Periplasmic depolymerase provides insight into ABC transporter-dependent secretion of bacterial capsular polysaccharides.

Capsules are surface layers of hydrated capsular polysaccharides (CPSs) produced by many bacteria. The human pathogen Salmonella enterica serovar Typhi produces "Vi antigen" CPS, which contributes to virulence. In a conserved strategy used by bacteria with diverse CPS structures, translocation of Vi antigen to the cell surface is driven by an ATP-binding cassette (ABC) transporter. These transporters are engaged in heterooligomeric complexes proposed to form an enclosed translocation conduit to the cell surface, allowing the transporter to power the entire process. We identified Vi antigen biosynthesis genetic loci in genera of the Burkholderiales, which are paradoxically distinguished from S. Typhi by encoding VexL, a predicted pectate lyase homolog. Biochemical analyses demonstrated that VexL is an unusual metal-independent endolyase with an acidic pH optimum that is specific for O-acetylated Vi antigen. A 1.22-Å crystal structure of the VexL-Vi antigen complex revealed features which distinguish common secreted catabolic pectate lyases from periplasmic VexL, which participates in cell-surface assembly. VexL possesses a right-handed parallel ß-superhelix, of which one face forms an electropositive glycan-binding groove with an extensive hydrogen bonding network that includes Vi antigen acetyl groups and confers substrate specificity. VexL provided a probe to interrogate conserved features of the ABC transporter-dependent export model. When introduced into S Typhi, VexL localized to the periplasm and degraded Vi antigen. In contrast, a cytosolic derivative had no effect unless export was disrupted. These data provide evidence that CPS assembled in ABC transporter-dependent systems is actually exposed to the periplasm during envelope translocation.

Molecular Characterization of N-glycan Degradation and Transport in Streptococcus pneumoniae and Its Contribution to Virulence.

The carbohydrate-rich coating of human tissues and cells provide a first point of contact for colonizing and invading bacteria. Commensurate with N-glycosylation being an abundant form of protein glycosylation that has critical functional roles in the host, some host-adapted bacteria possess the machinery to process N-linked glycans. The human pathogen Streptococcus pneumoniae depolymerizes complex N-glycans with enzymes that sequentially trim a complex N-glycan down to the Man3GlcNAc2 core prior to the release of the glycan from the protein by endo-ß-N-acetylglucosaminidase (EndoD), which cleaves between the two GlcNAc residues. Here we examine the capacity of S. pneumoniae to process high-mannose N-glycans and transport the products. Through biochemical and structural analyses we demonstrate that S. pneumoniae also possesses an α-(1,2)-mannosidase (SpGH92). This enzyme has the ability to trim the terminal α-(1,2)-linked mannose residues of high-mannose N-glycans to generate Man5GlcNAc2. Through this activity SpGH92 is able to produce a substrate for EndoD, which is not active on high-mannose glycans with α-(1,2)-linked mannose residues. Binding studies and X-ray crystallography show that NgtS, the solute binding protein of an ABC transporter (ABCNG), is able to bind Man5GlcNAc, a product of EndoD activity, with high affinity. Finally, we evaluated the contribution of EndoD and ABCNG to growth of S. pneumoniae on a model N-glycosylated glycoprotein, and the contribution of these enzymes and SpGH92 to virulence in a mouse model. We found that both EndoD and ABCNG contribute to growth of S. pneumoniae, but that only SpGH92 and EndoD contribute to virulence. Therefore, N-glycan processing, but not transport of the released glycan, is required for full virulence in S. pneumoniae. To conclude, we synthesize our findings into a model of N-glycan processing by S. pneumoniae in which both complex and high-mannose N-glycans are targeted, and in which the two arms of this degradation pathway converge at ABCNG.

Structural Analysis of a Family 101 Glycoside Hydrolase in Complex with Carbohydrates Reveals Insights into Its Mechanism.

