Structural and functional delineation of aerobactin biosynthesis in hypervirulent Klebsiella pneumoniae.

Aerobactin, a citryl-hydroxamate siderophore, is produced by a number of pathogenic Gram-negative bacteria to aid in iron assimilation. Interest in this well-known siderophore was reignited by recent investigations suggesting that it plays a key role in mediating the enhanced virulence of a hypervirulent pathotype of Klebsiella pneumoniae (hvKP). In contrast to classical opportunistic strains of K. pneumoniae, hvKP causes serious life-threatening infections in previously healthy individuals in the community. Multiple contemporary reports have confirmed fears that the convergence of multidrug-resistant and hvKP pathotypes has led to the evolution of a highly transmissible, drug-resistant, and virulent "super bug." Despite hvKP harboring four distinct siderophore operons, knocking out production of only aerobactin led to a significant attenuation of virulence. Herein, we continue our structural and functional studies on the biosynthesis of this crucial virulence factor. In vivo heterologous production and in vitro reconstitution of aerobactin biosynthesis from hvKP was carried out, demonstrating the specificity, stereoselectivity, and kinetic throughput of the complete pathway. Additionally, we present a steady-state kinetic analysis and the X-ray crystal structure of the second aerobactin synthetase IucC, as well as describe a surface entropy reduction strategy that was employed for structure determination. Finally, we show solution X-ray scattering data that support a unique dimeric quaternary structure for IucC. These new insights into aerobactin assembly will help inform potential antivirulence strategies and advance our understanding of siderophore biosynthesis.

Substrate Trapping in the Siderophore Tailoring Enzyme PvdQ.

Siderophore biosynthesis by Pseudomonas aeruginosa enhances virulence and represents an attractive drug target. PvdQ functions in the type-1 pyoverdine biosynthetic pathway by removing a myristoyl anchor from a pyoverdine precursor, allowing eventual release from the periplasm. A circularly permuted version of PvdQ bypasses the self-processing step of this Ntn-hydrolase and retains the activity, selectivity, and structure of wild-type PvdQ, as revealed by a 1.8 Å resolution X-ray crystal structure. A 2.55 Å resolution structure of the inactive S1A/N269D-cpPvdQ mutant in complex with the pyoverdine precursor PVDIq reveals a specific binding pocket for the d-Tyr of this modified peptide substrate. To our knowledge, this structure is the first of a pyoverdine precursor peptide bound to a biosynthetic enzyme. Details of the observed binding interactions have implications for control of pyoverdine biosynthesis and inform future drug design efforts.

Structures of a Nonribosomal Peptide Synthetase Module Bound to MbtH-like Proteins Support a Highly Dynamic Domain Architecture.

Nonribosomal peptide synthetases (NRPSs) produce a wide variety of peptide natural products. During synthesis, the multidomain NRPSs act as an assembly line, passing the growing product from one module to the next. Each module generally consists of an integrated peptidyl carrier protein, an amino acid-loading adenylation domain, and a condensation domain that catalyzes peptide bond formation. Some adenylation domains interact with small partner proteins called MbtH-like proteins (MLPs) that enhance solubility or activity. A structure of an MLP bound to an adenylation domain has been previously reported using a truncated adenylation domain, precluding any insight that might be derived from understanding the influence of the MLP on the intact adenylation domain or on the dynamics of the entire NRPS module. Here, we present the structures of the full-length NRPS EntF bound to the MLPs from Escherichia coli and Pseudomonas aeruginosa These new structures, along with biochemical and bioinformatics support, further elaborate the residues that define the MLP-adenylation domain interface. Additionally, the structures highlight the dynamic behavior of NRPS modules, including the module core formed by the adenylation and condensation domains as well as the orientation of the mobile thioesterase domain.

Structural and Functional Characterization of Aerobactin Synthetase IucA from a Hypervirulent Pathotype of Klebsiella pneumoniae.

