The high catalytic rate of the cold-active Vibrio alkaline phosphatase requires a hydrogen bonding network involving a large interface loop.

The role of surface loops in mediating communication through residue networks is still a relatively poorly understood part in the study of cold adaptation of enzymes, especially in terms of their quaternary interactions. Alkaline phosphatase (AP) from the psychrophilic marine bacterium Vibrio splendidus (VAP) is characterized by an analogous large surface loop in each monomer, referred to as the large loop, that hovers over the active site of the other monomer. It presumably has a role in the high catalytic efficiency of VAP which accompanies its extremely low thermal stability. Here, we designed several different variants of VAP with the aim of removing intersubunit interactions at the dimer interface. Breaking the intersubunit contacts from one residue in particular (Arg336) reduced the temperature stability of the catalytically potent conformation and caused a 40% drop in catalytic rate. The high catalytic rates of enzymes from cold-adapted organisms are often associated with increased dynamic flexibility. Comparison of the relative B-factors of the R336L crystal structure to that of the wild-type confirmed surface flexibility was increased in a loop on the opposite monomer, but not in the large loop. The increase in flexibility resulted in a reduced catalytic rate. The large loop increases the area of the interface between the subunits through its contacts and may facilitate an alternating structural cycle demanded by a half-of-sites reaction mechanism through stronger ties, as the dimer oscillates between high affinity (active) or low phosphoryl group affinity (inactive).

X-ray crystal structure of Vibrio alkaline phosphatase with the non-competitive inhibitor cyclohexylamine.

BACKGROUND: Para-nitrophenyl phosphate, the common substrate for alkaline phosphatase (AP), is available as a cyclohexylamine salt. Here, we report that cyclohexylamine is a non-competitive inhibitor of APs. METHODS: Cyclohexylamine inhibited four different APs. Co-crystallization with the cold-active Vibrio AP (VAP) was performed and the structure solved. RESULTS: Inhibition of VAP fitted a non-competitive kinetic model (Km unchanged, Vmax reduced) with IC50 45.3 mM at the pH optimum 9.8, not sensitive to 0.5 M NaCl, and IC50 27.9 mM at pH 8.0, where the addition of 0.5 M NaCl altered the inhibition to the level observed at pH 9.8. APs from E. coli and calf intestines were less sensitive to cyclohexylamine, whereas an Antarctic bacterial AP was similar to VAP in this respect. X-ray crystallography at 2.3 Å showed two binding sites, one in the active site channel and another at the surface close to dimer interface. Antarctic bacterial AP and VAP have Trp274 in common in their active-sites, that takes part in binding cyclohexylamine. VAP variants W274A, W274K, and W274H gave IC50 values of 179 mM, 188 mM and 187 mM, respectively, at pH 9.8. CONCLUSIONS: The binding of cyclohexylamine in locations at the dimeric interface and/or in the active site of APs may delay product release or reduce the rate of catalytic step(s) involving conformational changes and intersubunit communications. GENERAL SIGNIFICANCE: Cyclohexylamine is a common chemical in industries and used as a counterion in substrates for alkaline phosphatase, a clinically important and common enzyme in the biosphere.

Structural insights of the enzymes from the chitin utilization locus of Flavobacterium johnsoniae.

Chitin is one of the most abundant renewable organic materials found on earth. The chitin utilization locus in Flavobacterium johnsoniae, which encodes necessary proteins for complete enzymatic depolymerization of crystalline chitin, has recently been characterized but no detailed structural information on the enzymes was provided. Here we present protein structures of the F. johnsoniae chitobiase (FjGH20) and chitinase B (FjChiB). FjGH20 is a multi-domain enzyme with a helical domain not before observed in other chitobiases and a domain organization reminiscent of GH84 (ß-N-acetylglucosaminidase) family members. The structure of FjChiB reveals that the protein lacks loops and regions associated with exo-acting activity in other chitinases and instead has a more solvent accessible substrate binding cleft, which is consistent with its endo-chitinase activity. Additionally, small angle X-ray scattering data were collected for the internal 70 kDa region that connects the N- and C-terminal chitinase domains of the unique 158 kDa multi-domain chitinase A (FjChiA). The resulting model of the molecular envelope supports bioinformatic predictions of the region comprising six domains, each with similarities to either Fn3-like or Ig-like domains. Taken together, the results provide insights into chitin utilization by F. johnsoniae and reveal structural diversity in bacterial chitin metabolism.

