Serial macromolecular crystallography at ALBA Synchrotron Light Source. Erratum.

A revised version of Table 2 of Martin-Garcia et al. [J. Synchrotron Rad. (2022). 29, 896-907] is provided.

Serial macromolecular crystallography at ALBA Synchrotron Light Source.

The increase in successful adaptations of serial crystallography at synchrotron radiation sources continues. To date, the number of serial synchrotron crystallography (SSX) experiments has grown exponentially, with over 40 experiments reported so far. In this work, we report the first SSX experiments with viscous jets conducted at ALBA beamline BL13-XALOC. Small crystals (15-30 µm) of five soluble proteins (lysozyme, proteinase K, phycocyanin, insulin and α-spectrin-SH3 domain) were suspended in lipidic cubic phase (LCP) and delivered to the X-ray beam with a high-viscosity injector developed at Arizona State University. Complete data sets were collected from all proteins and their high-resolution structures determined. The high quality of the diffraction data collected from all five samples, and the lack of specific radiation damage in the structures obtained in this study, confirm that the current capabilities at the beamline enables atomic resolution determination of protein structures from microcrystals as small as 15 µm using viscous jets at room temperature. Thus, BL13-XALOC can provide a feasible alternative to X-ray free-electron lasers when determining snapshots of macromolecular structures.

Structural and biochemical characterization of the relaxosome auxiliary proteins encoded on the Bacillus subtilis plasmid pLS20.

Bacterial conjugation is an important route for horizontal gene transfer. The initial step in this process involves a macromolecular protein-DNA complex called the relaxosome, which in plasmids consists of the origin of transfer (oriT) and several proteins that prepare the transfer. The relaxosome protein named relaxase introduces a nick in one of the strands of the oriT to initiate the process. Additional relaxosome proteins can exist. Recently, several relaxosome proteins encoded on the Bacillus subtilis plasmid pLS20 were identified, including the relaxase, named RelpLS20, and two auxiliary DNA-binding factors, named Aux1pLS20 and Aux2pLS20. Here, we extend this characterization in order to define their function. We present the low-resolution SAXS envelope of the Aux1pLS20 and the atomic X-ray structure of the C-terminal domain of Aux2pLS20. We also study the interactions between the auxiliary proteins and the full-length RelpLS20, as well as its separate domains. The results show that the quaternary structure of the auxiliary protein Aux1pLS20 involves a tetramer, as previously determined. The crystal structure of the C-terminal domain of Aux2pLS20 shows that it forms a tetramer and suggests that it is an analog of TraMpF of plasmid F. This is the first evidence of the existence of a TraMpF analog in gram positive conjugative systems, although, unlike other TraMpF analogs, Aux2pLS20 does not interact with the relaxase. Aux1pLS20 interacts with the C-terminal domain, but not the N-terminal domain, of the relaxase RelpLS20. Thus, the pLS20 relaxosome exhibits some unique features despite the apparent similarity to some well-studied G- conjugation systems.

Structural and functional basis of phospholipid oxygenase activity of bacterial lipoxygenase from Pseudomonas aeruginosa.