O-Linked glycosylation is one of the most abundant post-translational modifications of proteins. Within the secretory pathway of higher eukaryotes, the core of these glycans is frequently an N-acetylgalactosamine residue that is α-linked to serine or threonine residues. Glycoside hydrolases in family 101 are presently the only known enzymes to be able to hydrolyze this glycosidic linkage. Here we determine the high-resolution structures of the catalytic domain comprising a fragment of GH101 from Streptococcus pneumoniae TIGR4, SpGH101, in the absence of carbohydrate, and in complex with reaction products, inhibitor, and substrate analogues. Upon substrate binding, a tryptophan lid (residues 724-WNW-726) closes on the substrate. The closing of this lid fully engages the substrate in the active site with Asp-764 positioned directly beneath C1 of the sugar residue bound within the -1 subsite, consistent with its proposed role as the catalytic nucleophile. In all of the bound forms of the enzyme, however, the proposed catalytic acid/base residue was found to be too distant from the glycosidic oxygen (>4.3 Å) to serve directly as a general catalytic acid/base residue and thereby facilitate cleavage of the glycosidic bond. These same complexes, however, revealed a structurally conserved water molecule positioned between the catalytic acid/base and the glycosidic oxygen. On the basis of these structural observations we propose a new variation of the retaining glycoside hydrolase mechanism wherein the intervening water molecule enables a Grotthuss proton shuttle between Glu-796 and the glycosidic oxygen, permitting this residue to serve as the general acid/base catalytic residue.

Conformational analysis of the Streptococcus pneumoniae hyaluronate lyase and characterization of its hyaluronan-specific carbohydrate-binding module.

For a subset of pathogenic microorganisms, including Streptococcus pneumoniae, the recognition and degradation of host hyaluronan contributes to bacterial spreading through the extracellular matrix and enhancing access to host cell surfaces. The hyaluronate lyase (Hyl) presented on the surface of S. pneumoniae performs this role. Using glycan microarray screening, affinity electrophoresis, and isothermal titration calorimetry we show that the N-terminal module of Hyl is a hyaluronan-specific carbohydrate-binding module (CBM) and the founding member of CBM family 70. The 1.2 Å resolution x-ray crystal structure of CBM70 revealed it to have a ß-sandwich fold, similar to other CBMs. The electrostatic properties of the binding site, which was identified by site-directed mutagenesis, are distinct from other CBMs and complementary to its acidic ligand, hyaluronan. Dynamic light scattering and solution small angle x-ray scattering revealed the full-length Hyl protein to exist as a monomer/dimer mixture in solution. Through a detailed analysis of the small angle x-ray scattering data, we report the pseudoatomic solution structures of the monomer and dimer forms of the full-length multimodular Hyl.

Substrate and metal ion promiscuity in mannosylglycerate synthase.

The enzymatic transfer of the sugar mannose from activated sugar donors is central to the synthesis of a wide range of biologically significant polysaccharides and glycoconjugates. In addition to their importance in cellular biology, mannosyltransferases also provide model systems with which to study catalytic mechanisms of glycosyl transfer. Mannosylglycerate synthase (MGS) catalyzes the synthesis of α-mannosyl-D-glycerate using GDP-mannose as the preferred donor species, a reaction that occurs with a net retention of anomeric configuration. Past work has shown that the Rhodothermus marinus MGS, classified as a GT78 glycosyltransferase, displays a GT-A fold and performs catalysis in a metal ion-dependent manner. MGS shows very unusual metal ion dependences with Mg(2+) and Ca(2+) and, to a lesser extent, Mn(2+), Ni(2+), and Co(2+), thus facilitating catalysis. Here, we probe these dependences through kinetic and calorimetric analyses of wild-type and site-directed variants of the enzyme. Mutation of residues that interact with the guanine base of GDP are correlated with a higher k(cat) value, whereas substitution of His-217, a key component of the metal coordination site, results in a change in metal specificity to Mn(2+). Structural analyses of MGS complexes not only provide insight into metal coordination but also how lactate can function as an alternative acceptor to glycerate. These studies highlight the role of flexible loops in the active center and the subsequent coordination of the divalent metal ion as key factors in MGS catalysis and metal ion dependence. Furthermore, Tyr-220, located on a flexible loop whose conformation is likely influenced by metal binding, also plays a critical role in substrate binding.

Mechanistic insights into a Ca2+-dependent family of alpha-mannosidases in a human gut symbiont.

