research papers
The
of mycothiol disulfide reductase (Mtr) provides mechanistic insight into the specific low-molecular-weight thiol reductase activity of ActinobacteriaaSection for Biochemistry and Molecular Biology, Department of Biosciences, University of Oslo, PO Box 1066, Blindern, 0316 Oslo, Norway, and bCentre for Molecular Medicine Norway, Nordic EMBL Partnership, University of Oslo, 0318 Oslo, Norway
*Correspondence e-mail: j.g.fernandez@ncmm.uio.no, h.p.hersleth@ibv.uio.no, marta.hammerstad@ibv.uio.no
Low-molecular-weight (LMW) Mycobacterium tuberculosis, use the LMW thiol mycothiol (MSH) to buffer the intracellular redox environment. The NADPH-dependent FAD-containing oxidoreductase mycothiol disulfide reductase (Mtr) is known to reduce oxidized mycothiol disulfide (MSSM) to MSH, which is crucial to maintain the cellular redox balance. In this work, the first crystal structures of Mtr are presented, expanding the structural knowledge and understanding of LMW thiol reductases. The structural analyses and docking calculations provide insight into the nature of Mtrs, with regard to the binding and reduction of the MSSM substrate, in the context of related The putative binding site for MSSM suggests a similar binding to that described for the homologous glutathione reductase and its respective substrate glutathione disulfide, but with distinct structural differences shaped to fit the bulkier MSSM substrate, assigning Mtrs as uniquely functioning reductases. As MSH has been acknowledged as an attractive antitubercular target, the structural findings presented in this work may contribute towards future antituberculosis drug development.
are involved in many processes in all organisms, playing a protective role against reactive species, heavy metals, toxins and antibiotics. Actinobacteria, such asKeywords: low-molecular-weight thiols; mycothiol disulfide reductase; oxidoreductases; redox homeostasis; flavoenzymes; X-ray crystallography; docking; protein structure; Actinobacteria.
1. Introduction
Eukaryotes, most Gram-negative bacteria and some Gram-positive bacteria use the well studied glutathione (GSH) as their major low-molecular-weight (LMW) thiol (Fahey et al., 1978; Loi et al., 2015). LMW are a group of reactive sulfhydryl-containing compounds that play critical roles as intracellular redox buffers that maintain cytosolic proteins in their reduced states and in protection against, for example, oxygen and antibiotic toxicity. Most Gram-positive bacteria do not produce GSH and rely on alternative thiol-redox buffers. Firmicutes (low-G+C Gram-positive bacteria), including Staphylococcus aureus and many bacilli, utilize bacillithiol (BSH) to protect the cells against a variety of reactive species (Newton et al., 2009; Sharma et al., 2011; Gaballa et al., 2010), whereas in most Actinobacteria (high-G+C Gram-positive bacteria), such as mycobacteria, Streptomycetes and corynebacteria, mycothiol (MSH) serves as the predominant LMW thiol (Newton et al., 1996; Reyes et al., 2018; Sakuda et al., 1994; Spies & Steenkamp, 1994). Many Actinobacteria are important in human pathogenesis (for example Mycobacterium tuberculosis, the causative agent of tuberculosis) and the detoxification of contaminants (for example Rhodococcus) (Nazari et al., 2022). MSH, formed by the conjugation of N-acetylcysteine with 1-D-myo-inosityl-2-amido-2-deoxy-α-D-glucopyranoside (Newton et al., 1995; Jothivasan & Hamilton, 2008) and present at millimolar concentrations in the cell (Newton et al., 1996), plays analogous roles to those of GSH in maintaining cellular redox homeostasis. One such role includes redox regulation of proteins and protection of protein through the formation (termed S-mycothiolation) of mixed disulfides between MSH and protein that are formed under conditions of oxidative stress (Chi et al., 2014; Reyes et al., 2018). Additionally, MSH participates in a variety of metabolic processes, such as protection against heavy metals, alkylating agents, reactive oxygen species (ROS) and reactive nitrogen species (RNS), as well as detoxification of antibiotics and electrophiles (Loi et al., 2015). Under oxidative stress conditions, MSH is oxidized to the disulfide (MSSM) state. For instance, S-mycothiolation is redox-controlled by the glutaredoxin homolog mycoredoxin-1 (Mrx1), which regenerates through the reduction of S-mycothiolated proteins, resulting in an Mrx1-SSM intermediate that is reduced by MSH, and ultimately leading to the formation of MSSM (Van Laer et al., 2012). Similarly, GSH is oxidized to glutathione disulfide (GSSG) and BSH is oxidized to bacillithiol disulfide (BSSB). In order to maintain intracellular redox homeostasis, the disulfide forms are reduced back to the reduced states by the respective flavoprotein disulfide reductases (FDRs): GSSG by glutathione reductase (GR) and BSSB by bacillithiol disulfide reductase (Bdr) (Fahey et al., 1978). The lack of GR activity in mycobacterial cell lysates suggested the occurrence of a distinct reductase in these bacteria that is functionally analogous to GR but that exhibits no activity for GSSG and has been shown to have an absolute requirement for the glucosamine moiety of the MSSM substrate for activity (Patel & Blanchard, 1998, 1999). To restore the MSH/MSSM redox balance, Actinobacteria encode the flavoenzyme mycothiol disulfide reductase (Mtr; also called mycothione reductase), which catalyzes the reduction of MSSM to MSH, as demonstrated for Mycobacterium smegmatis Mtr (Patel & Blanchard, 1998) and M. tuberculosis Mtr (Patel & Blanchard, 1999; Kumar et al., 2017), and was originally characterized using a truncated substrate. Mtr belongs to a group of NAD(P)H-dependent homodimeric with a tightly bound flavin adenine dinucleotide (FAD) cofactor per subunit. The overall structure composed of two dinucleotide-binding domains (the `two dinucleotide-binding domains' tDBDFs), which are responsible for binding FAD and NAD(P)H, respectively, is seen in many of these and is commonly described for the low-molecular-weight (low Mr) thioredoxin reductases (TrxRs), as typified by the Escherichia coli enzyme (Argyrou & Blanchard, 2004; Williams, 1995). In addition, each subunit contains a catalytic redox-active disulfide/Cys pair that is responsible for the reduction of their respective and structurally unique disulfide-bonded substrates. Although some flavin-dependent are solely composed of the low Mr TrxR architecture, other oxidoreductases contain the low Mr TrxR architecture with additional domains making up larger structures, as in the case of, for example, high Mr TrxR and GR (Hammerstad & Hersleth, 2021; Kuriyan, Krishna et al., 1991). of this group all share conserved amino-acid sequence motifs for the binding of NAD(P)H and FAD, and a characteristic His–Glu that is involved in proton transfer, and most share a similar catalytic mechanism (Patel & Blanchard, 1999, 2001; Fagan & Palfey, 2010). The initial characterization of Mtr suggested a mechanism similar to that of the prototypical GR, exhibiting a bi-bi ping-pong kinetic mechanism (Patel & Blanchard, 1999; Massey & Williams, 1965). In the reductive half-reaction, a two-electron-reduced enzyme is generated through