Pseudomonas aeruginosa expresses a secreted LOX-isoform (PA-LOX, LoxA) capable of oxidizing polyenoic fatty acids to hydroperoxy derivatives. Here we report high-level expression of this enzyme in E. coli and its structural and functional characterization. Recombinant PA-LOX oxygenates polyenoic fatty acids including eicosapentaenoic acid and docosahexaenoic acid to the corresponding (n-6)S-hydroperoxy derivatives. This reaction involves abstraction of the proS-hydrogen from the n-8 bisallylic methylene. PA-LOX lacks major leukotriene synthase activity but converts 5S-HETE and 5S,6R/S-DiHETE to anti-inflammatory and pro-resolving lipoxins. It also exhibits phospholipid oxygenase activity as indicated by the formation of a specific pattern of oxygenation products from different phospholipid subspecies. Multiple mutagenesis studies revealed that PA-LOX does not follow classical concepts explaining the reaction specificity of mammalian LOXs. The crystal structure of PA-LOX was solved with resolutions of up to 1.48Å and its polypeptide chain is folded as single domain. The substrate-binding pocket consists of two fatty acid binding subcavities and lobby. Subcavity-1 contains the catalytic non-heme iron. A phosphatidylethanolamine molecule occupies the substrate-binding pocket and its sn1 fatty acid is located close to the catalytic non-heme iron. His377, His382, His555, Asn559 and the C-terminal Ile685 function as direct iron ligands and a water molecule (hydroxyl) completes the octahedral ligand sphere. Although the biological relevance of PA-LOX is still unknown its functional characteristics (lipoxin synthase activity) implicate this enzyme in a bacterial evasion strategy aimed at downregulating the hosts' immune system.

Interaction with the Redox Cofactor MYW and Functional Role of a Mobile Arginine in Eukaryotic Catalase-Peroxidase.

Catalase-peroxidases (KatGs) are unique bifunctional heme peroxidases with an additional posttranslationally formed redox-active Met-Tyr-Trp cofactor that is essential for catalase activity. On the basis of studies of bacterial KatGs, controversial mechanisms of hydrogen peroxide oxidation were proposed. The recent discovery of eukaryotic KatGs with differing pH optima of catalase activity now allows us to scrutinize those postulated reaction mechanisms. In our study, secreted KatG from the fungus Magnaporthe grisea (MagKatG2) was used to analyze the role of a remote KatG-typical mobile arginine that was shown to interact with the Met-Tyr-Trp adduct in a pH-dependent manner in bacterial KatGs. Here we present crystal structures of MagKatG2 at pH 3.0, 5.5, and 7.0 and investigate the mobility of Arg461 by molecular dynamics simulation. Data suggest that at pH ≥4.5 Arg461 mostly interacts with the deprotonated adduct Tyr. Elimination of Arg461 by mutation to Ala slightly increases the thermal stability but does not alter the active site architecture or the kinetics of cyanide binding. However, the variant Arg461Ala lost the wild-type-typical optimum of catalase activity at pH 5.25 (kcat = 6450 s(-1)) but exhibits a broad plateau between pH 4.5 and 7.5 (kcat = 270 s(-1) at pH 5.5). Moreover, significant differences in the kinetics of interconversion of redox intermediates of wild-type and mutant protein mixed with either peroxyacetic acid or hydrogen peroxide are observed. These findings together with published data from bacterial KatGs allow us to propose a role of Arg461 in the H2O2 oxidation reaction of KatG.

Eukaryotic Catalase-Peroxidase: The Role of the Trp-Tyr-Met Adduct in Protein Stability, Substrate Accessibility, and Catalysis of Hydrogen Peroxide Dismutation.

Recently, it was demonstrated that bifunctional catalase-peroxidases (KatGs) are found not only in archaea and bacteria but also in lower eukaryotes. Structural studies and preliminary biochemical data of the secreted KatG from the rice pathogen Magnaporthe grisea (MagKatG2) suggested both similar and novel features when compared to those of the prokaryotic counterparts studied so far. In this work, we demonstrate the role of the autocatalytically formed redox-active Trp140-Tyr273-Met299 adduct of MagKatG2 in (i) the maintenance of the active site architecture, (ii) the catalysis of hydrogen peroxide dismutation, and (iii) the protein stability by comparing wild-type MagKatG2 with the single mutants Trp140Phe, Tyr273Phe, and Met299Ala. The impact of disruption of the covalent bonds between the adduct residues on the spectral signatures and heme cavity architecture was small. By contrast, loss of its integrity converts bifunctional MagKatG2 to a monofunctional peroxidase of significantly reduced thermal stability. It increases the accessibility of ligands due to the increased flexibility of the KatG-typical large loop 1 (LL1), which contributes to the substrate access channel and anchors at the adduct Tyr. We discuss these data with respect to those known from prokaryotic KatGs and in addition present a high-resolution structure of an oxoiron compound of MagKatG2.