Iron is a vital mineral nutrient required by virtually all life forms to prosper; pathogenic bacteria are no exception. Despite the abundance of iron within the human host, highly regulated iron physiology can result in exceedingly low levels of iron bioavailable to prospective invading bacteria. To combat this scarcity of iron, many pathogenic bacteria have acquired specific and efficient iron acquisition systems, which allow them to thrive in iron-deficient host environments. One of the more prominent bacterial iron acquisition systems involves the synthesis, secretion, and reuptake of small-molecule iron chelators known as siderophores. Aerobactin, a citrate-hydroxamate siderophore originally isolated nearly 50 years ago, is produced by a number of pathogenic Gram-negative bacteria. Aerobactin has recently been demonstrated to play a pivotal role in mediating the enhanced virulence of a particularly invasive pathotype of Klebsiella pneumoniae (hvKP). Toward further understanding of this key virulence factor, we report the structural and functional characterization of aerobactin synthetase IucA from a strain of hvKP. The X-ray crystal structures of unliganded and ATP-bound forms of IucA were solved, forming the foundation of our structural analysis. Small angle X-ray scattering (SAXS) data suggest that, unlike its closest structurally characterized homologues, IucA adopts a tetrameric assembly in solution. Finally, we employed activity assays to investigate the substrate specificity and determine the apparent steady-state kinetic parameters of IucA.

Structure-Function Studies of Hydrophobic Residues That Clamp a Basic Glutamate Side Chain during Catalysis by Triosephosphate Isomerase.

Kinetic parameters are reported for the reactions of whole substrates (kcat/Km, M(-1) s(-1)) (R)-glyceraldehyde 3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP) and for the substrate pieces [(kcat/Km)E·HPi/Kd, M(-2) s(-1)] glycolaldehyde (GA) and phosphite dianion (HPi) catalyzed by the I172A/L232A mutant of triosephosphate isomerase from Trypanosoma brucei brucei (TbbTIM). A comparison with the corresponding parameters for wild-type, I172A, and L232A TbbTIM-catalyzed reactions shows that the effect of I172A and L232A mutations on ΔG(⧧) for the wild-type TbbTIM-catalyzed reactions of the substrate pieces is nearly the same as the effect of the same mutations on TbbTIM previously mutated at the second side chain. This provides strong evidence that mutation of the first hydrophobic side chain does not affect the functioning of the second side chain in catalysis of the reactions of the substrate pieces. By contrast, the effects of I172A and L232A mutations on ΔG(⧧) for wild-type TbbTIM-catalyzed reactions of the whole substrate are different from the effect of the same mutations on TbbTIM previously mutated at the second side chain. This is due to the change in the rate-determining step that determines the barrier to the isomerization reaction. X-ray crystal structures are reported for I172A, L232A, and I172A/L232A TIMs and for the complexes of these mutants to the intermediate analogue phosphoglycolate (PGA). The structures of the PGA complexes with wild-type and mutant enzymes are nearly superimposable, except that the space opened by replacement of the hydrophobic side chain is occupied by a water molecule that lies ∼3.5 Å from the basic side chain of Glu167. The new water at I172A mutant TbbTIM provides a simple rationalization for the increase in the activation barrier ΔG(⧧) observed for mutant enzyme-catalyzed reactions of the whole substrate and substrate pieces. By contrast, the new water at the L232A mutant does not predict the decrease in ΔG(⧧) observed for the mutant enzyme-catalyzed reactions of the substrate piece GA.

1.2†Šresolution crystal structure of the periplasmic aminotransferase PvdN from Pseudomonas aeruginosa.

The Gram-negative pathogen Pseudomonas aeruginosa uses a nonribosomal peptide synthetase (NRPS) biosynthetic cluster for the production of a peptide siderophore. In addition to four multimodular NRPS proteins, the biosynthetic pathway also requires several additional enzymes involved in the production of nonproteinogenic amino acids and maturation of the peptide product. Among the proteins that are required for the final steps in pyoverdine synthesis is PvdN, a pyridoxal phosphate-dependent enzyme that catalyzes an uncharacterized step in pyoverdine production. This study reports the high-resolution structure of PvdN bound to a PLP cofactor solved by multi-wavelength anomalous dispersion (MAD). The PvdN model shows high structural homology to type I aspartate aminotransferases and also contains positive density that suggests an uncharacterized external aldimine.