the reduction of the redox-active Cys pair by NADPH, via FAD. Subsequent of the reducing equivalents in turn reduces the disulfide substrate through a dithiol–disulfide interchange step in the oxidative half-reaction. From a structural perspective, in GR electrons flow from NAD(P)H, bound on the re face of the FAD isoalloxazine ring, to the Cys pair on the si face of FAD, where the now reduced dithiol reacts with GSSG. Although similar in sequence to GR, the rate of the oxidative half-reaction was shown to be slightly faster than the reductive half-reaction in Mtr, unlike as is seen in most FDRs, where the oxidative half-reaction is commonly rate-limiting (Patel & Blanchard, 2001). More knowledge on the catalytic mechanism could potentially be attained from a Evidence that MSSM is recycled by the FAD-containing NAD(P)H-dependent oxidoreductase Mtr has provided insight into MSH-dependent mechanisms and the Mrx1/MSH/Mtr pathway in Actinobacteria; however, detailed structural data on Mtrs have been lacking to date. A low-resolution small-angle X-ray scattering (SAXS) solution structure of Mtr confirmed the dimeric state of the enzyme, however, indicating the presence of an asymmetric dimer (Kumar et al., 2017). A of Mtr would provide a missing link in the LMW thiol field and an important addition to the previously characterized crystal structures of LMW thiol-specific reductases such as GR (Karplus & Schulz, 1989), Bdr (Hammerstad et al., 2020) and coenzyme A disulfide reductase (CoADR; Mallett et al., 2006). How structurally similar are Mtrs to related oxidoreductases? Can a provide insight into the mechanism of MSSM reduction by Mtr, as compared with GSSG reduction by GR? Furthermore, MSH is the major LMW thiol involved in the maintenance of redox balance crucial for the survival of the human pathogen M. tuberculosis, and has been shown to contribute to its pathogenicity, infection and antibiotic-resistance mechanisms (Rawat et al., 2002; Trivedi et al., 2012; Nambi et al., 2015; Sareen et al., 2003; Sassetti & Rubin, 2003). MSH has already been recognized as an attractive antitubercular target (Nilewar & Kathiravan, 2014); however, with an increasing number of drug-resistant M. tuberculosis strains, new structural and mechanistic insight into enzymes involved in MSH redox biology, including Mtr, could provide valuable information for future antituberculosis drug development. Moreover, the ability of Mtr from Rhodococcus erythropolis to reduce tellurite () to elemental tellurium (Butz et al., 2021), as also reported for other reductases (Moore & Kaplan, 1992; Maltman et al., 2017; Arenas-Salinas et al., 2016), makes Mtr an interesting candidate for bioremediation research as well as for nanotechnology involving tellurium-based nanostructures.
In this work, we present the first reported crystal structures of Mtr from two homologous Actinobacteria: R. erythropolis and the close M. tuberculosis relative M. smegmatis. The overall structural architecture of homodimeric Mtr highly resembles that of the well characterized flavin-dependent oxidoreductase GR. Using docking calculations and inspection of the Mtr structures, as well as comparison with GR, we propose a putative binding site for the MSSM substrate. Our findings demonstrate that MSSM can bind placing its disulfide bond in the proximity of the FAD cofactor and redox-active Cys pair in Mtr, allowing reduction of the substrate. Although Mtrs and GRs share a similar catalytic mechanism and substrate-binding site, we have identified structural differences that are likely to account for the substrate specificity for MSSM in Mtrs. The highly conserved MSSM binding site in Mtr is considerably larger than in GR, in agreement with its bulkier natural substrate, making Mtrs distinct and unique in terms of function. This study provides an important missing link in the field of redox biology and LMW as well as NAD(P)H-dependent FAD-containing providing new insight into the biological function of Mtrs.
2. Materials and methods
2.1. Expression and purification of MtrRe and MtrMs
The genes for R. erythropolis PR4 Mtr (MtrRe; locus tag RER_26020) or for M. smegmatis MC2 155 Mtr (MtrMs; locus tag LJ00_12995) were cloned into a pET-22b(+) plasmid (constructed using NdeI and XhoI sites; GenScript) and transformed into competent E. coli One Shot BL21 (DE3) cells (Invitrogen, Thermo Fisher Scientific). The cells were grown in LB medium containing 100 µg ml−1 ampicillin. Protein expression was induced by adding isopropyl β-D-1-thiogalactopyranoside (IPTG) to a final concentration of 0.5 mM on reaching an OD600 nm of 0.4–0.6, and the cultures were incubated at 15°C for 24 h with shaking before the cells were harvested and stored at −80°C.
The cells were thawed, dissolved in 50 mM Tris–HCl pH 7.5, 1 mM DTT, 5 µg ml−1 DNase with a cOmplete Protease Inhibitor Cocktail tablet (Roche) at a 1:10 cell wet weight:buffer ratio and lysed by sonication. MtrRe and MtrMs were precipitated with 0.6 and 0.4 g ml−1 ammonium sulfate, respectively, dissolved in 50 mM Tris–HCl pH 7.5, 1 mM DTT and desalted by dialysis (SnakeSkin dialysis tubing, 10K molecular-weight cutoff, Thermo Fisher Scientific). The desalted proteins were filtered through a 0.45 µm filter (Merck Millipore), applied onto a HiTrap Q FF anion-exchange column (Cytiva) and eluted with a linear gradient to 50 mM Tris–HCl pH 7.5, 1 mM DTT, 1 M NaCl. Finally, the proteins were purified on a Superdex 200 Increase 10/300 GL column (Cytiva) in 50 mM HEPES pH 7.5, 150 mM NaCl. The eluted MtrRe and MtrMs both showed approximate molecular weights of 100 kDa, corresponding to the dimeric form of Mtr, as validated by calibration of the size-exclusion column (Gel Filtration LMW Calibration Kit, Cytiva). Protein fractions were pooled and concentrated to 30 mg ml−1 using Amicon Ultra-15 filter units (50 kDa molecular-weight cutoff, Merck Millipore), flash-frozen in liquid nitrogen and stored at −80°C. All chromatographic steps were performed on an ÄKTApurifier FPLC system (GE Healthcare) and all expression and purification steps were monitored by sodium dodecyl sulfate–polyacrylamide gel (SDS–PAGE). UV–visible (UV–Vis) spectra were recorded on an Agilent Cary 60 spectrophotometer and the protein concentrations were estimated using an extinction coefficient at λmax of ɛ465 nm = 11.5 mM−1 cm−1.
2.2. Protein crystallization and structure determination
Initial crystallization screening of both Mtr proteins was performed using the sitting-drop vapor-diffusion crystallization method with a Mosquito robot (SPT Labtech). Initial hits giving rod-shaped crystals of MtrRe (30 mg ml−1) of approximately 100 × 50 × 50 µm in size were obtained with the MIDASplus screen from Molecular Dimensions and were further optimized in 45%(w/v) polypropylene glycol 400, 4%(v/v) ethanol. Rod-shaped crystals of MtrMs (12.5 mg ml−1) of approximately 200 × 30 × 30 µm in size were obtained with Index from Hampton Research [0.02 M magnesium chloride hexahydrate, 0.1 M HEPES pH 7.5, 22%(w/v) poly(acrylic acid sodium salt) 5100]. The crystals were grown at 20°C, cryoprotected in 50%(w/v) polypropylene glycol 400 [and 4%(v/v) ethanol for MtrRe] and flash-cooled in liquid nitrogen.