The crystal structure of haemoglobin from Atlantic cod.

The crystal structure of haemoglobin from Atlantic cod has been solved to 2.54 Šresolution. The structure consists of two tetramers in the crystallographic asymmetric unit. The structure of haemoglobin obtained from one individual cod suggests polymorphism in the tetrameric assembly.

Features and structure of a cold active N-acetylneuraminate lyase.

N-acetylneuraminate lyases (NALs) are enzymes that catalyze the reversible cleavage and synthesis of sialic acids. They are therefore commonly used for the production of these high-value sugars. This study presents the recombinant production, together with biochemical and structural data, of the NAL from the psychrophilic bacterium Aliivibrio salmonicida LFI1238 (AsNAL). Our characterization shows that AsNAL possesses high activity and stability at alkaline pH. We confirm that these properties allow for the use in a one-pot reaction at alkaline pH for the synthesis of N-acetylneuraminic acid (Neu5Ac, the most common sialic acid) from the inexpensive precursor N-acetylglucosamine. We also show that the enzyme has a cold active nature with an optimum temperature for Neu5Ac synthesis at 20°C. The equilibrium constant for the reaction was calculated at different temperatures, and the formation of Neu5Ac acid is favored at low temperatures, making the cold active enzyme a well-suited candidate for use in such exothermic reactions. The specific activity is high compared to the homologue from Escherichia coli at three tested temperatures, and the enzyme shows a higher catalytic efficiency and turnover number for cleavage at 37°C. Mutational studies reveal that amino acid residue Asn 168 is important for the high kcat. The crystal structure of AsNAL was solved to 1.65 Å resolution and reveals a compact, tetrameric protein similar to other NAL structures. The data presented provides a framework to guide further optimization of its application in sialic acid production and opens the possibility for further design of the enzyme.

Structural insight into a CE15 esterase from the marine bacterial metagenome.

The family 15 carbohydrate esterase (CE15) MZ0003, which derives from a marine Arctic metagenome, has a broader substrate scope than other members of this family. Here we report the crystal structure of MZ0003, which reveals that residues comprising the catalytic triad differ from previously-characterized fungal homologs, and resolves three large loop regions that are unique to this bacterial sub-clade. The catalytic triad of the bacterial CE15, which includes Asp 332 as its third member, closely resembles that of family 1 carbohydrate esterases (CE1), despite the overall lower structural similarity with members of this family. Two of the three loop regions form a subdomain that deepens the active site pocket and includes several basic residues that contribute to the high positive charge surrounding the active site. Docking simulations predict specific interactions with the sugar moiety of glucuronic-acid substrates, and with aromatically-substituted derivatives that serve as model compounds for the lignin-carbohydrate complex of plant cell walls. Molecular dynamics simulations indicate considerable flexibility of the sub-domain in the substrate-bound form, suggesting plasticity to accommodate different substrates is possible. The findings from this first reported structure of a bacterial member of the CE15 family provide insight into the basis of its broader substrate specificity.

Multiple specialised goose-type lysozymes potentially compensate for an exceptional lack of chicken-type lysozymes in Atlantic cod.

Previous analyses of the Atlantic cod genome showed unique combinations of lacking and expanded number of genes for the immune system. The present study examined lysozyme activity, lysozyme gene distribution and expression in cod. Enzymatic assays employing specific bacterial lysozyme inhibitors provided evidence for presence of g-type, but unexpectedly not for c-type lysozyme activity. Database homology searches failed to identify any c-type lysozyme gene in the cod genome or in expressed sequence tags from cod. In contrast, we identified four g-type lysozyme genes (LygF1a-d) constitutively expressed, although differentially, in all cod organs examined. The active site glutamate residue is replaced by alanine in LygF1a, thus making it enzymatic inactive, while LygF1d was found in two active site variants carrying alanine or glutamate, respectively. In vitro and in vivo infection by the intracellular bacterium Francisella noatunensis gave a significantly reduced LygF1a and b expression but increased expression of the LygF1c and d genes as did also the interferon gamma (IFNγ) cytokine. These results demonstrate a lack of c-type lysozyme that is unprecedented among vertebrates. Our results further indicate that serial gene duplications have produced multiple differentially regulated cod g-type lysozymes with specialised functions potentially compensating for the lack of c-type lysozymes.