Structures of two distinct conformations of holo-non-ribosomal peptide synthetases.

Many important natural products are produced by multidomain non-ribosomal peptide synthetases (NRPSs). During synthesis, intermediates are covalently bound to integrated carrier domains and transported to neighbouring catalytic domains in an assembly line fashion. Understanding the structural basis for catalysis with non-ribosomal peptide synthetases will facilitate bioengineering to create novel products. Here we describe the structures of two different holo-non-ribosomal peptide synthetase modules, each revealing a distinct step in the catalytic cycle. One structure depicts the carrier domain cofactor bound to the peptide bond-forming condensation domain, whereas a second structure captures the installation of the amino acid onto the cofactor within the adenylation domain. These structures demonstrate that a conformational change within the adenylation domain guides transfer of intermediates between domains. Furthermore, one structure shows that the condensation and adenylation domains simultaneously adopt their catalytic conformations, increasing the overall efficiency in a revised structural cycle. These structures and the single-particle electron microscopy analysis demonstrate a highly dynamic domain architecture and provide the foundation for understanding the structural mechanisms that could enable engineering of novel non-ribosomal peptide synthetases.

Analysis of the linker region joining the adenylation and carrier protein domains of the modular nonribosomal peptide synthetases.

Nonribosomal peptide synthetases (NRPSs) are multimodular proteins capable of producing important peptide natural products. Using an assembly line process, the amino acid substrate and peptide intermediates are passed between the active sites of different catalytic domains of the NRPS while bound covalently to a peptidyl carrier protein (PCP) domain. Examination of the linker sequences that join the NRPS adenylation and PCP domains identified several conserved proline residues that are not found in standalone adenylation domains. We examined the roles of these proline residues and neighboring conserved sequences through mutagenesis and biochemical analysis of the reaction catalyzed by the adenylation domain and the fully reconstituted NRPS pathway. In particular, we identified a conserved LPxP motif at the start of the adenylation-PCP linker. The LPxP motif interacts with a region on the adenylation domain to stabilize a critical catalytic lysine residue belonging to the A10 motif that immediately precedes the linker. Further, this interaction with the C-terminal subdomain of the adenylation domain may coordinate movement of the PCP with the conformational change of the adenylation domain. Through this work, we extend the conserved A10 motif of the adenylation domain and identify residues that enable proper adenylation domain function.

Identification of inhibitors of PvdQ, an enzyme involved in the synthesis of the siderophore pyoverdine.

Pseudomonas aeruginosa produces the peptide siderophore pyoverdine, which is used to acquire essential Fe(3+) ions from the environment. PvdQ, an Ntn hydrolase, is required for the biosynthesis of pyoverdine. PvdQ knockout strains are not infectious in model systems, suggesting that disruption of siderophore production via PvdQ inhibition could be exploited as a target for novel antibacterial agents, by preventing cells from acquiring iron in the low iron environments of most biological settings. We have previously described a high-throughput screen to identify inhibitors of PvdQ that identified inhibitors with IC50 values of ∼100 µM. Here, we describe the discovery of ML318, a biaryl nitrile inhibitor of PvdQ acylase. ML318 inhibits PvdQ in vitro (IC50 = 20 nM) by binding in the acyl-binding site, as confirmed by the X-ray crystal structure of PvdQ bound to ML318. Additionally, the PvdQ inhibitor is active in a whole cell assay, preventing pyoverdine production and limiting the growth of P. aeruginosa under iron-limiting conditions.

Aerobactin mediates virulence and accounts for increased siderophore production under iron-limiting conditions by hypervirulent (hypermucoviscous) Klebsiella pneumoniae.