X-ray diffraction data were collected at 100 K on beamlines ID23-2 and ID30B for MtrRe and MtrMs, respectively, at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. Diffraction images were processed with XDS (Kabsch, 2010) and AIMLESS (Evans, 2011; Agirre et al., 2023) to resolutions of 2.9 Å for MtrRe and 4.7 Å for MtrMs. The structures contained two molecules per The structure of MtrRe was solved by using Phaser (McCoy et al., 2007) with an AlphaFold (Jumper et al., 2021) ab initio model of R. erythropolis DN1 Mtr as a template (pLDDT > 90), and the solved structure was further used as a template to solve the MtrMs structure, in which the chains were rebuilt with CHAINSAW (Stein, 2008). Several rounds of rebuilding and various strategies were performed using Coot (Emsley et al., 2010) and Phenix (Liebschner et al., 2019). The MtrRe data were twinned, and the h, −h − k, −l with a twin fraction of 0.47 was used in all refinements in Phenix. The best MtrRe structure was obtained by refining XYZ in both reciprocal and real space, translation–libration–screw (TLS) rotation factors and individual B factors, and applying (NCS) restraints. A similar was performed for MtrMs but using isotropic B factors and no TLS rotation factors. For the MtrMs structure, a round of with PDB-REDO was performed (Joosten et al., 2014). The presence of disulfide bonds between the redox-active Cys pairs was confirmed in both chains in MtrRe by omit maps, although some minor negative electron density was present indicating slightly incomplete occupancy of the disulfide bridges. PDBePISA was used to analyze the buried surface area of the Mtr dimer. The absorbed X-ray dose was calculated using RADDOSE-3D (Zeldin et al., 2013). All structural figures were prepared with PyMOL version 2.5 (Schrödinger).
2.3. Docking analysis and protein–ligand interactions
Re and MSH or MSSM were performed with CB-Dock2 (Cavity-detection guided Blind Docking; Liu et al., 2022). Structure-based blind-docking calculations were performed using the dimeric structure of MtrRe and the experimental MSH structure from M. tuberculosis mycothiol S-transferase (PDB entry 8f5v; Jayasinghe et al., 2023) or an MSSM model, with human GR with GSSG bound (PDB entry 1gra; Karplus & Schulz, 1989) as a reference model for template-based blind docking. To generate schematic diagrams of protein–ligand interactions, LigPlot+ (Wallace et al., 1995; Laskowski & Swindells, 2011) was used, applying a 4 Å cutoff radius, to show hydrogen bonds and hydrophobic contacts, which are represented by dashed lines and by arcs with spokes radiating towards the ligand atoms that they contact, respectively.
between Mtr2.4. Bioinformatics analysis: sequence-similarity networks, multiple sequence alignment and phylogenetic analysis
Sequence-similarity networks (SSNs) were generated with the EFI Enzyme Similarity Tool EFI-EST (https://efi.igb.illinois.edu/efi-est/; Oberg et al., 2023), using M. tuberculosis Mtr as a search sequence, in order to analyze the relation to other similar The search included bacterial, archaeal and eukaryotic taxonomic groups using the UniRef50 and UniRef90 sequence databases, retrieving a total of 16 589 homologous sequences. Sequences were grouped with an alignment score of 120 and nodes representing sequences sharing >60% identity, dividing the main homologous into separate clusters. Figures illustrating SSN analyses were created in Cytoscape (version 3.9) with the organic layout (Shannon et al., 2003). The ten largest clusters of sequences were selected and assembled into seven groups with respect to their annotated function. Multiple sequence alignments were performed in JalView (Waterhouse et al., 2009) with Clustal Omega (Sievers et al., 2011) and phylogenetic tree analysis with average distances using the BLOSUM62 matrix on two or three selected sequences from each of the ten clusters, including the sequences for the top DALI hits.
2.5. Structure comparison: structural alignment search with DALI
A search for similar structures to Mtr in the Protein Data Bank (PDB) was performed with the DALI (Distance-matrix ALIgnment) protein structure-comparison server (Holm, 2022) using the MtrRe structure as a search template. For the top selected hits, a multiple structure sequence alignment was generated with DALI including secondary-structure assignments by DSSP (Kabsch & Sander, 1983; Touw et al., 2015) through DALI and was presented with JalView.
2.6. Analysis of conserved residues
The degree of conservation of residues in the MtrRe and GR (PDB entry 1gra; Karplus & Schulz, 1989) structures was evaluated with ConSurf (Ashkenazy et al., 2010, 2016; Celniker et al., 2013). The analysis was based on identification of homologous sequences from the UniRef90 database using the HMMER algorithm (Finn et al., 2015) and multiple sequence alignment with MAFFT (Katoh & Standley, 2013). Of the homologous sequences passing the standard threshold, ConSurf selected a sample of 150 representative sequences. The sequences were inspected in JalView and all 150 sequences from the GR search were annotated as GRs, while only 97 of the 150 sequences selected from the Mtr search were annotated as Mtrs. Therefore, ConSurf was re-run using the 97 Mtr sequences to obtain a conservation degree based only on annotated Mtrs. A nine-bin colored scale was used to show the conservation of each residue, from most variable (turquoise) to most conserved (maroon), when generating three-dimensional figures with PyMOL or coloring the residues in LigPlot+.
3. Results and discussion
3.1. Overall structures of MtrRe and MtrMs
Two structures of Mtrs from homologous Actinobacteria are presented in this work; MtrMs from the M. tuberculosis model organism M. smegmatis and MtrRe from R. erythropolis, a biocatalyst used for the bioremediation of toxic compounds. The crystal structures of MtrRe and MtrMs confirm that Mtr is a homodimer, as also supported by molecular-weight estimations during protein purification. Each monomer, composed of 458 and 461 residues in MtrRe and MtrMs, respectively, belongs to the pyridine nucleotide-disulfide oxidoreductase (PNDO) superfamily (Lu et al., 2020) and consists of three domains, namely a NAD(P)H-binding domain, a FAD-binding domain and a dimerization/interface domain (Fig. 1). In FAD-containing NAD(P)H-dependent two globular dinucleotide-binding three-layer ββα-sandwich Rossmann-like fold domains are fused into a single chain responsible for the binding of FAD and NAD(P)H (Ojha et al., 2007) through conserved amino-acid sequence motifs (Susanti et al., 2017; Dym & Eisenberg, 2001; Hammerstad & Hersleth, 2021), as also seen in the Mtr structures. Insight into the dimeric state of Mtr from M. tuberculosis has previously been given by Grüber and coworkers, demonstrated by a low-resolution structure derived from SAXS data, as well as dynamic light-scattering (DLS) studies (Kumar et al., 2017). The latter study, however, proposed an extended conformation of the NAD(P)H-binding domain of one of the monomers, resulting in an asymmetric dimer assembly which indicates domain flexibility in solution. This feature is not seen in crystal structures of other homologous or in the MtrRe or MtrMs crystal structures, which are both composed of a symmetrical dimer assembly.