Discovery of Novel Inhibitor Scaffolds against the Metallo-ß-lactamase VIM-2 by Surface Plasmon Resonance (SPR) Based Fragment Screening.

Metallo-ß-lactamase (MBL) inhibitors can restore the function of carbapenem antibiotics and therefore help to treat infections of antibiotic resistant bacteria. In this study, we report novel fragments inhibiting the clinically relevant MBL Verona integron-encoded metallo-ß-lactamase (VIM-2). The fragments were identified from a library of 490 fragments using an orthogonal screening approach based on a surface plasmon resonance (SPR) based assay combined with an enzyme inhibition assay. The identified fragments showed IC50 values between 14 and 1500 µM and ligand efficiencies (LE) between 0.48 and 0.23 kcal/mol per heavy atom. For two of the identified fragments, crystal structures in complex with VIM-2 were obtained. The identified fragments represent novel inhibitor scaffolds and are good starting points for the design of potent MBL inhibitors. Furthermore, the established SPR based assay and the screening approach can be adapted to other MBLs and in this way improve the drug discovery process for this important class of drug targets.

Psoriasis pathogenesis - Pso p27 constitutes a compact structure forming large aggregates.

Psoriasis is a chronic inflammatory skin disease. The absence of microbial organisms as potential causal agents has given rise to the hypothesis that the inflammation is due to an autoimmune reaction. The defined inflamed areas of the skin lesions argue for an immunological disease with a local production of a causal antigen. Pso p27 is a protein generated in mast cells in psoriatic plaques, but not in uninvolved skin. We recently demonstrated that the Pso p27 is generated by cleavage of SerpinB3 (SCCA1) in the presence of mast cell associated chymase. In this communication we demonstrate by X-ray crystallographic analysis that the cleavage products associate into a complex similar to SCCA1, but with the reactive centre loop inserted into a 5-stranded central ß-sheet. Native gel electrophoresis show that these Pso p27 complexes form large aggregates which may be of significance with respect to an immunogenic role of Pso p27.

Structural and functional characterization of a conserved pair of bacterial cellulose-oxidizing lytic polysaccharide monooxygenases.

For decades, the enzymatic conversion of cellulose was thought to rely on the synergistic action of hydrolytic enzymes, but recent work has shown that lytic polysaccharide monooxygenases (LPMOs) are important contributors to this process. We describe the structural and functional characterization of two functionally coupled cellulose-active LPMOs belonging to auxiliary activity family 10 (AA10) that commonly occur in cellulolytic bacteria. One of these LPMOs cleaves glycosidic bonds by oxidation of the C1 carbon, whereas the other can oxidize both C1 and C4. We thus demonstrate that C4 oxidation is not confined to fungal AA9-type LPMOs. X-ray crystallographic structures were obtained for the enzyme pair from Streptomyces coelicolor, solved at 1.3 Å (ScLPMO10B) and 1.5 Å (CelS2 or ScLPMO10C) resolution. Structural comparisons revealed differences in active site architecture that could relate to the ability to oxidize C4 (and that also seem to apply to AA9-type LPMOs). Despite variation in active site architecture, the two enzymes exhibited similar affinities for Cu(2+) (12-31 nM), redox potentials (242 and 251 mV), and electron paramagnetic resonance spectra, with only the latter clearly different from those of chitin-active AA10-type LPMOs. We conclude that substrate specificity depends not on copper site architecture, but rather on variation in substrate binding and orientation. During cellulose degradation, the members of this LPMO pair act in synergy, indicating different functional roles and providing a rationale for the abundance of these enzymes in biomass-degrading organisms.

CorA is a copper repressible surface-associated copper(I)-binding protein produced in Methylomicrobium album BG8.

