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PubMed2006

Nanoscale resolved infrared probing of crystal structure and of plasmon-phonon coupling.

We show that slight variations of a crystal lattice cause significant spectral modifications of phonon-polariton resonant near-field interaction between polar semiconductor crystals and a scanning metal tip. Exploiting the effect for near-field imaging a SiC polytype boundary, we establish infrared mapping of crystal structure and crystal defects at 20 nm spatial resolution (lambda/500). By spectroscopic probing of doped SiC polytypes, we find that phonon-polariton resonant near-field interaction is also sensitive to electronic properties due to plasmon-phonon coupling in the crystals.

Crystallography↗
PubMed1996

Crystal structures of guinea-pig, goat and bovine alpha-lactalbumin highlight the enhanced conformational flexibility of regions that are significant for its action in lactose synthase.

BACKGROUND: The regulation of milk lactose biosynthesis is highly dependent on the action of a specifier protein, alpha-lactalbumin (LA). Together with a glycosyltransferase, LA forms the enzyme complex lactose synthase. LA promotes the binding of glucose to the complex and facilitates the biosynthesis of lactose. To gain further insight into the molecular basis of LA function in lactose synthase we have determined the structures of three species variants of LA. RESULTS: The crystal structures of guinea-pig, goat and a recombinant from of bovine LA have been determined using molecular replacement techniques. Overall, the structures are very similar and reflect their high degree of amino acid sequence identity (66-94%). Nonetheless, the structures show that a portion of the molecule (residues 105-110), known to be important for function, exhibits a variety of distinct conformers. This region lies adjacent to two residues (Phe31 and His32) that have been implicated in monosaccharide binding by lactose synthase and its conformation has significant effects on the environments of these functional groups. The crystal structures also demonstrate that residues currently implicated in LA's modulatory properties are located in a region of the structure that has relatively high thermal parameters and is therefore probably flexible in vivo. CONCLUSIONS: LA's proposed interaction site for the catalytic component of the lactose synthase complex is primarily located in the flexible C-terminal portion of the molecule. This general observation implies that conformational adjustments may be important for the formation and function of lactose synthase.

Animals↗
PubMed2000

Crystal structure of the matrix protein VP40 from Ebola virus.

Ebola virus maturation occurs at the plasma membrane of infected cells and involves the clustering of the viral matrix protein VP40 at the assembly site as well as its interaction with the lipid bilayer. Here we report the X-ray crystal structure of VP40 from Ebola virus at 2.0 A resolution. The crystal structure reveals that Ebola virus VP40 is topologically distinct from all other known viral matrix proteins, consisting of two domains with unique folds, connected by a flexible linker. The C-terminal domain, which is absolutely required for membrane binding, contains large hydrophobic patches that may be involved in the interaction with lipid bilayers. Likewise, a highly basic region is shared between the two domains. The crystal structure reveals how the molecule may be able to switch from a monomeric conformation to a hexameric form, as observed in vitro. Its implications for the assembly process are discussed.

Amino Acid Sequence↗
PubMed2003

The 0.93A crystal structure of sphericase: a calcium-loaded serine protease from Bacillus sphaericus.

We have previously isolated sphericase (Sph), an extracellular mesophilic serine protease produced by Bacillus sphaericus. The Sph amino acid sequence is highly homologous to two cold-adapted subtilisins from Antarctic bacilli S39 and S41 (76% and 74% identity, respectively). Sph is calcium-dependent, 310 amino acid residues long and has optimal activity at pH 10.0. S41 and S39 have not as yet been structurally analysed. In the present work, we determined the crystal structure of Sph by the Eu/multiwavelength anomalous diffraction method. The structure was extended to 0.93A resolution and refined to a crystallographic R-factor of 9.7%. The final model included all 310 amino acid residues, one disulfide bond, 679 water molecules and five calcium ions. Although Sph is a mesophilic subtilisin, its amino acid sequence is similar to that of the psychrophilic subtilisins, which suggests that the crystal structure of these subtilisins is very similar. The presence of five calcium ions bound to a subtilisin molecule, as found here for Sph, has not been reported for the subtilisin superfamily. None of these calcium-binding sites correlates with the well-known high-affinity calcium-binding site (site I or site A), and only one site has been described previously. This calcium-binding pattern suggests that a reduction in the flexibility of the surface loops of Sph by calcium binding may be responsible for its adaptation to mesophilic organisms.

Amino Acid Sequence↗
PubMed1994

Structural similarity of cytochrome c2 from Rhodopseudomonas viridis to mitochondrial cytochromes c revealed by its crystal structure at 2.7 A resolution.