The crystal contacts between the monomers were analyzed with PISA using the MtrRe structure, showing a total buried surface area of 3405 Å2, which is in strong agreement with the total interface area calculated for the dimeric GR (Karplus & Schulz, 1987). Moreover, the total solvent-accessible surface area of Mtr is 34 240 Å2, the complex-formation significance score (CSS) is 0.668 and the solvation free-energy gain upon the formation of the interface (ΔiG) is −38.0 kcal mol−1. Its formation entails a solvation free energy (ΔGint) of −51.6 kcal mol−1, while the free energy of dissociation (ΔGdiss) is 42.2 kcal mol−1. Therefore, the dimeric Mtr shown in this work strongly corresponds to the in vivo biological assembly.
Little deviation is observed between MtrRe (2.9 Å resolution) and MtrMs (4.7 Å resolution), which show highly similar overall folds with an r.m.s.d. of 1.0 Å (Table 1 and Fig. 2). Despite the low resolution obtained for the latter structure, it is clear that it adopts the structurally conserved topology that is seen for members of the PNDO superfamily and that major features are invariant between the two structures in this study. In addition, a strongly comparable A465 nm/A280 nm ratio for both proteins, as well as the yellow crystals obtained of MtrRe and MtrMs, support the presence of FAD in both proteins, although FAD could not be built into the MtrMs structure due to its low resolution and lack of significant density. Together, these results demonstrate that the detailed structural features observed and described for the structure of MtrRe are representative of both MtrRe and MtrMs, as well as other Mtrs across species (Fig. 2). In MtrRe, both monomers show strong density for the FAD cofactor, which is tightly stabilized in its binding pocket through several polar interactions. The conserved and redox-active Cys pair responsible for substrate reduction is located on the si face of the isoalloxazine ring of FAD, with the cysteines (Cys39 and Cys44) forming a disulfide bond. The conserved His–Glu (His442 and Glu447) essential for proton transfer is located at the C-terminal end, positioning it in close proximity to the FAD cofactor of the opposite monomer (Fig. 1). No electron density was observed for NADPH in the Mtr structures reported in this study, as is often the case for oxidoreductase structures where no NADPH has been included in the crystallization setup. However, the putative NAD(P)H-binding site in Mtr is lined by the conserved amino-acid sequence motif [GXGXXA/G for the pyrophosphate group of NAD(P)H] commonly found in utilizing this cofactor (Dym & Eisenberg, 2001; Hammerstad & Hersleth, 2021). Part of the HRRXXXR binding motif for the 2′ phosphate group of NADPH, found, for example, in E. coli low Mr TrxR (Laurent et al., 1964), is lacking in Mtr. However, amino-acid substitutions in this motif are commonly seen in NADPH-consuming reductases, and the basic amino acids present in MtrRe (for example Arg199 and Arg205) are likely to serve a homologous role in the stabilization of the 2′ phosphate group of the pyridine nucleotide. Moreover, Mtr has previously been shown to be selective for NADPH over NADH, as well as indicating a strict preference for the 2′-phosphate regioisomer when assayed with 3′-NADPH (Patel & Blanchard, 1999).
|
3.2. Sequence and structure comparison of Mtr with homologous oxidoreductases
The sequence and structure of Mtr were compared with those of other enzymes using different approaches. Structural comparison of Mtr (MtrRe) with deposited PDB structures using the DALI protein structure comparison server shows that Mtr is highly similar to other disulfide Hits with the highest Z-score for each of the eight most similar protein types are shown in Table 2, which correlates to the results from the SSN analysis. High structural similarity is observed to MerA (PDB entry 5x1y; Bafana et al., 2017), DLD (PDB entry 1ebd; Mande et al., 1996), GR (PDB entry 6b4o; Center for Structural Genomics of Infectious Diseases, unpublished work), TryR (PDB entry 2tpr; Kuriyan, Kong et al., 1991), GAR (PDB entry 2rab; Van Petegem et al., 2007) and TrxR (PDB entry 3dgz; B. E. Eckenroth, R. J. Hondal & S. J. Everse, unpublished work) (r.m.s.d.s of 1.8–2.2 Å), with somewhat lower similarity to quinone reductase (LpdA; PDB entry 1xdi; Argyrou et al., 2004) and CoADR (PDB entry 5l1n; Sea et al., 2018) (r.m.s.d.s of 2.7–2.8 Å).
|
The overall fold of Mtr highly resembles that of previously characterized a). All structures share the overall conformation and arrangement of their three domains and the same orientation of the FAD in the FAD-binding domain, as well as the location of the NAD(P)H-binding site. The probable and conserved positioning of NADPH in Mtr can be demonstrated using the coordinates of GR (PDB entry 1grb; Karplus & Schulz, 1989; Fig. 3b). A conformational change of an aromatic residue, Phe178, would be required for NADPH binding in Mtr, a rearrangement that has previously been described for GR. In GR, the equivalent Tyr197 on the re face of FAD shields the flavin cofactor, blocking the nicotinamide-binding pocket, but rotates away from the isoalloxazine ring of FAD upon NADPH binding (Fig. 3b), further allowing hydride transfer from NADPH to FAD and ultimately transferring an electron pair to the proximal cysteine of the redox-active Cys pair (Karplus & Schulz, 1989; Berry et al., 1989). Hence, by direct comparison, the NADPH nicotinamide ring of MtrRe would be stabilized between the re face of the isoalloxazine ring of FAD and the phenyl group of Phe178 through stacking interactions. In Mtr, Arg199 and Arg205, which are also conserved in GR (Arg218 and Arg224), are likely to be involved in electrostatic interactions with the 2′ phosphate group of NADPH, which can also be noted as an RXXXXXR motif (Figs. 3b and 4). The Mtr crystal structures presented in this work confirm that Mtr is composed of the low Mr TrxR-like fold with an additional C-terminal interface domain, as described for related such as GR (Hammerstad & Hersleth, 2021).
as seen from the structural alignment of Mtr with selected homologous structures (Fig. 3To more generally characterize Mtr with respect to other homologous enzymes, a sequence-similarity network (SSN) was generated. The SSN investigation showed that the top ten clusters consisted of other NAD(P)H-dependent disulfide/thiol Mr TrxR fold with an additional interface domain. The clusters were divided into seven groups according to their annotated function (Fig. 5). Actinobacterial Mtrs cluster together with a group of archaeal dihydrolipoamide dehydrogenases (DLDs; colored orange); the latter is part of a larger group of three DLD clusters (colored red). The close relationship between Mtrs and archaeal DLDs is also seen from the phylogenetic analysis (Supplementary Fig. S1). GRs cluster into a separate cluster, however, containing a few sequences of the related glutathione amide reductases (GARs) and trypanothione reductases (TryRs). Mercuric reductases (MerAs), eukaryotic high Mr TrxRs and the most distant group of CoADRs each make up distinct clusters, as seen in the SSN and phylogenetic analysis. CoADRs differ functionally from the remaining enzymatic clusters in view of their single active-site Cys residue used for catalysis and cluster into an independent clade in the phylogenetic tree. Three clusters containing sequences annotated as DLDs, PNDOs and FAD-dependent or MerAs, with the latter lacking the metal-binding NmerA domain, make up a final miscellaneous group. The SSN shows that the largest groups of homologous enzymes to Mtr are the GRs, DLDs, MerAs and high Mr TrxRs; however, the closest group that cluster together with Mtrs are the archeal group annotated as DLDs. This close relationship is interesting and will need further investigation to reveal whether there are some additional functional relationships between these groups.
