Solution structure and biochemical characterization of a spare part protein that restores activity to an oxygen-damaged glycyl radical enzyme

Modeling the Initiation Phase of the Catalytic Cycle in the Glycyl-Radical Enzyme Benzylsuccinate Synthase

The reaction of benzylsuccinate synthase, the radical-based addition of toluene to a fumarate cosubstrate, is initiated by hydrogen transfer from a conserved cysteine to the nearby glycyl radical in the active center of the enzyme. In this study, we analyze this step by comprehensive computer modeling, predicting (i) the influence of bound substrates or products, (ii) the energy profiles of forward- and backward hydrogen-transfer reactions, (iii) their kinetic constants and potential mechanisms, (iv) enantiospecificity differences, and (v) kinetic isotope effects. Moreover, we support several of the computational predictions experimentally, providing evidence for the predicted H/D-exchange reactions into the product and at the glycyl radical site. Our data indicate that the hydrogen transfer reactions between the active site glycyl and cysteine are principally reversible, but their rates differ strongly depending on their stereochemical orientation, transfer of protium or deuterium, and the presence or absence of substrates or products in the active site. This is particularly evident for the isotope exchange of the remaining protium atom of the glycyl radical to deuterium, which appears dependent on substrate or product binding, explaining why the exchange is observed in some, but not all, glycyl-radical enzymes.

The formate-hydrogen axis and its impact on the physiology of enterobacterial fermentation

Formic acid (HCOOH) and dihydrogen (H2) are characteristic products of enterobacterial mixed-acid fermentation, with H2 generation increasing in conjunction with a decrease in extracellular pH. Formate and acetyl-CoA are generated by radical-based and coenzyme A-dependent cleavage of pyruvate catalysed by pyruvate formate-lyase (PflB). Formate is also the source of H2, which is generated along with carbon dioxide through the action of the membrane-associated, cytoplasmically-oriented formate hydrogenlyase (FHL-1) complex. Synthesis of the FHL-1 complex is completely dependent on the cytoplasmic accumulation of formate. Consequently, formate determines its own disproportionation into H2 and CO2 by the FHL-1 complex. Cytoplasmic formate levels are controlled by FocA, a pentameric channel that translocates formic acid/formate bidirectionally between the cytoplasm and periplasm. Each protomer of FocA has a narrow hydrophobic pore through which neutral formic acid can pass. Two conserved amino acid residues, a histidine and a threonine, at the center of the pore control directionality of translocation. The histidine residue is essential for pH-dependent influx of formic acid. Studies with the formate analogue hypophosphite and amino acid variants of FocA suggest that the mechanisms of formic acid efflux and influx differ. Indeed, current data suggest, depending on extracellular formate levels, two separate uptake mechanisms exist, both likely contributing to maintain pH homeostasis. Bidirectional formate/formic acid translocation is dependent on PflB and influx requires an active FHL-1 complex. This review describes the coupling of formate and H2 production in enterobacteria.

Type III secretion system effector YfiD inhibits the activation of host poly(ADP-ribose) polymerase-1 to promote bacterial infection

Modulation of cell death is a powerful strategy employed by pathogenic bacteria to evade host immune clearance and occupy profitable replication niches during infection. Intracellular pathogens employ the type III secretion system (T3SS) to deliver effectors, which interfere with regulated cell death pathways to evade immune defenses. Here, we reveal that poly(ADP-ribose) polymerase-1 (PARP1)-dependent cell death restrains Edwardsiella piscicida's proliferation in mouse monocyte macrophages J774A.1, of which PARP1 activation results in the accumulation of poly(ADP-ribose) (PAR) and enhanced inflammatory response. Moreover, E. piscicida, an important intracellular pathogen, leverages a T3SS effector YfiD to impair PARP1's activity and inhibit PAR accumulation. Once translocated into the host nucleus, YfiD binds to the ADP-ribosyl transferase (ART) domain of PARP1 to suppress its PARylation ability as the pharmacological inhibitor of PARP1 behaves. Furthermore, the interaction between YfiD and ART mainly relies on the complete unfolding of the helical domain, which releases the inhibitory effect on ART. In addition, YfiD impairs the inflammatory response and cell death in macrophages and promotes in vivo colonization and virulence of E. piscicida. Collectively, our results establish the functional mechanism of YfiD as a potential PARP1 inhibitor and provide more insights into host defense against bacterial infection.