CorA is a copper repressible protein previously identified in the methanotrophic bacterium Methylomicrobium album BG8. In this work, we demonstrate that CorA is located on the cell surface and binds one copper ion per protein molecule, which, based on X-ray Absorption Near Edge Structure analysis, is in the reduced state (Cu(I)). The structure of endogenously expressed CorA was solved using X-ray crystallography. The 1.6 Å three-dimensional structure confirmed the binding of copper and revealed that the copper atom was coordinated in a mononuclear binding site defined by two histidines, one water molecule, and the tryptophan metabolite, kynurenine. This arrangement of the copper-binding site is similar to that of its homologous protein MopE* from Metylococcus capsulatus Bath, confirming the importance of kynurenine for copper binding in these proteins. Our findings show that CorA has an overall fold similar to MopE, including the unique copper(I)-binding site and most of the secondary structure elements. We suggest that CorA plays a role in the M. album BG8 copper acquisition.

CD40L--a costimulatory molecule involved in the maturation of antigen presenting cells in Atlantic salmon (Salmo salar).

The CD40L/CD40 signalling pathway is critically involved in the final stage of the maturation of DCs. This paper reports the identification and functional characterization of CD40L and CD40 from Atlantic salmon (Salmo salar). Salmon CD40L is a type II membrane-bound protein with a TNF homology domain in its extracellular C-terminal region, while CD40 is a type I membrane-bound receptor with a sequence pattern of four cysteine-rich domains in its extracellular N-terminal region. The salmon CD40L and CD40 were widely expressed, particularly in immune tissues, and while CD40L expression was induced by in vitro stimulation of HKLs with PHA and ConA, CpG increased CD40 expression. A CD40L construct was overexpressed in the CHSE-214 cell line and co-cultivation of the CD40L-CHSE transfectants with HKL induced a rapid and long-lasting upregulation of important costimulatory molecules like CD40, CD83, B7-H1 and the cytokines IL-12p40, IL-10, IL-1ß and IFNs, which all are involved in T-helper cell responses. Furthermore, the CD40L transfected cells increased the percentage of HKLs expressing surface MHCIIß but unlike other APC maturation stimuli, like CpG, they did not reduce the capacity to internalise antigen. Our results provide the first evidence for the existence of a functional CD40L mediated costimulatory pathway in Atlantic salmon.

The Methylococcus capsulatus (Bath) secreted protein, MopE*, binds both reduced and oxidized copper.

Under copper limiting growth conditions the methanotrophic bacterium Methylococcus capsulatus (Bath) secrets essentially only one protein, MopE*, to the medium. MopE* is a copper-binding protein whose structure has been determined by X-ray crystallography. The structure of MopE* revealed a unique high affinity copper binding site consisting of two histidine imidazoles and one kynurenine, the latter an oxidation product of Trp130. In this study, we demonstrate that the copper ion coordinated by this strong binding site is in the Cu(I) state when MopE* is isolated from the growth medium of M. capsulatus. The conclusion is based on X-ray Near Edge Absorption spectroscopy (XANES), and Electron Paramagnetic Resonance (EPR) studies. EPR analyses demonstrated that MopE*, in addition to the strong copper-binding site, also binds Cu(II) at two weaker binding sites. Both Cu(II) binding sites have properties typical of non-blue type II Cu (II) centres, and the strongest of the two Cu(II) sites is characterised by a relative high hyperfine coupling of copper (A(||) =20 mT). Immobilized metal affinity chromatography binding studies suggests that residues in the N-terminal part of MopE* are involved in forming binding site(s) for Cu(II) ions. Our results support the hypothesis that MopE plays an important role in copper uptake, possibly making use of both its high (Cu(I) and low Cu(II) affinity properties.

Thermodynamics and structure of a salmon cold active goose-type lysozyme.