containing the canonical low3.3. The putative MSSM binding site
Through 1gra; Karplus & Schulz, 1989), the binding of MSSM to the active site of Mtr was examined. Mtr shows high structural similarity to GR, and a similar to that of GR has been proposed for Mtr in the reduction of MSSM (Patel & Blanchard, 1999). Therefore, we expected MSSM to bind to Mtr in a similar way as GSSG binds to GR, placing the substrate disulfide in the vicinity of the redox-active Cys pair on the si face of FAD, allowing reduction of MSSM. Moreover, as GR is only functional as a homodimer, with each substrate-binding site being formed by both subunits (Schulz et al., 1978), this is also expected for Mtr.
and structural comparison with the of human GR (with GSSG bound; PDB entryInitial structure-based blind-docking calculations were performed between homodimeric MtrRe and the reduced product MSH (PDB entry 8f5v; Jayasinghe et al., 2023), returning two significant solutions with Vina scores of −6.8 and −5.8. The MSH molecules are docked into the cavity corresponding to the GSSG binding cleft in GR, placing the MSH thiol near the catalytic Cys pair in Mtr, stabilized by a significant number of putative polar contacts (data not shown).
Further , Supplementary Fig. S2 and Fig. 4). This positioning of the substrate would facilitate the initial nucleophilic attack performed by the N-terminal cysteine of Mtr (Cys39) on the MSSM disulfide bond, forming a mixed disulfide, commonly described as the first catalytic step performed by GR and similar FDRs (Fagan & Palfey, 2010; Deponte, 2013; Berry et al., 1989; Pai & Schulz, 1983; Kallis & Holmgren, 1980). Analogous to GR, deprotonation of the Cys39 thiol group in Mtr, leading to the formation of this intermolecular disulfide bond, could be accelerated owing to the histidine residue (His442) of the conserved His–Glu located in the opposite subunit of the Mtr homodimer, which is positioned close to the redox-active Cys pair and FAD cofactor (Figs. 1b and 6a). In GR, the interaction between the analogous histidine and glutamate residues has been proposed to facilitate deprotonation in a similar way as in serine and the histidine has furthermore been suggested to protonate the thiolate of the first GSH molecule leaving the active site (Pai & Schulz, 1983; Veine et al., 1998; Wong & Blanchard, 1989; Wong et al., 1988; Arscott et al., 2000; Fig. 6c). A nearby tyrosine residue (Tyr114, human GR numbering) was proposed to be involved in assisting the acid catalyst histidine (Krauth-Siegel et al., 1998); however, this has subsequently been disproved by others (Deonarain et al., 1989). This tyrosine is, however, only conserved in 9% of GRs in the ConSurf search and is not conserved in Mtrs, where it is replaced by a glycine residue (Gly95) that is unlikely to play a catalytic role in the oxidative half-reaction of MSSM reduction.
revealed that MSSM can also fit into the expected substrate-binding site with its disulfide positioned in close proximity to the catalytic cysteines of Mtr (Vina score of −7.0; Fig. 6The α-helix (numbered α3) enclosing one side of the substrate-binding pocket (Trp77–Asp102 in MtrRe) is slightly shifted in the Mtr structure (Figs. 6e and 6f) compared with GR (PDB entry 1gra; Karplus & Schulz, 1989), creating a larger binding cavity lined with highly conserved residues (Figs. 6b and 6d and Supplementary Fig. S2). Consequently, this allows more space for the larger MSSM substrate to bind, in agreement with the larger size of MSSM compared with GSSG. It is notable that the residues in the α3 helix are more conserved among Mtrs than among GRs (Figs. 6b and 6d). That the α3 helix is more straight in GR, while it is bent in Mtrs, could be due to the three glycine residues (known to disorder helices) found in both MtrRe and MtrMs but not in GR (Figs. 6e and 4).
Overall, our docking calculations provide insight into a likely MSSM binding site in Mtr that is compatible with the expected mechanism for MSSM reduction, supported by functional studies as well as by structural similarity to GR (Patel & Blanchard, 1999; Holsclaw et al., 2011; Kumar et al., 2017; Karplus & Schulz, 1989).
4. Conclusions
The first crystallographic structures of the FAD-containing NADPH-dependent oxidoreductase Mtr, presented in this work, display a homodimeric architecture, assigning Mtrs to the group of Mr TrxRs, Mtr contains an additional dimerization domain that is not present in, for example, low Mr TrxR or Bdr. In contrast to the previously reported asymmetric SAXS structure of Mtr, the crystallographic Mtr structures from M. smegmatis and R. erythropolis presented in this work display a symmetrical topology, as described for most homologous oxidoreductase structures.
consisting of two dinucleotide-binding Rossmann-like fold domains fused into a single chain: the tDBDFs. As also found in the structurally related GRs, DLDs and highMtr shares high structural and sequence similarity with GR, the functionally related reductase of GSSG. Although similar overall to GR, the , 1999). The high degree of amino-acid conservation and the binding-site architecture may also contribute to a highly tailored substrate stabilization, possibly contributing to the faster rate reported for the oxidative half-reaction in Mtr (Patel & Blanchard, 2001).
of Mtr reveals a larger substrate-binding cleft that is adapted to accommodate the larger and bulkier MSSM substrate. The enlarged, altered and conserved substrate-binding site, partly due to the shifting of a helix, in Mtrs facilitates our proposed binding mode of MSSM, as demonstrated through docking calculations. The shape and size of the binding site may partly explain the previously reported minimum requirement for the glucosamine moiety of MSSM, as well as the lack of GSSG activity (Patel & Blanchard, 1998A large number of tDBDFs comprise
that act on sulfur-containing substrates, and the majority of FDRs belong to this FDRs represent a family of enzymes with high sequence and structural homology that catalyze the NAD(P)H-dependent reduction of sulfide-bonded substrates, such as the reduction of thioredoxin catalyzed by TrxRs. The sulfide-bonded substrates also comprise LMW such as GSSG, BSSB, coenzyme A disulfide (CoASSCoA) and MSSM, which are reduced by GR, Bdr, CoADR and Mtr, respectively. The structures of GR, Bdr and CoADR have been described previously, including comprehensive and extensive structural and functional studies of GR and the GSH redox system. No crystallographic data for Mtr have been available to date, creating a knowledge gap in the field of bacterial LMW thiol redox biology. The Mtr structures presented in this work extend our knowledge of Mtrs and tDBDFs, adding an important missing piece to the structural understanding of and the substrate specificity among reductases of structurally distinct sulfur-containing substrates, in particular LMW Our structural data, as well as the insight into the MSSM binding mode in Mtrs, may also contribute to future antituberculosis drug development or to new advancements in bioremediation processes, two important areas of research related to actinobacterial survival mechanisms.Supporting information
Link https://doi.org/10.15151/ESRF-DC-1468846919
Diffraction data.
Supplementary Figures. DOI: https://doi.org/10.1107/S205979832400113X/gi5042sup1.pdf
Acknowledgements
The authors thank the Regional Core Facility for Structural Biology at the Oslo University Hospital for access to crystallization screening and the UiO Structural Biology Core Facilities (PX-Oslo) node at the Department of Biosciences. We acknowledge the European Synchrotron Radiation Facility (ESRF) for the provision of synchrotron-radiation facilities and the staff of beamlines ID30B and ID23-2 for their assistance. The diffraction data for MtrRe and MtrMs were collected at the ESRF and have been archived in the ESRF depository (https://doi.org/10.15151/ESRF-DC-1468846919).