An ionizable active-site tryptophan imparts catalase activity to a peroxidase core.

Catalase peroxidases (KatG's) are bifunctional heme proteins that can disproportionate hydrogen peroxide (catalatic reaction) despite their structural dissimilarity with monofunctional catalases. Using X-ray crystallography and QM/MM calculations, we demonstrate that the catalatic reaction of KatG's involves deprotonation of the active-site Trp, which plays a role similar to that of the distal His in monofunctional catalases. The interaction of a nearby mobile arginine with the distal Met-Tyr-Trp essential adduct (in/out) acts as an electronic switch, triggering deprotonation of the adduct Trp.

Binding of the antitubercular pro-drug isoniazid in the heme access channel of catalase-peroxidase (KatG). A combined structural and metadynamics investigation.

Isonicotinic acid hydrazide (isoniazid or INH) is a front line antitubercular pro-drug that is converted to its active form, isonicotinyl-NAD, by the bacterial catalase-peroxidase KatG. Understanding the role of KatG in the INH activation process has been hampered by a lack of knowledge of the actual drug binding site. In this work, we have investigated the binding of INH in the main access channel of KatG with a combination of molecular dynamics, using an enhanced-sampling technique (metadynamics), X-ray crystallography, and site-directed mutagenesis. The metadynamics simulations show that there are several weak drug binding sites along the access channel. Moreover, the simulations evidence that complete entrance to the heme active site is impeded by an aspartate residue (D141) located above the heme. This has been confirmed by structural and functional analysis of the D141A mutant, leading to the first X-ray crystallography evidence of INH at the heme access channel.

Structure and interaction with phospholipids of a prokaryotic lipoxygenase from Pseudomonas aeruginosa.

Lipoxygenases (LOXs), which are essential in eukaryotes, have no confirmed function in prokaryotes that are devoid of polyunsaturated fatty acids. The structure of a secretable LOX from Pseudomonas aeruginosa (Pa_LOX), the first available from a prokaryote, presents significant differences with respect to eukaryotic LOXs, including a cluster of helices acting as a lid to the active center. The mobility of the lid and the structural variability of the N-terminal region of Pa_LOX was confirmed by comparing 2 crystal forms. The binding pocket contains a phosphatidylethanolamine phospholipid with branches of 18 (sn-1) and 14/16 (sn-2) carbon atoms in length. Carbon atoms from the sn-1 chain approach the catalytic iron in a manner that sheds light on how the enzymatic reaction might proceed. The findings in these studies suggest that Pa_LOX has the capacity to extract and modify unsaturated phospholipids from eukaryotic membranes, allowing this LOX to play a role in the interaction of P. aeruginosa with host cells.

Structure of glycerol-3-phosphate dehydrogenase (GPD1) from Saccharomyces cerevisiae at 2.45†Šresolution.

The interconversion of glycerol 3-phosphate and dihydroxyacetone phosphate by glycerol-3-phosphate dehydrogenases provides a link between carbohydrate and lipid metabolism and provides Saccharomyces cerevisiae with protection against osmotic and anoxic stress. The first structure of a glycerol-3-phosphate dehydrogenase from S. cerevisiae, GPD1, is reported at 2.45 Šresolution. The asymmetric unit contains two monomers, each of which is organized with N- and C-terminal domains. The N-terminal domain contains a classic Rossmann fold with the (ß-α-ß-α-ß)2 motif typical of many NAD+-dependent enzymes, while the C-terminal domain is mainly α-helical. Structural and phylogenetic comparisons reveal four main structure types among the five families of glycerol-3-phosphate and glycerol-1-phosphate dehydrogenases and reveal that the Clostridium acetobutylican protein with PDB code 3ce9 is a glycerol-1-phosphate dehydrogenase.