Colonic bacteria, exemplified by Bacteroides thetaiotaomicron, play a key role in maintaining human health by harnessing large families of glycoside hydrolases (GHs) to exploit dietary polysaccharides and host glycans as nutrients. Such GH family expansion is exemplified by the 23 family GH92 glycosidases encoded by the B. thetaiotaomicron genome. Here we show that these are alpha-mannosidases that act via a single displacement mechanism to utilize host N-glycans. The three-dimensional structure of two GH92 mannosidases defines a family of two-domain proteins in which the catalytic center is located at the domain interface, providing acid (glutamate) and base (aspartate) assistance to hydrolysis in a Ca(2+)-dependent manner. The three-dimensional structures of the GH92s in complex with inhibitors provide insight into the specificity, mechanism and conformational itinerary of catalysis. Ca(2+) plays a key catalytic role in helping distort the mannoside away from its ground-state (4)C(1) chair conformation toward the transition state.

Degradation of chondroitin sulfate A by a PUL-like operon in Tannerella forsythia.

Advanced periodontitis has been shown to have strong association with the residence of the bacterial consortia known as the red complex comprised by Porphyromonas gingivalis, Tannerella forsythia, and Treponema denticola. T. forsythia shares a distant genetic linkage to Bacteroidetes thetaiotaomicron and may therefore produce analogous polysaccharide utilization loci (PUL) which enable complex carbohydrate degradation, import, and use, although this capacity has yet to be demonstrated. Chondroitin sulfate A is a linear, sulfated carbohydrate linked to periodontal disease as the principal species of glycosaminoglycan appended on the surface of cortical bone of teeth and in supporting dental ligaments. Through genomic comparisons with B. thetaiotaomicron, a new PUL-like operon (Bfo2285-Bfo2295, and Bfo3043) was identified in T. forsythia and the crystal structure of two proteins from this PUL-like operon, Bfo2290 and Bfo2294, were reported using X-ray crystallography. Enzyme kinetics for Bfo2290 were reported using a pH-dependent assay and suggested a Km of 0.75 mg/ml ± 0.60 mg/ml, Kcat of 3.74 min-1 ± 0.88 min-1, and Vmax of 7.48 µM/min ± 1.76 µM/min with partially degraded chondroitin sulfate A. Fluorophore-assisted carbohydrate electrophoresis was used to show the processive degradation of chondroitin sulfate A by the proteins encoded in T. forsythia PUL-like operon, and revealed Bfo2291 and Bfo2290 to be an endolytic chondroitin sulfate A lyase and exolytic ΔDi-4S chondroitin sulfate A sulfatase, respectively.

Analysis of Two SusE-Like Enzymes From Bacteroides thetaiotaomicron Reveals a Potential Degradative Capacity for This Protein Family.

Bacteroides thetaiotaomicron is a major constituent of the human gut microbiome and recognized as a prolific degrader of diverse and complex carbohydrates. This capacity is due to the large number of glycan-depolymerization and acquisition systems that are encoded by gene clusters known as polysaccharide utilization loci (PUL), with the starch utilization system (Sus) serving as the established model. Sharing features with the Sus are Sus-like systems, that require the presence of a specific membrane transporter and surface lipoprotein to be classified as Sus-like. Sus-like import loci are extremely varied with respect to any additional protein components encoded, that would effectively modify the functionality of the degradative and import action of each locus. Herein we have identified eight Sus-like systems in B. thetaiotaomicron that share the feature of a homologous SusE-like factor encoded immediately downstream from the transporter/lipoprotein duo susC/D. Two SusE-like proteins from these systems, BT2857 and BT3158, were characterized by X-ray crystallography and BT2857 was further analyzed by small-angle X-ray scattering. The SusE-like proteins were found to be composed of a conserved three domain architecture: a partially disordered N-terminal domain that is predicted to be proximal to the membrane and structurally homologous to an FN3-like bundle, a middle ß-sandwich domain, and a C-terminal domain homologous to family 32 carbohydrate-binding modules, that bind to galactose. Structural comparisons of SusE with SusE-like proteins suggested only a small structural divergence has occurred. However, functional analyses with BT2857 and BT3158 revealed that the SusE-like proteins exhibited galactosidase activity with para-nitrophenyl-ß-D-galactopyranoside and α-(1,4)-lactose substrates, that has not been demonstrated for SusE proteins. Using a series of domain truncations of BT2857, the predominant ß-D-galactosidase activity is suggested to be localized to the C-terminal DUF5126 domain that would be most distal from the outer membrane. The expanded functionality we have observed with these SusE-like proteins provides a plausible explanation of how Sus-like systems are adapted to target more diverse groups of carbohydrates, when compared to their Sus counterparts.