Pyruvate formate-lyase activating enzyme: The catalytically active 5'-deoxyadenosyl radical caught in the act of H-atom abstraction

Enzymes of the radical S-adenosyl-l-methionine (radical SAM, RS) superfamily, the largest in nature, catalyze remarkably diverse reactions initiated by H-atom abstraction. Glycyl radical enzyme activating enzymes (GRE-AEs) are a growing class of RS enzymes that generate the catalytically essential glycyl radical of GREs, which in turn catalyze essential reactions in anaerobic metabolism. Here, we probe the reaction of the GRE-AE pyruvate formate-lyase activating enzyme (PFL-AE) with the peptide substrate RVSG734YAV, which mimics the site of glycyl radical formation on the native substrate, pyruvate formate-lyase. Time-resolved freeze-quench electron paramagnetic resonance spectroscopy shows that at short mixing times reduced PFL-AE + SAM reacts with RVSG734YAV to form the central organometallic intermediate, Ω, in which the adenosyl 5'C is covalently bound to the unique iron of the [4Fe-4S] cluster. Freeze-trapping the reaction at longer times reveals the formation of the peptide G734• glycyl radical product. Of central importance, freeze-quenching at intermediate times reveals that the conversion of Ω to peptide glycyl radical is not concerted. Instead, homolysis of the Ω Fe-C5' bond generates the nominally "free" 5'-dAdo• radical, which is captured here by freeze-trapping. During cryoannealing at 77 K, the 5'-dAdo• directly abstracts an H-atom from the peptide to generate the G734• peptide radical trapped in the PFL-AE active site. These observations reveal the 5'-dAdo• radical to be a well-defined intermediate, caught in the act of substrate H-atom abstraction, providing new insights into the mechanistic steps of radical initiation by RS enzymes.

Development of in-line anoxic small-angle X-ray scattering and structural characterization of an oxygen-sensing transcriptional regulator

Oxygen-sensitive metalloenzymes are responsible for many of the most fundamental biochemical processes in nature, from the reduction of dinitrogen in nitrogenase to the biosynthesis of photosynthetic pigments. However, biophysical characterization of such proteins under anoxic conditions can be challenging, especially at noncryogenic temperatures. In this study, we introduce the first in-line anoxic small-angle X-ray scattering (anSAXS) system at a major national synchrotron source, featuring both batch-mode and chromatography-mode capabilities. To demonstrate chromatography-coupled anSAXS, we investigated the oligomeric interconversions of the fumarate and nitrate reduction (FNR) transcription factor, which is responsible for the transcriptional response to changing oxygen conditions in the facultative anaerobe Escherichia coli. Previous work has shown that FNR contains a labile [4Fe-4S] cluster that is degraded when oxygen is present and that this change in cluster composition leads to the dissociation of the DNA-binding dimeric form. Using anSAXS, we provide the first direct structural evidence for the oxygen-induced dissociation of the E. coli FNR dimer and its correlation with cluster composition. We further demonstrate how complex FNR-DNA interactions can be studied by investigating the promoter region of the anaerobic ribonucleotide reductase genes, nrdDG, which contains tandem FNR-binding sites. By coupling size-exclusion chromatography-anSAXS with full-spectrum UV-Vis analysis, we show that the [4Fe-4S] cluster-containing dimeric form of FNR can bind to both sites in the nrdDG promoter region. The development of in-line anSAXS greatly expands the toolbox available for the study of complex metalloproteins and provides a foundation for future expansions.