Influence of main channel structure on H(2)O(2) access to the heme cavity of catalase KatE of Escherichia coli.

The main channel for H(2)O(2) access to the heme cavity in large subunit catalases is twice as long as in small subunit catalases and is divided into two distinct parts. Like small subunit catalases, the 15Å of the channel adjacent to the heme has a predominantly hydrophobic surface with only weak water occupancy, but the next 15Å extending to the protein surface is hydrophilic and contains a complex water matrix in multiple passages. At the approximate junction of these two sections are a conserved serine and glutamate that are hydrogen bonded and associated with H(2)O(2) in inactive variants. Mutation of these residues changed the dimensions of the channel, both enlarging and constricting it, and also changed the solvent occupancy in the hydrophobic, inner section of the main channel. Despite these structural changes and the prominent location of the residues in the channel, the variants exhibited less than a 2-fold change in the k(cat) and apparent K(M) kinetic constants. These results reflect the importance of the complex multi-passage structure of the main channel. Surprisingly, mutation of either the serine or glutamate to an aliphatic side chain interfered with heme oxidation to heme d.

High conformational stability of secreted eukaryotic catalase-peroxidases: answers from first crystal structure and unfolding studies.

Catalase-peroxidases (KatGs) are bifunctional heme enzymes widely spread in archaea, bacteria, and lower eukaryotes. Here we present the first crystal structure (1.55 Å resolution) of an eukaryotic KatG, the extracellular or secreted enzyme from the phytopathogenic fungus Magnaporthe grisea. The heme cavity of the homodimeric enzyme is similar to prokaryotic KatGs including the unique distal (+)Met-Tyr-Trp adduct (where the Trp is further modified by peroxidation) and its associated mobile arginine. The structure also revealed several conspicuous peculiarities that are fully conserved in all secreted eukaryotic KatGs. Peculiarities include the wrapping at the dimer interface of the N-terminal elongations from the two subunits and cysteine residues that cross-link the two subunits. Differential scanning calorimetry and temperature- and urea-mediated unfolding followed by UV-visible, circular dichroism, and fluorescence spectroscopy combined with site-directed mutagenesis demonstrated that secreted eukaryotic KatGs have a significantly higher conformational stability as well as a different unfolding pattern when compared with intracellular eukaryotic and prokaryotic catalase-peroxidases. We discuss these properties with respect to the structure as well as the postulated roles of this metalloenzyme in host-pathogen interactions.

Thirty years of heme catalases structural biology.

About thirty years ago the crystal structures of the heme catalases from Penicillium vitale (PVC) and, a few months later, from bovine liver (BLC) were published. Both enzymes were compact tetrameric molecules with subunits that, despite their size differences and the large phylogenetic separation between the two organisms, presented a striking structural similarity for about 460 residues. The high conservation, confirmed in all the subsequent structures determined, suggested a strong pressure to preserve a functional catalase fold, which is almost exclusively found in these mono-functional heme catalases. However, even in the absence of the catalase fold an efficient catalase activity is also found in the heme containing catalase-peroxidase proteins. The structure of these broad substrate range enzymes, reported for the first time less than ten years ago from the halophilic archaebacterium Haloarcula marismortui (HmCPx) and from the bacterium Burkholderia pseudomallei (BpKatG), showed a heme pocket closely related to that of plant peroxidases, though with a number of unique modifications that enable the catalase reaction. Despite the wealth of structural information already available, for both monofunctional catalases and catalase-peroxidases, a number of unanswered major questions require continuing structural research with truly innovative approaches.

Modulation of heme orientation and binding by a single residue in catalase HPII of Escherichia coli.

