoxin isoform (CfTryX2) in two crystal forms and compare them with structures determined previously. Efforts to chemically generate crystals of reduced TryX1 were unsuccessful, and we carried out a novel experiment to break the redox-active disulfide, formed between Cys-40 and Cys-43, utilizing the intense x-radiation from a third generation synchrotron undulator beamline. A time course study of the S-S bond cleavage is reported with the structure of a TryX1 C43A mutant as the control. When freed from the constraints of a disulfide link to Cys-43, Cys-40 pivots to become slightly more solvent-accessible. In addition, we have determined the structure of Trypanosoma brucei TryX, which, influenced by the molecular packing in the crystal lattice, displays a significantly different orientation of the active site tryptophan lid. This structural change may be of functional significance when TryX interacts with tryparedoxin peroxidase, the final protein in the trypanothione-dependent peroxidase pathway. Comparisons with chloroplast TrX and its substrate fructose 1,6-bisphosphate phosphatase suggest that this movement may represent a general feature of redox regulation in the trypanothione and thioredoxin peroxidase pathways.