Development of in-line anoxic small-angle X-ray scattering and structural characterization of an oxygen-sensing transcriptional regulator

Oxygen-sensitive metalloenzymes are responsible for many of the most fundamental biochemical processes in nature, from the reduction of di-nitrogen in nitrogenase to the biosynthesis of photosynthetic pigments. However, biophysical characterization of such proteins under anoxic conditions can be challenging, especially at non-cryogenic temperatures. In this study, we introduce the first in-line anoxic small-angle X-ray scattering (anSAXS) system at a major national synchrotron source, featuring both batch-mode and chromatography-mode capabilities. To demonstrate chromatography-coupled anSAXS, we investigated the oligomeric interconversions of the Fumarate and Nitrate Reduction (FNR) transcription factor, which is responsible for the transcriptional response to changing oxygen conditions in the facultative anaerobe Escherichia coli . Previous work has shown that FNR contains a labile [4Fe-4S] cluster that is degraded when oxygen is present, and that this change in cluster composition leads to the dissociation of the DNA-binding dimeric form. Using anSAXS, we provide the first direct structural evidence for the oxygen-induced dissociation of the E. coli FNR dimer and its correlation with cluster composition. We further demonstrate how complex FNR-DNA interactions can be studied by investigating the promoter region of the anaerobic ribonucleotide reductase genes, nrdDG , which contains tandem FNR binding sites. By coupling SEC-anSAXS with full spectrum UV-Vis analysis, we show that the [4Fe-4S] clustercontaining dimeric form of FNR can bind to both sites in the nrdDG promoter region. The development of in-line anSAXS greatly expands the toolbox available for the study of complex metalloproteins and provides a foundation for future expansions.

Genomic evaluation of novel Kenyan virulent phage isolates infecting carbapenemase-producing Klebsiella pneumoniae and safety determination of their lysates in Balb/c mice

This study aimed to evaluate the genomic features of novel Kenyan virulent phage isolates infecting carbapenemase-producing Klebsiella pneumoniae and to determine the safety of their lysates using mice model in a preclinical study. The genomics showed that the Klebsiella phages vB_KpM_CPRSA and vB_KpM_CPRSB belonged to the genus Slopekvirus with a similarity index of less than 92% compared to the most closest relative species. Their genomes did not contain antimicrobial resistance and toxin genes. Then endotoxin levels in the Klebsiella phage lysates were statistically significant (p value ˃ 0.05). The serum activities of aspartate aminotransferase, alanine aminotransferase and urea in the group of balb/c mice injected with bacteriophage lysates through the intravenous route were higher compared to that of the intranasal route. Unexpectedly, there was mild congestion of the central veins of kidneys and liver without damage to renal tubules and hepatocytes and a lack of physical discomfort and pain in the mice. Our study isolated and characterised Klebsiella phages against carbapenem-resistant K. pneumoniae, which are promising therapeutic agents for the treatment of respiratory tract infections using the topical mode of administration as the preferred route of bacteriophage delivery.

Activation of Glycyl Radical Enzymes─Multiscale Modeling Insights into Catalysis and Radical Control in a Pyruvate Formate-Lyase-Activating Enzyme

Pyruvate formate-lyase (PFL) is a glycyl radical enzyme (GRE) playing a pivotal role in the metabolism of strict and facultative anaerobes. Its activation is carried out by a PFL-activating enzyme, a member of the radical S-adenosylmethionine (rSAM) superfamily of metalloenzymes, which introduces a glycyl radical into the Gly radical domain of PFL. The activation mechanism is still not fully understood and is structurally based on a complex with a short model peptide of PFL. Here, we present extensive molecular dynamics simulations in combination with quantum mechanics/molecular mechanics (QM/MM)-based kinetic and thermodynamic reaction evaluations of a more complete activation model comprising the 49 amino acid long C-terminus region of PFL. We reveal the benefits and pitfalls of the current activation model, providing evidence that the bound peptide conformation does not resemble the bound protein-protein complex conformation with PFL, with implications for the activation process. Substitution of the central glycine with (S)- and (R)-alanine showed excellent binding of (R)-alanine over unstable binding of (S)-alanine. Radical stabilization calculations indicate that a higher radical stability of the glycyl radical might not be the sole origin of the evolutionary development of GREs. QM/MM-derived radical formation kinetics further demonstrate feasible activation barriers for both peptide and C-terminus activation, demonstrating why the crystalized model peptide system is an excellent inhibitory system for natural activation. This new evidence supports the theory that GREs converged on glycyl radical formation due to the better conformational accessibility of the glycine radical loop, rather than the highest radical stability of the formed peptide radicals.