Funding information
This project has been funded by a grant from the Research Council of Norway (grant No. 301584) through support from the University of Oslo and the UiO Structural Biology Core Facilities (PX-Oslo). The project received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 801133.
References
Agirre, J., Atanasova, M., Bagdonas, H., Ballard, C. B., Baslé, A., Beilsten-Edmands, J., Borges, R. J., Brown, D. G., Burgos-Mármol, J. J., Berrisford, J. M., Bond, P. S., Caballero, I., Catapano, L., Chojnowski, G., Cook, A. G., Cowtan, K. D., Croll, T. I., Debreczeni, J. É., Devenish, N. E., Dodson, E. J., Drevon, T. R., Emsley, P., Evans, G., Evans, P. R., Fando, M., Foadi, J., Fuentes-Montero, L., Garman, E. F., Gerstel, M., Gildea, R. J., Hatti, K., Hekkelman, M. L., Heuser, P., Hoh, S. W., Hough, M. A., Jenkins, H. T., Jiménez, E., Joosten, R. P., Keegan, R. M., Keep, N., Krissinel, E. B., Kolenko, P., Kovalevskiy, O., Lamzin, V. S., Lawson, D. M., Lebedev, A. A., Leslie, A. G. W., Lohkamp, B., Long, F., Malý, M., McCoy, A. J., McNicholas, S. J., Medina, A., Millán, C., Murray, J. W., Murshudov, G. N., Nicholls, R. A., Noble, M. E. M., Oeffner, R., Pannu, N. S., Parkhurst, J. M., Pearce, N., Pereira, J., Perrakis, A., Powell, H. R., Read, R. J., Rigden, D. J., Rochira, W., Sammito, M., Sánchez Rodríguez, F., Sheldrick, G. M., Shelley, K. L., Simkovic, F., Simpkin, A. J., Skubak, P., Sobolev, E., Steiner, R. A., Stevenson, K., Tews, I., Thomas, J. M. H., Thorn, A., Valls, J. T., Uski, V., Usón, I., Vagin, A., Velankar, S., Vollmar, M., Walden, H., Waterman, D., Wilson, K. S., Winn, M. D., Winter, G., Wojdyr, M. & Yamashita, K. (2023). Acta Cryst. D79, 449–461. Web of Science CrossRef IUCr Journals Google Scholar
Arenas-Salinas, M., Vargas-Pérez, J. I., Morales, W., Pinto, C., Muñoz-Díaz, P., Cornejo, F. A., Pugin, B., Sandoval, J. M., Díaz-Vásquez, W. A., Muñoz-Villagrán, C., Rodríguez-Rojas, F., Morales, E. H., Vásquez, C. C. & Arenas, F. A. (2016). Front. Microbiol. 7, 1160. PubMed Google Scholar
Argyrou, A. & Blanchard, J. S. (2004). Prog. Nucleic Acid Res. Mol. Biol. 78, 89–142. Web of Science CrossRef PubMed CAS Google Scholar
Argyrou, A., Vetting, M. W. & Blanchard, J. S. (2004). J. Biol. Chem. 279, 52694–52702. CrossRef PubMed CAS Google Scholar
Arscott, L. D., Veine, D. M. & Williams, C. H. (2000). Biochemistry, 39, 4711–4721. CrossRef PubMed CAS Google Scholar
Ashkenazy, H., Abadi, S., Martz, E., Chay, O., Mayrose, I., Pupko, T. & Ben-Tal, N. (2016). Nucleic Acids Res. 44, W344–W350. Web of Science CrossRef CAS PubMed Google Scholar
Ashkenazy, H., Erez, E., Martz, E., Pupko, T. & Ben-Tal, N. (2010). Nucleic Acids Res. 38, W529–W533. Web of Science CrossRef CAS PubMed Google Scholar
Bafana, A., Khan, F. & Suguna, K. (2017). Biometals, 30, 809–819. CrossRef CAS PubMed Google Scholar
Berry, A., Scrutton, N. S. & Perham, R. N. (1989). Biochemistry, 28, 1264–1269. CrossRef CAS PubMed Google Scholar
Butz, Z. J., Hendricks, A., Borgognoni, K. & Ackerson, C. J. (2021). FEMS Microbiol. Ecol. 97, fiaa220. Google Scholar
Celniker, G., Nimrod, G., Ashkenazy, H., Glaser, F., Martz, E., Mayrose, I., Pupko, T. & Ben-Tal, N. (2013). Isr. J. Chem. 53, 199–206. Web of Science CrossRef CAS Google Scholar
Chi, B. K., Busche, T., Van Laer, K., Bäsell, K., Becher, D., Clermont, L., Seibold, G. M., Persicke, M., Kalinowski, J., Messens, J. & Antelmann, H. (2014). Antioxid. Redox Signal. 20, 589–605. CrossRef CAS PubMed Google Scholar
Deonarain, M. P., Berry, A., Scrutton, N. S. & Perham, R. N. (1989). Biochemistry, 28, 9602–9607. CrossRef CAS PubMed Google Scholar
Deponte, M. (2013). Biochim. Biophys. Acta, 1830, 3217–3266. CrossRef CAS PubMed Google Scholar
Dym, O. & Eisenberg, D. (2001). Protein Sci. 10, 1712–1728. Web of Science CrossRef PubMed CAS Google Scholar
Emsley, P., Lohkamp, B., Scott, W. G. & Cowtan, K. (2010). Acta Cryst. D66, 486–501. Web of Science CrossRef CAS IUCr Journals Google Scholar
Evans, P. R. (2011). Acta Cryst. D67, 282–292. Web of Science CrossRef CAS IUCr Journals Google Scholar
Fagan, R. L. & Palfey, B. A. (2010). Flavin-Dependent Enzymes. Oxford: Elsevier. Google Scholar
Fahey, R. C., Brown, W. C., Adams, W. B. & Worsham, M. B. (1978). J. Bacteriol. 133, 1126–1129. CrossRef CAS PubMed Web of Science Google Scholar
Finn, R. D., Clements, J., Arndt, W., Miller, B. L., Wheeler, T. J., Schreiber, F., Bateman, A. & Eddy, S. R. (2015). Nucleic Acids Res. 43, W30–W38. Web of Science CrossRef CAS PubMed Google Scholar
Gaballa, A., Newton, G. L., Antelmann, H., Parsonage, D., Upton, H., Rawat, M., Claiborne, A., Fahey, R. C. & Helmann, J. D. (2010). Proc. Natl Acad. Sci. USA, 107, 6482–6486. Web of Science CrossRef CAS PubMed Google Scholar
Hammerstad, M., Gudim, I. & Hersleth, H.-P. (2020). Biochemistry, 59, 4793–4798. CrossRef CAS PubMed Google Scholar
Hammerstad, M. & Hersleth, H.-P. (2021). Arch. Biochem. Biophys. 702, 108826. CrossRef PubMed Google Scholar
Holm, L. (2022). Nucleic Acids Res. 50, W210–W215. Web of Science CrossRef CAS PubMed Google Scholar