Heme-containing catalases have been extensively studied, revealing the roles of many residues, the existence of two heme orientations, flipped 180° relative to one another along the propionate-vinyl axis, and the presence of both heme b and heme d. The focus of this report is a residue, situated adjacent to the vinyl groups of the heme at the entrance of the lateral channel, with an unusual main chain geometry that is conserved in all catalase structures so far determined. In Escherichia coli catalase HPII, the residue is Ile274, and replacing it with Gly, Ala, and Val, found at the same location in other catalases, results in a reduction in catalytic efficiency, a reduced intensity of the Soret absorbance band, and a mixture of heme orientations and species. The reduced turnover rates and higher H(2)O(2) concentrations required to attain equivalent reaction velocities are explained in terms of less efficient containment of substrate H(2)O(2) in the heme cavity arising from easier escape through the more open entrance to the lateral channel created by the smaller side chains of Gly and Ala. Inserting a Cys at position 274 resulted in the heme being covalently linked to the protein through a Cys-vinyl bond that is hypersensitive to X-ray irradiation being largely degraded within seconds of exposure to the X-ray beam. Two heme orientations, flipped along the propionate-vinyl axis, are found in the Ala, Val, and Cys variants.

Isonicotinic acid hydrazide conversion to Isonicotinyl-NAD by catalase-peroxidases.

Activation of the pro-drug isoniazid (INH) as an anti-tubercular drug in Mycobacterium tuberculosis involves its conversion to isonicotinyl-NAD, a reaction that requires the catalase-peroxidase KatG. This report shows that the reaction proceeds in the absence of KatG at a slow rate in a mixture of INH, NAD(+), Mn(2+), and O(2), and that the inclusion of KatG increases the rate by >7 times. Superoxide, generated by either Mn(2+)- or KatG-catalyzed reduction of O(2), is an essential intermediate in the reaction. Elimination of the peroxidatic process by mutation slows the rate of reaction by 60% revealing that the peroxidatic process enhances, but is not essential for isonicotinyl-NAD formation. The isonicotinyl-NAD(*+) radical is identified as a reaction intermediate, and its reduction by superoxide is proposed. Binding sites for INH and its co-substrate, NAD(+), are identified for the first time in crystal complexes of Burkholderia pseudomallei catalase-peroxidase with INH and NAD(+) grown by co-crystallization. The best defined INH binding sites were identified, one in each subunit, on the opposite side of the protein from the entrance to the heme cavity in a funnel-shaped channel. The NAD(+) binding site is approximately 20 A from the entrance to the heme cavity and involves interactions primarily with the AMP portion of the molecule in agreement with the NMR saturation transfer difference results.

The dynamic role of distal side residues in heme hydroperoxidase catalysis. Interplay between X-ray crystallography and ab initio MD simulations.

The enzymatic cycle of hydroperoxidases involves the resting Fe(III) state of the enzyme and the high-valent iron intermediates Compound I and Compound II. These states might be characterized by X-ray crystallography and the transition pathways between each state can be investigated using atomistic simulations. Here we review our recent work in the modeling of two key steps of the enzymatic reaction of hydroperoxidases: the formation of Cpd I in peroxidase and the reduction of Cpd I in catalase. It will be shown that small conformational motions of distal side residues (His in peroxidases and His/Asn in catalases), not,or only partially, revealed by the available X-ray structures, play an important role in the catalytic processes examined.

Electronic state of the molecular oxygen released by catalase.

In catalases, the high redox intermediate known as compound I (Cpd I) is reduced back to the resting state by means of hydrogen peroxide in a 2-electron reaction [Cpd I (Por(*+)-Fe(IV)O) + H(2)O(2) --> Enz (Por-Fe(III)) + H(2)O + O(2)]. It has been proposed that this reaction takes place via proton transfer toward the distal His and hydride transfer toward the oxoferryl oxygen (H(+)/H(-) scheme) and some authors have related it to singlet oxygen generation. Here, we consider the possible reaction schemes and qualitatively analyze the electronic state of the species involved to show that the commonly used association of the H(+)/H(-) scheme with singlet oxygen production is not justified. The analysis is complemented with density functional theory (DFT) calculations for a gas-phase active site model of the reactants and products.