Hypervirulent (hypermucoviscous) Klebsiella pneumoniae (hvKP) strains are an emerging variant of "classical" K. pneumoniae (cKP) that cause organ and life-threatening infection in healthy individuals. An understanding of hvKP-specific virulence mechanisms that enabled evolution from cKP is limited. Observations by our group and previously published molecular epidemiologic data led us to hypothesize that hvKP strains produced more siderophores than cKP strains and that this trait enhanced hvKP virulence. Quantitative analysis of 12 hvKP strains in iron-poor minimal medium or human ascites fluid showed a significant and distinguishing 6- to 10-fold increase in siderophore production compared to that for 14 cKP strains. Surprisingly, high-pressure liquid chromatography (HPLC)-mass spectrometry and characterization of the hvKP strains hvKP1, A1142, and A1365 and their isogenic aerobactin-deficient (ΔiucA) derivatives established that aerobactin accounted for the overwhelming majority of increased siderophore production and that this was not due to gene copy number. Further, aerobactin was the primary factor in conditioned medium that enhanced the growth/survival of hvKP1 in human ascites fluid. Importantly the ex vivo growth/survival of hvKP1 ΔiucA was significantly less than that of hvKP1 in human ascites fluid, and the survival of outbred CD1 mice challenged subcutaneously or intraperitoneally with hvKP1 was significantly less than that of mice challenged with hvKP1 ΔiucA. The lowest subcutaneous and intraperitoneal challenge inocula of 3 × 10(2) and 3.2 × 10(1) CFU, respectively, resulted in 100% mortality, demonstrating the virulence of hvKP1 and its ability to cause infection at a low dose. These data strongly support that aerobactin accounts for increased siderophore production in hvKP compared to cKP (a potential defining trait) and is an important virulence factor.

Structure-based engineering of streptavidin monomer with a reduced biotin dissociation rate.

We recently reported the engineering of monomeric streptavidin, mSA, corresponding to one subunit of wild type (wt) streptavidin tetramer. The monomer was designed by homology modeling, in which the streptavidin and rhizavidin sequences were combined to engineer a high affinity binding pocket containing residues from a single subunit only. Although mSA is stable and binds biotin with nanomolar affinity, its fast off rate (koff ) creates practical challenges during applications. We obtained a 1.9 Å crystal structure of mSA bound to biotin to understand their interaction in detail, and used the structure to introduce targeted mutations to improve its binding kinetics. To this end, we compared mSA to shwanavidin, which contains a hydrophobic lid containing F43 in the binding pocket and binds biotin tightly. However, the T48F mutation in mSA, which introduces a comparable hydrophobic lid, only resulted in a modest 20-40% improvement in the measured koff . On the other hand, introducing the S25H mutation near the bicyclic ring of bound biotin increased the dissociation half life (t½ ) from 11 to 83 min at 20°C. Molecular dynamics (MD) simulations suggest that H25 stabilizes the binding loop L3,4 by interacting with A47, and protects key intermolecular hydrogen bonds by limiting solvent entry into the binding pocket. Concurrent T48F or T48W mutation clashes with H25 and partially abrogates the beneficial effects of H25. Taken together, this study suggests that stabilization of the binding loop and solvation of the binding pocket are important determinants of the dissociation kinetics in mSA.

Non-nucleoside inhibitors of BasE, an adenylating enzyme in the siderophore biosynthetic pathway of the opportunistic pathogen Acinetobacter baumannii.

Siderophores are small-molecule iron chelators produced by bacteria and other microorganisms for survival under iron limiting conditions such as found in a mammalian host. Siderophore biosynthesis is essential for the virulence of many important Gram-negative pathogens including Acinetobacter baumannii, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Escherichia coli. We performed high-throughput screening against BasE, which is involved in siderophore biosynthesis in A. baumannii, and identified 6-phenyl-1-(pyridin-4-ylmethyl)-1H-pyrazolo[3,4-b]pyridine-4-carboxylic acid 15. Herein we report the synthesis, biochemical, and microbiological evaluation of a systematic series of analogues of the HTS hit 15. Analogue 67 is the most potent analogue with a KD of 2 nM against BasE. Structural characterization of the inhibitors with BasE reveals that they bind in a unique orientation in the active site, occupying all three substrate binding sites, and thus can be considered as multisubstrate inhibitors. These results provide a foundation for future studies aimed at increasing both enzyme potency and antibacterial activity.

Rational design of proteolytically stable, cell-permeable peptide-based selective Mcl-1 inhibitors.