Holsclaw, C. M., Muse, W. B., Carroll, K. S. & Leary, J. A. (2011). Int. J. Mass Spectrom. 305, 151–156. CrossRef CAS PubMed Google Scholar
Jayasinghe, Y. P., Banco, M. T., Lindenberger, J. J., Favrot, L., Palčeková, Z., Jackson, M., Manabe, S. & Ronning, D. R. (2023). RSC Med. Chem. 14, 491–500. CrossRef CAS PubMed Google Scholar
Joosten, R. P., Long, F., Murshudov, G. N. & Perrakis, A. (2014). IUCrJ, 1, 213–220. Web of Science CrossRef CAS PubMed IUCr Journals Google Scholar
Jothivasan, V. K. & Hamilton, C. J. (2008). Nat. Prod. Rep. 25, 1091–1117. CrossRef PubMed CAS Google Scholar
Jumper, J., Evans, R., Pritzel, A., Green, T., Figurnov, M., Ronneberger, O., Tunyasuvunakool, K., Bates, R., Žídek, A., Potapenko, A., Bridgland, A., Meyer, C., Kohl, S. A. A., Ballard, A. J., Cowie, A., Romera-Paredes, B., Nikolov, S., Jain, R., Adler, J., Back, T., Petersen, S., Reiman, D., Clancy, E., Zielinski, M., Steinegger, M., Pacholska, M., Berghammer, T., Bodenstein, S., Silver, D., Vinyals, O., Senior, A. W., Kavukcuoglu, K., Kohli, P. & Hassabis, D. (2021). Nature, 596, 583–589. Web of Science CrossRef CAS PubMed Google Scholar
Kabsch, W. (2010). Acta Cryst. D66, 125–132. Web of Science CrossRef CAS IUCr Journals Google Scholar
Kabsch, W. & Sander, C. (1983). Biopolymers, 22, 2577–2637. CrossRef CAS PubMed Web of Science Google Scholar
Kallis, G. B. & Holmgren, A. (1980). J. Biol. Chem. 255, 10261–10265. CrossRef CAS PubMed Web of Science Google Scholar
Karplus, P. A. & Schulz, G. E. (1987). J. Mol. Biol. 195, 701–729. CrossRef CAS PubMed Google Scholar
Karplus, P. A. & Schulz, G. E. (1989). J. Mol. Biol. 210, 163–180. CrossRef CAS PubMed Web of Science Google Scholar
Katoh, K. & Standley, D. M. (2013). Mol. Biol. Evol. 30, 772–780. Web of Science CrossRef CAS PubMed Google Scholar
Krauth-Siegel, R. L., Arscott, L. D., Schönleben-Janas, A., Schirmer, R. H. & Williams, C. H. (1998). Biochemistry, 37, 13968–13977. CAS PubMed Google Scholar
Kumar, A., Nartey, W., Shin, J., Manimekalai, M. S. S. & Grüber, G. (2017). Biochim. Biophys. Acta, 1861, 2354–2366. CrossRef CAS Google Scholar
Kuriyan, J., Kong, X. P., Krishna, T. S. R., Sweet, R. M., Murgolo, N. J., Field, H., Cerami, A. & Henderson, G. B. (1991). Proc. Natl Acad. Sci. USA, 88, 8764–8768. CrossRef PubMed CAS Google Scholar
Kuriyan, J., Krishna, T. S. R., Wong, L., Guenther, B., Pahler, A., Williams, C. H. & Model, P. (1991). Nature, 352, 172–174. CrossRef CAS PubMed Google Scholar
Laskowski, R. A. & Swindells, M. B. (2011). J. Chem. Inf. Model. 51, 2778–2786. Web of Science CrossRef CAS PubMed Google Scholar
Laurent, T. C., Moore, E. C. & Reichard, P. (1964). J. Biol. Chem. 239, 3436–3444. CrossRef PubMed CAS Web of Science Google Scholar
Ledwidge, R., Patel, B., Dong, A. P., Fiedler, D., Falkowski, M., Zelikova, J., Summers, A. O., Pai, E. F. & Miller, S. M. (2005). Biochemistry, 44, 11402–11416. CrossRef PubMed CAS Google Scholar
Liebschner, D., Afonine, P. V., Baker, M. L., Bunkóczi, G., Chen, V. B., Croll, T. I., Hintze, B., Hung, L.-W., Jain, S., McCoy, A. J., Moriarty, N. W., Oeffner, R. D., Poon, B. K., Prisant, M. G., Read, R. J., Richardson, J. S., Richardson, D. C., Sammito, M. D., Sobolev, O. V., Stockwell, D. H., Terwilliger, T. C., Urzhumtsev, A. G., Videau, L. L., Williams, C. J. & Adams, P. D. (2019). Acta Cryst. D75, 861–877. Web of Science CrossRef IUCr Journals Google Scholar
Liu, Y., Yang, X. C., Gan, J. H., Chen, S., Xiao, Z. X. & Cao, Y. (2022). Nucleic Acids Res. 50, W159–W164. CrossRef CAS PubMed Google Scholar
Loi, V. V., Rossius, M. & Antelmann, H. (2015). Front. Microbiol. 6, 187. CrossRef PubMed Google Scholar
Lu, S. N., Wang, J. Y., Chitsaz, F., Derbyshire, M. K., Geer, R. C., Gonzales, N. R., Gwadz, M., Hurwitz, D. I., Marchler, G. H., Song, J. S., Thanki, N., Yamashita, R. A., Yang, M. Z., Zhang, D. C., Zheng, C. J., Lanczycki, C. J. & Marchler-Bauer, A. (2020). Nucleic Acids Res. 48, D265–D268. CrossRef CAS PubMed Google Scholar
Mallett, T. C., Wallen, J. R., Karplus, P. A., Sakai, H., Tsukihara, T. & Claiborne, A. (2006). Biochemistry, 45, 11278–11289. CrossRef PubMed CAS Google Scholar
Maltman, C., Donald, L. J. & Yurkov, V. (2017). Microorganisms, 5, 20. CrossRef PubMed Google Scholar
Mande, S. S., Sarfaty, S., Allen, M. D., Perham, R. N. & Hol, W. G. J. (1996). Structure, 4, 277–286. CrossRef CAS PubMed Web of Science Google Scholar
Massey, V. & Williams, C. H. (1965). J. Biol. Chem. 240, 4470–4480. CrossRef CAS PubMed Google Scholar
McCoy, A. J., Grosse-Kunstleve, R. W., Adams, P. D., Winn, M. D., Storoni, L. C. & Read, R. J. (2007). J. Appl. Cryst. 40, 658–674. Web of Science CrossRef CAS IUCr Journals Google Scholar
Moore, M. D. & Kaplan, S. (1992). J. Bacteriol. 174, 1505–1514. CrossRef PubMed CAS Google Scholar
Nambi, S., Long, J. E., Mishra, B. B., Baker, R., Murphy, K. C., Olive, A. J., Nguyen, H. P., Shaffer, S. A. & Sassetti, C. M. (2015). Cell Host Microbe, 17, 829–837. CrossRef CAS PubMed Google Scholar
Nazari, M. T., Simon, V., Machado, B. S., Crestani, L., Marchezi, G., Concolato, G., Ferrari, V., Colla, L. M. & Piccin, J. S. (2022). J. Environ. Manage. 323, 116220. CrossRef PubMed Google Scholar