Bacteroides thetaiotaomicron generates diverse α-mannosidase activities through subtle evolution of a distal substrate-binding motif.

A dominant human gut microbe, the well studied symbiont Bacteroides thetaiotaomicron (Bt), is a glyco-specialist that harbors a large repertoire of genes devoted to carbohydrate processing. Despite strong similarities among them, many of the encoded enzymes have evolved distinct substrate specificities, and through the clustering of cognate genes within operons termed polysaccharide-utilization loci (PULs) enable the fulfilment of complex biological roles. Structural analyses of two glycoside hydrolase family 92 α-mannosidases, BT3130 and BT3965, together with mechanistically relevant complexes at 1.8-2.5 Šresolution reveal conservation of the global enzyme fold and core catalytic apparatus despite different linkage specificities. Structure comparison shows that Bt differentiates the activity of these enzymes through evolution of a highly variable substrate-binding region immediately adjacent to the active site. These observations unveil a genetic/biochemical mechanism through which polysaccharide-processing bacteria can evolve new and specific biochemical activities from otherwise highly similar gene products.

Methods for Determining Glycosyltransferase Kinetics.

Glycosyltransferases are a class of biosynthetic enzymes that transfer individual activated monosaccharide units to specific acceptors. Colorimetric assays using the detection of released products such as para-nitrophenol and coupled assays for inorganic phosphate detection allow for convenient and quantifiable kinetic characterization. These techniques may be applied to determine the enzymatic activity of glycosyltransferases by indirectly measuring the transfer of nucleotide-activated donor carbohydrate units to various cognate acceptor molecules. In addition to an overview of these methods, the protocol for quantifying the glycosyltransferase activity used for the characterization of penicillin-binding proteins (PBPs) involving the transfer of lipid II to form elongated murein chains during bacterial cell wall synthesis is described herein.

An alternative reaction for heme degradation catalyzed by the Escherichia coli O157:H7 ChuS protein: Release of hematinic acid, tripyrrole and Fe(III).

As part of the machinery to acquire, internalize and utilize heme as a source of iron from the host, some bacteria possess a canonical heme oxygenase, where heme plays the dual role of substrate and cofactor, the later catalyzing the cleavage of the heme moiety using O2 and electrons, and resulting in biliverdin, carbon monoxide and ferrous non-heme iron. We have previously reported that the Escherichia coli O157:H7 ChuS protein, which is not homologous to heme oxygenases, can bind and degrade heme in a reaction that releases carbon monoxide. Here, we have pursued a detailed characterization of such heme degradation reaction using stopped-flow UV-visible absorption spectrometry, the characterization of the intermediate species formed in such reaction by EPR spectroscopy and the identification of reaction products by NMR spectroscopy and Mass spectrometry. We show that hydrogen peroxide (in molar equivalent) is the key player in the degradation reaction, at variance to canonical heme oxygenases. While the initial intermediates of the reaction of ChuS with hydrogen peroxide (a ferrous keto π neutral radical and ferric verdoheme, both identified by EPR spectroscopy) are in common with heme oxygenases, a further and unprecedented reaction step, involving the cleavage of the porphyrin ring at adjacent meso-carbons, results in the release of hematinic acid (a monopyrrole moiety identified by NMR spectroscopy), a tripyrrole product (identified by Mass spectrometry) and non-heme iron in the ferric oxidation state (identified by EPR spectroscopy). Overall, the unprecedented reaction of E. coli O157:H7 ChuS provides evidence for a novel heme degradation activity in a Gram-negative bacterium.