Atlantic salmon goose-type lysozyme (SalG) was previously shown to display features of cold-adaptation as well as renaturation following heat treatment. In this study differential scanning calorimetry (DSC) was carried out to investigate unfolding and potential refolding, while X-ray crystallography was used to study structural factors contributing to the temperature-related characteristics. The recombinant SalG has a melting temperature (T(m)) of 36.8 degrees C under thermal denaturation conditions and regains activity after returning to permissive (low) temperature. Furthermore, refolding is dramatically reduced in solutions with high SalG concentrations, coupled with significant protein precipitation. The structural features of SalG closely resemble those of other g-type lysozymes. However, the N-terminal region of SalG is less anchored to the rest of the molecule due to the absence of disulphide bonds, thus, contributing significantly to the low T(m) of SalG. The absence of disulphide bonds and the distribution of salt bridges may at the same time ease refolding leading to renaturation.

Crystal structures of g-type lysozyme from Atlantic cod shed new light on substrate binding and the catalytic mechanism.

Crystal structures of Atlantic cod lysozyme have been solved with and without ligand bound in the active site to 1.7 and 1.9 A resolution, respectively. The structures reveal the presence of NAG in the substrate binding sites at both sides of the catalytic Glu73, hence allowing the first crystallographic description of the goose-type (g-type) lysozyme E-G binding sites. In addition, two aspartic acid residues suggested to participate in catalysis (Asp101 and Asp90) were mutated to alanine. Muramidase activity data for two single mutants and one double mutant demonstrates that both residues are involved in catalysis, but Asp101 is the more critical of the two. The structures and activity data suggest that a water molecule is the nucleophile completing the catalytic reaction, and the roles of the aspartic acids are to ensure proper positioning of the catalytic water.

Structure of a highly stable mutant of human fibroblast growth factor 1.

Fibroblast growth factors (FGFs) are involved in diverse cellular processes such as cell migration, angiogenesis, osteogenesis, wound healing and embryonic and foetal development. Human acidic fibroblast growth factor (FGF-1) is the only member of the FGF family that binds with high affinity to all four FGF receptors and thus is considered to be the human mitogen with the broadest specificity. However, pharmacological applications of FGF-1 are limited owing to its low stability. It has previously been reported that the introduction of single mutations can significantly improve the stability of FGF-1 and its resistance to proteolytic degradation. Here, the structure of the Q40P/S47I/H93G triple mutant of FGF-1, which exhibits much higher stability, a prolonged half-life and enhanced mitogenic activity, is presented. Compared with the wild-type structure, three localized conformational changes in the stable triple mutant were observed, which is in agreement with the perfect energetic additivity of the single mutations described in a previous study. The huge change in FGF-1 stability (the denaturation temperature increased by 21.5 K, equivalent to DeltaDeltaG(den) = 24.3 kJ mol(-1)) seems to result from the formation of a short 3(10)-helix (position 40), an improvement in the propensity of amino acids to form beta-sheets (position 47) and the rearrangement of a local hydrogen-bond network (positions 47 and 93).

The 1.4 A crystal structure of the large and cold-active Vibrio sp. alkaline phosphatase.

Alkaline phosphatase (AP) from the cold-adapted Vibrio strain G15-21 is among the AP variants with the highest known k(cat) value. Here the structure of the enzyme at 1.4 A resolution is reported and compared to APs from E. coli, human placenta, shrimp and the Antarctic bacterium strain TAB5. The Vibrio AP is a dimer although its monomers are without the long N-terminal helix that embraces the other subunit in many other APs. The long insertion loop, previously noted as a special feature of the Vibrio AP, serves a similar function. The surface does not have the high negative charge density as observed in shrimp AP, but a positively charged patch is observed around the active site that may be favourable for substrate binding. The dimer interface has a similar number of non-covalent interactions as other APs and the "crown"-domain is the largest observed in known APs. Part of it slopes over the catalytic site suggesting that the substrates may be small molecules. The catalytic serines are refined with multiple conformations in both monomers. One of the ligands to the catalytic zinc ion in binding site M1 is directly connected to the crown-domain and is closest to the dimer interface. Subtle movements in metal ligands may help in the release of the product and/or facilitate prior dephosphorylation of the covalent intermediate. Intersubunit interactions may be a major factor for promoting active site geometries that lead to the high catalytic activity of Vibrio AP at low temperatures.

Structural adaptation of endonuclease I from the cold-adapted and halophilic bacterium Vibrio salmonicida.