Newton, G. L., Arnold, K., Price, M. S., Sherrill, C., Delcardayre, S. B., Aharonowitz, Y., Cohen, G., Davies, J., Fahey, R. C. & Davis, C. (1996). J. Bacteriol. 178, 1990–1995. CrossRef CAS PubMed Web of Science Google Scholar
Newton, G. L., Bewley, C. A., Dwyer, T. J., Horn, R., Aharonowitz, Y., Cohen, G., Davies, J., Faulkner, D. J. & Fahey, R. C. (1995). Eur. J. Biochem. 230, 821–825. CrossRef CAS PubMed Google Scholar
Newton, G. L., Rawat, M., La Clair, J. J., Jothivasan, V. K., Budiarto, T., Hamilton, C. J., Claiborne, A., Helmann, J. D. & Fahey, R. C. (2009). Nat. Chem. Biol. 5, 625–627. Web of Science CrossRef PubMed CAS Google Scholar
Nilewar, S. S. & Kathiravan, M. K. (2014). Bioorg. Chem. 52, 62–68. CrossRef CAS PubMed Google Scholar
Oberg, N., Zallot, R. & Gerlt, J. A. (2023). J. Mol. Biol. 435, 168018. CrossRef PubMed Google Scholar
Ojha, S., Meng, E. C. & Babbitt, P. C. (2007). PLoS Comput. Biol. 3, e121. CrossRef PubMed Google Scholar
Pai, E. F. & Schulz, G. E. (1983). J. Biol. Chem. 258, 1752–1757. CrossRef CAS PubMed Google Scholar
Patel, M. P. & Blanchard, J. S. (1998). J. Am. Chem. Soc. 120, 11538–11539. CrossRef CAS Google Scholar
Patel, M. P. & Blanchard, J. S. (1999). Biochemistry, 38, 11827–11833. CrossRef PubMed CAS Google Scholar
Patel, M. P. & Blanchard, J. S. (2001). Biochemistry, 40, 5119–5126. CrossRef PubMed CAS Google Scholar
Rawat, M., Newton, G. L., Ko, M., Martinez, G. J., Fahey, R. C. & Av-Gay, Y. (2002). Antimicrob. Agents Chemother. 46, 3348–3355. CrossRef PubMed CAS Google Scholar
Reyes, A. M., Pedre, B., De Armas, M. I., Tossounian, M. A., Radi, R., Messens, J. & Trujillo, M. (2018). Antioxid. Redox Signal. 28, 487–504. CrossRef CAS PubMed Google Scholar
Sakuda, S., Zhou, Z. Y. & Yamada, Y. (1994). Biosci. Biotechnol. Biochem. 58, 1347–1348. CrossRef CAS PubMed Google Scholar
Sareen, D., Newton, G. L., Fahey, R. C. & Buchmeier, N. A. (2003). J. Bacteriol. 185, 6736–6740. CrossRef PubMed CAS Google Scholar
Sassetti, C. M. & Rubin, E. J. (2003). Proc. Natl Acad. Sci. USA, 100, 12989–12994. Web of Science CrossRef PubMed CAS Google Scholar
Schulz, G. E., Schirmer, R. H., Sachsenheimer, W. & Pai, E. F. (1978). Nature, 273, 120–124. CrossRef CAS PubMed Google Scholar
Sea, K., Lee, J., To, D., Chen, B., Sazinsky, M. H. & Crane, E. J. (2018). FEBS Open Bio, 8, 1083–1092. CrossRef CAS PubMed Google Scholar
Shannon, P., Markiel, A., Ozier, O., Baliga, N. S., Wang, J. T., Ramage, D., Amin, N., Schwikowski, B. & Ideker, T. (2003). Genome Res. 13, 2498–2504. Web of Science CrossRef PubMed CAS Google Scholar
Sharma, S. V., Jothivasan, V. K., Newton, G. L., Upton, H., Wakabayashi, J. I., Kane, M. G., Roberts, A. A., Rawat, M., La Clair, J. J. & Hamilton, C. J. (2011). Angew. Chem. Int. Ed. 50, 7101–7104. CrossRef CAS Google Scholar
Sievers, F., Wilm, A., Dineen, D., Gibson, T. J., Karplus, K., Li, W. Z., Lopez, R., McWilliam, H., Remmert, M., Söding, J., Thompson, J. D. & Higgins, D. G. (2011). Mol. Syst. Biol. 7, 539. CrossRef PubMed Google Scholar
Spies, H. S. C. & Steenkamp, D. J. (1994). Eur. J. Biochem. 224, 203–213. CrossRef CAS PubMed Google Scholar
Stein, N. (2008). J. Appl. Cryst. 41, 641–643. Web of Science CrossRef CAS IUCr Journals Google Scholar
Susanti, D., Loganathan, U., Compton, A. & Mukhopadhyay, B. (2017). ACS Omega, 2, 4180–4187. CrossRef CAS PubMed Google Scholar
Touw, W. G., Baakman, C., Black, J., te Beek, T. A., Krieger, E., Joosten, R. P. & Vriend, G. (2015). Nucleic Acids Res. 43, D364–D368. Web of Science CrossRef CAS PubMed Google Scholar
Trivedi, A., Singh, N., Bhat, S. A., Gupta, P. & Kumar, A. (2012). Adv. Microb. Physiol. 60, 263–324. CrossRef CAS PubMed Google Scholar
Van Laer, K., Buts, L., Foloppe, N., Vertommen, D., Van Belle, K., Wahni, K., Roos, G., Nilsson, L., Mateos, L. M., Rawat, M., van Nuland, N. A. J. & Messens, J. (2012). Mol. Microbiol. 86, 787–804. CrossRef CAS PubMed Google Scholar
Van Petegem, F., De Vos, D., Savvides, S., Vergauwen, B. & Van Beeumen, J. (2007). J. Mol. Biol. 374, 883–889. Web of Science CrossRef PubMed CAS Google Scholar
Veine, D. M., Arscott, L. D. & Williams, C. H. (1998). Biochemistry, 37, 15575–15582. CrossRef CAS PubMed Google Scholar
Wallace, A. C., Laskowski, R. A. & Thornton, J. M. (1995). Protein Eng. Des. Sel. 8, 127–134. CrossRef CAS Web of Science Google Scholar
Waterhouse, A. M., Procter, J. B., Martin, D. M. A., Clamp, M. & Barton, G. J. (2009). Bioinformatics, 25, 1189–1191. Web of Science CrossRef PubMed CAS Google Scholar
Williams, C. H. (1995). FASEB J. 9, 1267–1276. CrossRef CAS PubMed Google Scholar
Wong, K. K. & Blanchard, J. S. (1989). Biochemistry, 28, 3586–3590. CrossRef CAS PubMed Google Scholar
Wong, K. K., Vanoni, M. A. & Blanchard, J. S. (1988). Biochemistry, 27, 7091–7096. CrossRef CAS PubMed Google Scholar
Zeldin, O. B., Gerstel, M. & Garman, E. F. (2013). J. Appl. Cryst. 46, 1225–1230. Web of Science CrossRef CAS IUCr Journals Google Scholar
This is an open-access article distributed under the terms of the Creative Commons Attribution (CC-BY) Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are cited.