Improving biophysical properties of a bispecific antibody scaffold to aid developability: quality by molecular design.

While the concept of Quality-by-Design is addressed at the upstream and downstream process development stages, we questioned whether there are advantages to addressing the issues of biologics quality early in the design of the molecule based on fundamental biophysical characterization, and thereby reduce complexities in the product development stages. Although limited number of bispecific therapeutics are in clinic, these developments have been plagued with difficulty in producing materials of sufficient quality and quantity for both preclinical and clinical studies. The engineered heterodimeric Fc is an industry-wide favorite scaffold for the design of bispecific protein therapeutics because of its structural, and potentially pharmacokinetic, similarity to the natural antibody. Development of molecules based on this concept, however, is challenged by the presence of potential homodimer contamination and stability loss relative to the natural Fc. We engineered a heterodimeric Fc with high heterodimeric specificity that also retains natural Fc-like biophysical properties, and demonstrate here that use of engineered Fc domains that mirror the natural system translates into an efficient and robust upstream stable cell line selection process as a first step toward a more developable therapeutic.

Structure and kinetic investigation of Streptococcus pyogenes family GH38 alpha-mannosidase.

BACKGROUND: The enzymatic hydrolysis of alpha-mannosides is catalyzed by glycoside hydrolases (GH), termed alpha-mannosidases. These enzymes are found in different GH sequence-based families. Considerable research has probed the role of higher eukaryotic "GH38" alpha-mannosides that play a key role in the modification and diversification of hybrid N-glycans; processes with strong cellular links to cancer and autoimmune disease. The most extensively studied of these enzymes is the Drosophila GH38 alpha-mannosidase II, which has been shown to be a retaining alpha-mannosidase that targets both alpha-1,3 and alpha-1,6 mannosyl linkages, an activity that enables the enzyme to process GlcNAc(Man)(5)(GlcNAc)(2) hybrid N-glycans to GlcNAc(Man)(3)(GlcNAc)(2). Far less well understood is the observation that many bacterial species, predominantly but not exclusively pathogens and symbionts, also possess putative GH38 alpha-mannosidases whose activity and specificity is unknown. METHODOLOGY/PRINCIPAL FINDINGS: Here we show that the Streptococcus pyogenes (M1 GAS SF370) GH38 enzyme (Spy1604; hereafter SpGH38) is an alpha-mannosidase with specificity for alpha-1,3 mannosidic linkages. The 3D X-ray structure of SpGH38, obtained in native form at 1.9 A resolution and in complex with the inhibitor swainsonine (K(i) 18 microM) at 2.6 A, reveals a canonical GH38 five-domain structure in which the catalytic "-1" subsite shows high similarity with the Drosophila enzyme, including the catalytic Zn(2+) ion. In contrast, the "leaving group" subsites of SpGH38 display considerable differences to the higher eukaryotic GH38s; features that contribute to their apparent specificity. CONCLUSIONS/SIGNIFICANCE: Although the in vivo function of this streptococcal GH38 alpha-mannosidase remains unknown, it is shown to be an alpha-mannosidase active on N-glycans. SpGH38 lies on an operon that also contains the GH84 hexosaminidase (Spy1600) and an additional putative glycosidase. The activity of SpGH38, together with its genomic context, strongly hints at a function in the degradation of host N- or possibly O-glycans. The absence of any classical signal peptide further suggests that SpGH38 may be intracellular, perhaps functioning in the subsequent degradation of extracellular host glycans following their initial digestion by secreted glycosidases.

Structure and heme binding properties of Escherichia coli O157:H7 ChuX.