The Autonomous Glycyl Radical Protein GrcA Restores Activity to Inactive Full-Length Pyruvate Formate-Lyase In Vivo

During glucose fermentation, Escherichia coli and many other microorganisms employ the glycyl radical enzyme (GRE) pyruvate formate-lyase (PflB) to catalyze the coenzyme A-dependent cleavage of pyruvate to formate and acetyl-coenzyme A (CoA). Due to its extreme reactivity, the radical in PflB must be controlled carefully and, once generated, is particularly susceptible to dioxygen. Exposure to oxygen of the radical on glycine residue 734 of PflB results in cleavage of the polypeptide chain and consequent inactivation of the enzyme. Two decades ago, a small 14-kDa protein called YfiD (now called autonomous glycyl radical cofactor [GrcA]) was shown to be capable of restoring activity to O₂-inactivated PflB in vitro; however, GrcA has never been shown to have this function in vivo. By constructing a strain with a chromosomally encoded PflB enzyme variant with a G734A residue exchange, we could show that cells retained near-wild type fermentative growth, as well as formate and H₂ production; H₂ is derived by enzymatic disproportionation of formate. Introducing a grcA deletion mutation into this strain completely prevented formate and H₂ generation and reduced anaerobic growth. We could show that the conserved glycine at position 102 on GrcA was necessary for GrcA to restore PflB activity and that this recovered activity depended on the essential cysteine residues 418 and 419 in the active site of PflB. Together, our findings demonstrate that GrcA is capable of restoring in vivo activity to inactive full-length PflB and support a model whereby GrcA displaces the C-terminal glycyl radical domain to rescue the catalytic function of PflB. IMPORTANCE Many facultative anaerobic microorganisms use glycyl radical enzymes (GREs) to catalyze chemically challenging reactions under anaerobic conditions. Pyruvate formate-lyase (PflB) is a GRE that catalyzes cleavage of the carbon-carbon bond of pyruvate during glucose fermentation. The problem is that glycyl radicals are destroyed readily, especially by oxygen. To protect and restore activity to inactivated PflB, bacteria like Escherichia coli have a small autonomous glycyl radical cofactor protein called GrcA, which functions to rescue inactivated PflB. To date, this proposed function of GrcA has only been demonstrated in vitro. Our data reveal that GrcA rescues and restores enzyme activity to an inactive full-length form of PflB in vivo. These results have important implications for the evolution of radical-based enzyme mechanisms.

When anaerobes encounter oxygen: mechanisms of oxygen toxicity, tolerance and defence

The defining trait of obligate anaerobes is that oxygen blocks their growth, yet the underlying mechanisms are unclear. A popular hypothesis was that these microorganisms failed to evolve defences to protect themselves from reactive oxygen species (ROS) such as superoxide and hydrogen peroxide, and that this failure is what prevents their expansion to oxic habitats. However, studies reveal that anaerobes actually wield most of the same defences that aerobes possess, and many of them have the capacity to tolerate substantial levels of oxygen. Therefore, to understand the structures and real-world dynamics of microbial communities, investigators have examined how anaerobes such as Bacteroides, Desulfovibrio, Pyrococcus and Clostridium spp. struggle and cope with oxygen. The hypoxic environments in which these organisms dwell - including the mammalian gut, sulfur vents and deep sediments - experience episodic oxygenation. In this Review, we explore the molecular mechanisms by which oxygen impairs anaerobes and the degree to which bacteria protect their metabolic pathways from it. The emergent view of anaerobiosis is that optimal strategies of anaerobic metabolism depend upon radical chemistry and low-potential metal centres. Such catalytic sites are intrinsically vulnerable to direct poisoning by molecular oxygen and ROS. Observations suggest that anaerobes have evolved tactics that either minimize the extent to which oxygen disrupts their metabolism or restore function shortly after the stress has dissipated.