Direct chemical modifications provide a simple and effective means to "translate" bioactive helical peptides into potential therapeutics targeting intracellular protein-protein interactions. We previously showed that distance-matching bisaryl cross-linkers can reinforce peptide helices containing two cysteines at the i and i+7 positions and confer cell permeability to the cross-linked peptides. Here we report the first crystal structure of a biphenyl-cross-linked Noxa peptide in complex with its target Mcl-1 at 2.0 Å resolution. Guided by this structure, we remodeled the surface of this cross-linked peptide through side-chain substitution and N-methylation and obtained a pair of cross-linked peptides with substantially increased helicity, cell permeability, proteolytic stability, and cell-killing activity in Mcl-1-overexpressing U937 cells.

Structural characterization and high-throughput screening of inhibitors of PvdQ, an NTN hydrolase involved in pyoverdine synthesis.

The human pathogen Pseudomonas aeruginosa produces a variety of virulence factors including pyoverdine, a nonribosomally produced peptide siderophore. The maturation pathway of the pyoverdine peptide is complex and provides a unique target for inhibition. Within the pyoverdine biosynthetic cluster is a periplasmic hydrolase, PvdQ, that is required for pyoverdine production. However, the precise role of PvdQ in the maturation pathway has not been biochemically characterized. We demonstrate herein that the initial module of the nonribosomal peptide synthetase PvdL adds a myristate moiety to the pyoverdine precursor. We extracted this acylated precursor, called PVDIq, from a pvdQ mutant strain and show that the PvdQ enzyme removes the fatty acid catalyzing one of the final steps in pyoverdine maturation. Incubation of PVDIq with crystals of PvdQ allowed us to capture the acylated enzyme and confirm through structural studies the chemical composition of the incorporated acyl chain. Finally, because inhibition of siderophore synthesis has been identified as a potential antibiotic strategy, we developed a high-throughput screening assay and tested a small chemical library for compounds that inhibit PvdQ activity. Two compounds that block PvdQ have been identified, and their binding within the fatty acid binding pocket was structurally characterized.

Biochemical and structural characterization of bisubstrate inhibitors of BasE, the self-standing nonribosomal peptide synthetase adenylate-forming enzyme of acinetobactin synthesis.

The human pathogen Acinetobacter baumannii produces a siderophore called acinetobactin that is derived from one molecule each of threonine, histidine, and 2,3-dihydroxybenzoic acid (DHB). The activity of several nonribosomal peptide synthetase (NRPS) enzymes is used to combine the building blocks into the final molecule. The acinetobactin synthesis pathway initiates with a self-standing adenylation enzyme, BasE, that activates the DHB molecule and covalently transfers it to the pantetheine cofactor of an aryl-carrier protein of BasF, a strategy that is shared with many siderophore-producing NRPS clusters. In this reaction, DHB reacts with ATP to form the aryl adenylate and pyrophosphate. In a second partial reaction, the DHB is transferred to the carrier protein. Inhibitors of BasE and related enzymes have been identified that prevent growth of bacteria on iron-limiting media. Recently, a new inhibitor of BasE has been identified via high-throughput screening using a fluorescence polarization displacement assay. We present here biochemical and structural studies to examine the binding mode of this inhibitor. The kinetics of the wild-type BasE enzyme is shown, and inhibition studies demonstrate that the new compound exhibits competitive inhibition against both ATP and 2,3-dihydroxybenzoate. Structural examination of BasE bound to this inhibitor illustrates a novel binding mode in which the phenyl moiety partially fills the enzyme pantetheine binding tunnel. Structures of rationally designed bisubstrate inhibitors are also presented.

Three-dimensional structures of Pseudomonas aeruginosa PvcA and PvcB, two proteins involved in the synthesis of 2-isocyano-6,7-dihydroxycoumarin.