For many pathogenic microorganisms, iron acquisition from host heme sources stimulates growth, multiplication, ultimately enabling successful survival and colonization. In gram-negative Escherichia coli O157:H7, Shigella dysenteriae and Yersinia enterocolitica the genes encoded within the heme utilization operon enable the effective uptake and utilization of heme as an iron source. While the complement of proteins responsible for heme internalization has been determined in these organisms, the fate of heme once it has reached the cytoplasm has only recently begun to be resolved. Here we report the first crystal structure of ChuX, a member of the conserved heme utilization operon from pathogenic E. coli O157:H7 determined at 2.05 A resolution. ChuX forms a dimer which remarkably given low sequence homology, displays a very similar fold to the monomer structure of ChuS and HemS, two other heme utilization proteins. Absorption spectral analysis of heme reconstituted ChuX demonstrates that ChuX binds heme in a 1:1 manner implying that each ChuX homodimer has the potential to coordinate two heme molecules in contrast to ChuS and HemS where only one heme molecule is bound. Resonance Raman spectroscopy indicates that the heme of ferric ChuX is composed of a mixture of coordination states: 5-coordinate and high-spin, 6-coordinate and low-spin, and 6-coordinate and high-spin. In contrast, the reduced ferrous form displays mainly a 5-coordinate and high-spin state with a minor contribution from a 6-coordinate and low-spin state. The nu(Fe-CO) and nu(C-O) frequencies of ChuX-bound CO fall on the correlation line expected for histidine-coordinated hemoproteins indicating that the fifth axial ligand of the ferrous heme is the imidazole ring of a histidine residue. Based on sequence and structural comparisons, we designed a number of site-directed mutations in ChuX to probe the heme binding sites and dimer interface. Spectral analysis of ChuX and mutants suggests involvement of H65 and H98 in heme coordination as mutations of both residues were required to abolish the formation of the hexacoordination state of heme-bound ChuX.

Novel structure of the conserved gram-negative lipopolysaccharide transport protein A and mutagenesis analysis.

Lipopolysaccharide (LPS) transport protein A (LptA) is an essential periplasmic localized transport protein that has been implicated together with MsbA, LptB, and the Imp/RlpB complex in LPS transport from the inner membrane to the outer membrane, thereby contributing to building the cell envelope in Gram-negative bacteria and maintaining its integrity. Here we present the first crystal structures of processed Escherichia coli LptA in two crystal forms, one with two molecules in the asymmetric unit and the other with eight. In both crystal forms, severe anisotropic diffraction was corrected, which facilitated model building and structural refinement. The eight-molecule form of LptA is induced when LPS or Ra-LPS (a rough chemotype of LPS) is included during crystallization. The unique LptA structure represents a novel fold, consisting of 16 consecutive antiparallel beta-strands, folded to resemble a slightly twisted beta-jellyroll. Each LptA molecule interacts with an adjacent LptA molecule in a head-to-tail fashion to resemble long fibers. Site-directed mutagenesis of conserved residues located within a cluster that delineate the N-terminal beta-strands of LptA does not impair the function of the protein, although their overexpression appears more detrimental to LPS transport compared with wild-type LptA. Moreover, altered expression of both wild-type and mutated proteins interfered with normal LPS transport as witnessed by the production of an anomalous form of LPS. Structural analysis suggests that head-to-tail stacking of LptA molecules could be destabilized by the mutation, thereby potentially contributing to impair LPS transport.

The Cellvibrio japonicus mannanase CjMan26C displays a unique exo-mode of action that is conferred by subtle changes to the distal region of the active site.

The microbial degradation of the plant cell wall is a pivotal biological process that is of increasing industrial significance. One of the major plant structural polysaccharides is mannan, a beta-1,4-linked d-mannose polymer, which is hydrolyzed by endo- and exo-acting mannanases. The mechanisms by which the exo-acting enzymes target the chain ends of mannan and how galactose decorations influence activity are poorly understood. Here we report the crystal structure and biochemical properties of CjMan26C, a Cellvibrio japonicus GH26 mannanase. The exo-acting enzyme releases the disaccharide mannobiose from the nonreducing end of mannan and mannooligosaccharides, harnessing four mannose-binding subsites extending from -2 to +2. The structure of CjMan26C is very similar to that of the endo-acting C. japonicus mannanase CjMan26A. The exo-activity displayed by CjMan26C, however, reflects a subtle change in surface topography in which a four-residue extension of surface loop creates a steric block at the distal glycone -2 subsite. endo-Activity can be introduced into enzyme variants through truncation of an aspartate side chain, a component of a surface loop, or by removing both the aspartate and its flanking residues. The structure of catalytically competent CjMan26C, in complex with a decorated manno-oligosaccharide, reveals a predominantly unhydrolyzed substrate in an approximate (1)S(5) conformation. The complex structure helps to explain how the substrate "side chain" decorations greatly reduce the activity of the enzyme; the galactose side chain at the -1 subsite makes polar interactions with the aglycone mannose, possibly leading to suboptimal binding and impaired leaving group departure. This report reveals how subtle differences in the loops surrounding the active site of a glycoside hydrolase can lead to a change in the mode of action of the enzyme.