RhaU of Rhizobium leguminosarum is a rhamnose mutarotase.

Of the nine genes comprising the L-rhamnose operon of Rhizobium leguminosarum, rhaU has not been assigned a function. The construction of a Delta rhaU strain revealed a growth phenotype that was slower than that of the wild-type strain, although the ultimate cell yields were equivalent. The transport of L-rhamnose into the cell and the rate of its phosphorylation were unaffected by the mutation. RhaU exhibits weak sequence similarity to the formerly hypothetical protein YiiL of Escherichia coli that has recently been characterized as an L-rhamnose mutarotase. To characterize RhaU further, a His-tagged variant of the protein was prepared and subjected to mass spectrometry analysis, confirming the subunit size and demonstrating its dimeric structure. After crystallization, the structure was refined to a 1.6-A resolution to reveal a dimer in the asymmetric unit with a very similar structure to that of YiiL. Soaking a RhaU crystal with L-rhamnose resulted in the appearance of beta-L-rhamnose in the active site.

Versatility of the electronic structure of compound I in catalase-peroxidases.

Catalase-peroxidases (KatGs) are bifunctional heme proteins, belonging to the family of class I peroxidases, that are able to catalyze both catalatic and peroxidatic reactions within a peroxidase-like structure. We investigated the electronic structure of reaction intermediates of the catalytic cycle of KatGs by means of density functional theory (DFT) QM/MM calculations. The outcome was that the ionization state of the KatG-specific covalent adduct (Met264-Tyr238-Trp111) affects the radical character of compound I (Cpd I). Specifically, in the optimized structures, substantial radical character is observed on the proximal Trp330 when Tyr238 is protonated, whereas when Tyr238 is deprotonated the radical localizes on the Met+-Tyr(O-)-Trp adduct. These findings are not affected by protein thermal fluctuations, although details of the spin density distribution are affected by the geometry of the active site. Calculations provide structures in good agreement with the crystal structure of BpKatG Cpd I. They also provide an explanation for the experimental findings of the mobile and catalatic-specific residue Arg426 being 100% in conformation R in the X-ray structure of BpKatG treated with organic peroxides. The role of different Cpd I forms in the catalase and peroxidase reaction pathways is discussed.

Catalase-peroxidases (KatG) exhibit NADH oxidase activity.

Catalase-peroxidases (KatG) produced by Burkholderia pseudomallei, Escherichia coli, and Mycobacterium tuberculosis catalyze the oxidation of NADH to form NAD+ and either H2O2 or superoxide radical depending on pH. The NADH oxidase reaction requires molecular oxygen, does not require hydrogen peroxide, is not inhibited by superoxide dismutase or catalase, and has a pH optimum of 8.75, clearly differentiating it from the peroxidase and catalase reactions with pH optima of 5.5 and 6.5, respectively, and from the NADH peroxidase-oxidase reaction of horseradish peroxidase. B. pseudomallei KatG has a relatively high affinity for NADH (Km=12 microm), but the oxidase reaction is slow (kcat=0.54 min(-1)) compared with the peroxidase and catalase reactions. The catalase-peroxidases also catalyze the hydrazinolysis of isonicotinic acid hydrazide (INH) in an oxygen- and H2O2-independent reaction, and KatG-dependent radical generation from a mixture of NADH and INH is two to three times faster than the combined rates of separate reactions with NADH and INH alone. The major products from the coupled reaction, identified by high pressure liquid chromatography fractionation and mass spectrometry, are NAD+ and isonicotinoyl-NAD, the activated form of isoniazid that inhibits mycolic acid synthesis in M. tuberculosis. Isonicotinoyl-NAD synthesis from a mixture of NAD+ and INH is KatG-dependent and is activated by manganese ion. M. tuberculosis KatG catalyzes isonicotinoyl-NAD formation from NAD+ and INH more efficiently than B. pseudomallei KatG.