The pvcABCD operon of Pseudomonas aeruginosa encodes four proteins (PA2254, PA2255, PA2256, and PA2257) that form a cluster that is responsible for the synthesis of a cyclized isocyano derivative of tyrosine. These proteins, which were identified originally as being responsible for a step in the maturation of the chromophore of the peptide siderophore pyoverdine, have been identified recently as belonging to a family of proteins that produce small organic isonitriles. We report that strains harboring a disruption in the pvcA or pvcB genes are able to grow in iron-depleted conditions and to produce pyoverdine. Additionally, we have determined the three-dimensional crystal structures of PvcA and PvcB. The structure of PvcA demonstrates a novel enzyme architecture that is built upon a Rossmann fold. We have analyzed the sequence conservation of enzymes within this family and identified six conserved motifs. These regions of the protein cluster around a putative active site cavity. The structure of the PvcB protein confirms it is a member of the Fe2+/alpha-ketoglutarate-dependent oxygenase family of enzymes. The active site of PvcB is compared to the structures of other family members and suggests that a conformational change to order several loops will accompany the binding of ligands.

The 1.8 A crystal structure of PA2412, an MbtH-like protein from the pyoverdine cluster of Pseudomonas aeruginosa.

Many bacteria use nonribosomal peptide synthetase (NRPS) proteins to produce peptide antibiotics and siderophores. The catalytic domains of the NRPS proteins are usually linked in large multidomain proteins. Often, additional proteins are coexpressed with NRPS proteins that modify the NRPS peptide products, ensure the availability of substrate building blocks, or play a role in the import or export of the NRPS product. Many NRPS clusters include a small protein of approximately 80 amino acids with homology to the MbtH protein of mycobactin synthesis in Mycobacteria tuberculosis; no function has been assigned to these proteins. Pseudomonas aeruginosa utilizes an NRPS cluster to synthesize the siderophore pyoverdine. The pyoverdine peptide contains a dihydroxyquinoline-based chromophore, as well as two formyl-N-hydroxyornithine residues, which are involved in iron binding. The pyoverdine cluster contains four modular NRPS enzymes and 10-15 additional proteins that are essential for pyoverdine production. Coexpressed with the pyoverdine synthetic enzymes is a 72-amino acid MbtH-like family member designated PA2412. We have determined the three-dimensional structure of the PA2412 protein and describe here the structure and the location of conserved regions. Additionally, we have further analyzed a deletion mutant of the PA2412 protein for growth and pyoverdine production. Our results demonstrate that PA2412 is necessary for the production or secretion of pyoverdine at normal levels. The PA2412 deletion strain is able to use exogenously produced pyoverdine, showing that there is no defect in the uptake or utilization of the iron-pyoverdine complex.

Determination of the crystal structure of EntA, a 2,3-dihydro-2,3-dihydroxybenzoic acid dehydrogenase from Escherichia coli.

The Escherichia coli enterobactin synthetic cluster is composed of six proteins, EntA-EntF, that form the enterobactin molecule from three serine molecules and three molecules of 2,3-dihydroxybenzoic acid (DHB). EntC, EntB and EntA catalyze the three-step synthesis of DHB from chorismate. EntA is a member of the short-chain oxidoreductase (SCOR) family of proteins and catalyzes the final step in DHB synthesis, the NAD+-dependent oxidation of 2,3-dihydro-2,3-dihydroxybenzoic acid to DHB. The structure of EntA has been determined by multi-wavelength anomalous dispersion methods. Here, the 2.0 A crystal structure of EntA in the unliganded form is presented. Analysis of the structure in light of recent structural and bioinformatic analysis of other members of the SCOR family provides insight into the residues involved in cofactor and substrate binding.

Structure of the EntB multidomain nonribosomal peptide synthetase and functional analysis of its interaction with the EntE adenylation domain.

Nonribosomal peptide synthetases are modular proteins that operate in an assembly line fashion to bind, modify, and link amino acids. In the E. coli enterobactin NRPS system, the EntE adenylation domain catalyzes the transfer of a molecule of 2,3-dihydroxybenzoic acid to the pantetheine cofactor of EntB. We present here the crystal structure of the EntB protein that contains an N-terminal isochorismate lyase domain that functions in the synthesis of 2,3-dihydroxybenzoate and a C-terminal carrier protein domain. Functional analysis showed that the EntB-EntE interaction was surprisingly tolerant of a number of point mutations on the surface of EntB and EntE. Mutational studies on EntE support our previous hypothesis that members of the adenylate-forming family of enzymes adopt two distinct conformations to catalyze the two-step reactions.