Piecing together the structure-function puzzle: experiences in structure-based functional annotation of hypothetical proteins.

The combination of genomic sequencing with structural genomics has provided a wealth of new structures for previously uncharacterized ORFs, more commonly referred to as hypothetical proteins. This rapid growth has been the direct result of high-throughput, automated approaches in both the identification of new ORFs and the determination of high-resolution 3-D protein structures. A significant bottleneck is reached, however, at the stage of functional annotation in that the assignment of function is not readily automatable. It is often the case that the initial structural analysis at best indicates a functional family for a given hypothetical protein, but further identification of a relevant ligand or substrate is impeded by the diversity of function in a particular structural classification of proteins family, a highly selective and specific ligand-binding site, or the identification of a novel protein fold. Our approach to the functional annotation of hypothetical proteins relies on the combination of structural information with additional bioinformatics evidence garnered from operon prediction, loose functional information of additional operon members, conservation of catalytic residues, as well as cocrystallization trials and virtual ligand screening. The synthesis of all available information for each protein has permitted the functional annotation of several hypothetical proteins from Escherichia coli and each assignment has been confirmed through generally accepted biochemical methods.

Structure of the Escherichia coli O157:H7 heme oxygenase ChuS in complex with heme and enzymatic inactivation by mutation of the heme coordinating residue His-193.

Heme oxygenases catalyze the oxidation of heme to biliverdin, CO, and free iron. For pathogenic microorganisms, heme uptake and degradation are critical mechanisms for iron acquisition that enable multiplication and survival within hosts they invade. Here we report the first crystal structure of the pathogenic Escherichia coli O157:H7 heme oxygenase ChuS in complex with heme at 1.45 A resolution. When compared with other heme oxygenases, ChuS has a unique fold, including structural repeats and a beta-sheet core. Not surprisingly, the mode of heme coordination by ChuS is also distinct, whereby heme is largely stabilized by residues from the C-terminal domain, assisted by a distant arginine from the N-terminal domain. Upon heme binding, there is no large conformational change beyond the fine tuning of a key histidine (His-193) residue. Most intriguingly, in contrast to other heme oxygenases, the propionic side chains of heme are orientated toward the protein core, exposing the alpha-meso carbon position where O(2) is added during heme degradation. This unique orientation may facilitate presentation to an electron donor, explaining the significantly reduced concentration of ascorbic acid needed for the reaction. Based on the ChuS-heme structure, we converted the histidine residue responsible for axial coordination of the heme group to an asparagine residue (H193N), as well as converting a second histidine to an alanine residue (H73A) for comparison purposes. We employed spectral analysis and CO measurement by gas chromatography to analyze catalysis by ChuS, H193N, and H73A, demonstrating that His-193 is the key residue for the heme-degrading activity of ChuS.

Identification of an Escherichia coli O157:H7 heme oxygenase with tandem functional repeats.

Heme oxygenases (HOs) catalyze the oxidation of heme to biliverdin, carbon monoxide (CO), and free iron. Iron acquisition is critical for invading microorganisms to enable survival and growth. Here we report the crystal structure of ChuS, which displays a previously uncharacterized fold and is unique compared with other characterized HOs. Despite only 19% sequence identity between the N- and C-terminal halves, these segments of ChuS represent a structural duplication, with a root-mean-square deviation of 2.1 A between the two repeats. ChuS is capable of using ascorbic acid or cytochrome P450 reductase-NADPH as electron sources for heme oxygenation. CO detection confirmed that ChuS is a HO, and we have identified it in pathogenic Escherichia coli O157:H7. Based on sequence analysis, this HO is present in many bacteria, although not in the E. coli K-12 strain. The N- and C-terminal halves of ChuS are each a functional HO.