Rescuing activity of oxygen-damaged pyruvate formate-lyase by a spare part protein

Pyruvate formate-lyase (PFL) is a glycyl radical enzyme (GRE) that converts pyruvate and coenzyme A into acetyl-CoA and formate in a reaction that is crucial to the primary metabolism of many anaerobic bacteria. The glycyl radical cofactor, which is posttranslationally installed by a radical S-adenosyl-L-methionine (SAM) activase, is a simple and effective catalyst, but is also susceptible to oxidative damage in microaerobic environments. Such damage occurs at the glycyl radical cofactor, resulting in cleaved PFL (cPFL). Bacteria have evolved a spare part protein termed YfiD that can be used to repair cPFL. Previously, we obtained a structure of YfiD by NMR spectroscopy and found that the N-terminus of YfiD was disordered and that the C-terminus of YfiD duplicates the structure of the C-terminus of PFL, including a β-strand that is not removed by the oxygen-induced cleavage. We also showed that cPFL is highly susceptible to proteolysis, suggesting that YfiD rescue of cPFL competes with protein degradation. Here, we probe the mechanism by which YfiD can bind and restore activity to cPFL through enzymatic and spectroscopic studies. Our data show that the disordered N-terminal region of YfiD is important for YfiD glycyl radical installation but not for catalysis, and that the duplicate β-strand does not need to be cleaved from cPFL for YfiD to bind. In fact, truncation of this PFL region prevents YfiD rescue. Collectively our data suggest the molecular mechanisms by which YfiD activation is precluded both when PFL is not damaged and when it is highly damaged.

The PhoPQ Two-Component System Is the Major Regulator of Cell Surface Properties, Stress Responses and Plant-Derived Substrate Utilisation During Development of Pectobacterium versatile-Host Plant Pathosystems

Pectobacterium versatile (formerly P. carotovorum) is a recently defined species of soft rot enterobacteria capable of infecting many plant hosts and damaging different tissues. Complex transcriptional regulation of virulence properties can be expected for such a versatile pathogen. However, the relevant information is available only for related species and is rather limited. The PhoPQ two-component system, originally described in pectobacteria as PehRS, was previously shown to regulate a single gene, pehA. Using an insertional phoP mutant of Pectobacterium versatile (earlier-P. carotovorum), we demonstrate that PhoP regulates at least 115 genes with a majority of them specific for pectobacteria. The functions performed by PhoP-controlled genes include degradation, transport and metabolism of plant-derived carbon sources (polygalacturonate, arabinose-containing polysaccharides and citrate), modification of bacterial cell envelope and stress resistance. We also demonstrated PhoP involvement in establishing the order of plant cell wall decomposition and utilisation of the corresponding breakdown products. Based on experimental data and in silico analysis, we defined a PhoP binding site motif and provided proof for its universality in enteric bacteria. Scanning P. versatile genome for the locations of this motif suggested a much larger PhoP regulon enriched with the genes important for a plant pathogen, which makes PhoP a global virulence regulator. Potential PhoP targets include many regulatory genes and PhoP control over one of them, expI, was confirmed experimentally, highlighting the link between the PhoPQ two-component and quorum sensing systems. High concentrations of calcium and magnesium ions were found to abolish the PhoPQ-dependent transcription activation but did not relieve repression. Reduced PhoP expression and minimisation of PhoP dependence of regulon members' expression in P. versatile cells isolated from potato tuber tissues suggest that PhoPQ system is a key switch of expression levels of multiple virulence-related genes fine-tuned to control the development of P. versatile-host plant pathosystem.

Prokaryotic Organelles: Bacterial Microcompartments in E. coli and Salmonella

Bacterial microcompartments (MCPs) are proteinaceous organelles consisting of a metabolic pathway encapsulated within a selectively permeable protein shell. Hundreds of species of bacteria produce MCPs of at least nine different types, and MCP metabolism is associated with enteric pathogenesis, cancer, and heart disease. This review focuses chiefly on the four types of catabolic MCPs (metabolosomes) found in Escherichia coli and Salmonella: the propanediol utilization (pdu), ethanolamine utilization (eut), choline utilization (cut), and glycyl radical propanediol (grp) MCPs. Although the great majority of work done on catabolic MCPs has been carried out with Salmonella and E. coli, research outside the group is mentioned where necessary for a comprehensive understanding. Salient characteristics found across MCPs are discussed, including enzymatic reactions and shell composition, with particular attention paid to key differences between classes of MCPs. We also highlight relevant research on the dynamic processes of MCP assembly, protein targeting, and the mechanisms that underlie selective permeability. Lastly, we discuss emerging biotechnology applications based on MCP principles and point out challenges, unanswered questions, and future directions.