The crystal structure of the periplasmic/extracellular endonuclease I from Vibrio salmonicida has been solved to 1.5 A resolution and, in comparison to the corresponding endonucleases from V. cholerae and V. vulnificus, serves as a model system for the investigation of the structural determinants involved in the temperature and NaCl adaptation of this enzyme class. The overall fold of the three enzymes is essentially similar, but the V. salmonicida endonuclease displays a significantly more positive surface potential than the other two enzymes owing to the presence of ten more Lys residues. However, if the optimum salt concentrations for the V. salmonicida and V. cholerae enzymes are taken into consideration in the electrostatic surface-potential calculation, the potentials of the two enzymes become surprisingly similar. The higher number of basic residues in the V. salmonicida protein is therefore likely to be a result, at least in part, of adaptation to the more saline habitat of V. salmonicida (seawater) than V. cholerae (brackish water). The hydrophobic core of all three enzymes is almost identical, but the V. salmonicida endonuclease has a slightly lower number of internal hydrogen bonds. This, together with repulsive forces between the basic residues on the protein surface of V. salmonicida endonuclease I and differences in the distribution of salt bridges, probably results in higher flexibility of regions of the V. salmonicida protein. This is likely to influence both the catalytic activity and the stability of the protein.

Effects of salt on the kinetics and thermodynamic stability of endonuclease I from Vibrio salmonicida and Vibrio cholerae.

Adaptation to extreme environments affects the stability and catalytic efficiency of enzymes, often endowing them with great industrial potential. We compared the environmental adaptation of the secreted endonuclease I from the cold-adapted marine fish pathogen Vibrio salmonicida (VsEndA) and the human pathogen Vibrio cholerae (VcEndA). Kinetic analysis showed that VsEndA displayed unique halotolerance. It retained a considerable amount of activity from low concentrations to at least 0.6 m NaCl, and was adapted to work at higher salt concentrations than VcEndA by maintaining a low K(m) value and increasing k(cat). In differential scanning calorimetry, salt stabilized both enzymes, but the effect on the calorimetric enthalpy and cooperativity of unfolding was larger for VsEndA, indicating salt dependence. Mutation of DNA binding site residues (VsEndA, Q69N and K71N; VcEndA, N69Q and N71K) affected the kinetic parameters. The VsEndA Q69N mutation also increased the T(m) value, whereas other mutations affected mainly DeltaH(cal). The determined crystal structure of VcEndA N69Q revealed the loss of one hydrogen bond present in native VcEndA, but also the formation of a new hydrogen bond involving residue 69 that could possibly explain the similar T(m) values for native and N69Q-mutated VcEndA. Structural analysis suggested that the stability, catalytic efficiency and salt tolerance of EndA were controlled by small changes in the hydrogen bonding networks and surface electrostatic potential. Our results indicate that endonuclease I adaptation is closely coupled to the conditions of the habitats of natural Vibrio, with VsEndA displaying a remarkable salt tolerance unique amongst the endonucleases characterized so far.

An oxidized tryptophan facilitates copper binding in Methylococcus capsulatus-secreted protein MopE.

Proteins can coordinate metal ions with endogenous nitrogen and oxygen ligands through backbone amino and carbonyl groups, but the amino acid side chains coordinating metals do not include tryptophan. Here we show for the first time the involvement of the tryptophan metabolite kynurenine in a protein metal-binding site. The crystal structure to 1.35 angstroms of MopE* from the methane-oxidizing Methylococcus capsulatus (Bath) provided detailed information about its structure and mononuclear copper-binding site. MopE* contains a novel protein fold of which only one-third of the structure displays similarities to other known folds. The geometry around the copper ion is distorted tetrahedral with one oxygen ligand from a water molecule, two histidine imidazoles (His-132 and His-203), and at the fourth distorted tetrahedral position, the N1 atom of the kynurenine, an oxidation product of Trp-130. Trp-130 was not oxidized to kynurenine in MopE* heterologously expressed in Escherichia coli, nor did this protein bind copper. Our findings indicate that the modification of tryptophan to kynurenine and its involvement in copper binding is an innate property of M. capsulatus MopE*.