The crystal structure of cytochrome c2 from Rhodopseudomonas viridis has been refined using molecular dynamics and restrained least-squares methods to a crystallographic R-factor of 0.216 at 2.7 A resolution. A structural comparison between Rps. viridis cytochrome c2 and the other bacterial cytochromes c2 or mitochondrial cytochromes c indicates that the overall protein foldings are very similar to each other with the exception of the surface loop and terminal region of the polypeptide chain. However, the position and hydrogen-bond pattern of the evolutionarily conserved water molecule buried within the heme binding pocket in Rps. viridis cytochrome c2 are common to those in the mitochondrial cytochromes c. This fact indicates that Rps. viridis cytochrome c2 is structurally more similar to mitochondrial cytochromes c than to the other bacterial cytochromes c2.

Crystallography, X-Ray↗
PubMed2003

Comparison of X-ray crystal structure of the 30S subunit-antibiotic complex with NMR structure of decoding site oligonucleotide-paromomycin complex.

Aminoglycoside antibiotics that bind to 16S ribosomal RNA in the aminoacyl-tRNA site (A site) cause misreading of the genetic code and inhibit translocation. Structures of an A site RNA oligonucleotide free in solution and bound to the aminoglycosides paromomycin or gentamicin C1a have been determined by NMR. Recently, the X-ray crystal structure of the entire 30S subunit has been determined, free and bound to paromomycin. Distinct differences were observed in the crystal structure, particularly at A1493. Here, the NMR structure of the oligonucleotide-paromomycin complex was determined with higher precision and is compared with the X-ray crystal structure of the 30S subunit complex. The comparison shows the validity of both structures in identifying critical interactions that affect ribosome function.

Anti-Bacterial Agents↗
PubMed2000

Binding of the general anesthetics propofol and halothane to human serum albumin. High resolution crystal structures.

Human serum albumin (HSA) is one of the most abundant proteins in the circulatory system and plays a key role in the transport of fatty acids, metabolites, and drugs. For many drugs, binding to serum albumin is a critical determinant of their distribution and pharmacokinetics; however, there have as yet been no high resolution crystal structures published of drug-albumin complexes. Here we describe high resolution crystal structures of HSA with two of the most widely used general anesthetics, propofol and halothane. In addition, we describe a crystal structure of HSA complexed with both halothane and the fatty acid, myristate. We show that the intravenous anesthetic propofol binds at two discrete sites on HSA in preformed pockets that have been shown to accommodate fatty acids. Similarly we show that the inhalational agent halothane binds (at concentrations in the pharmacologically relevant range) at three sites that are also fatty acid binding loci. At much higher halothane concentrations, we have identified additional sites that are occupied. All of the higher affinity anesthetic binding sites are amphiphilic in nature, with both polar and apolar parts, and anesthetic binding causes only minor changes in local structure.

Anesthetics, General↗
PubMed2005

A new molecular-dynamics based approach for molecular crystal structure search.

A new molecular-dynamics based approach is proposed to search for candidate crystal structures of molecular solids. The procedure is based on the observation of spontaneous transitions between ordered and disordered states in molecular-dynamics simulations of an artificial periodic system with a small unit cell. In such a way only the most stable structures are automatically selected. The method can be applied to the solution of crystal structures from low-quality or very complex diffraction data. Tests are presented for H2O-ice polymorphs.

Journal Article↗
PubMed1988

Structure-activity relationship of aldose reductase inhibitors based on X-ray crystal structures of oxazolecarbamate derivatives.

In order to elucidate the key atoms and/or stereostructures necessary for the inhibitory emergence of aldose reductase, crystal structure determinations were carried out for 11 oxazolecarbamate analogues, which have similar chemical and physicochemical properties but different inhibitory activities. The molecular conformations, revealed by X-ray analyses, were also ascertained to be energetically stable from theoretical conformational energy calculations. A surprising degree of conformational similarity was observed for the potent inhibitors. The analyses of the quantitative structure--activity relationships showed that the molecular conformation and the dipole moment, as well as the hydrophobicity at the oxazole C5-site, were important for high activity.

Aldehyde Reductase↗
PubMed2002

CH3CN: X-ray structural investigation of a unique single crystal. beta --> alpha phase transition and crystal structure.

The phase transition from the low- (beta) to the high-temperature (alpha) form of acetonitrile, CH(3)CN, has been directly observed and studied on a unique single crystal. Both the beta and alpha structures have been determined at temperatures close to the transition temperature (206 K and 201 K), taking advantage of the hysteresis. A single crystal of the beta form was obtained for the first time.

Journal Article↗
PubMed2002

The x-ray crystal structure of the NF-kappa B p50.p65 heterodimer bound to the interferon beta -kappa B site.

We have determined the x-ray crystal structure of the transcription factor NF-kappaB p50.p65 heterodimer complexed to kappaB DNA from the cytokine interferon beta enhancer (IFNbeta-kappaB). To better understand how the binding modes of NF-kappaB on its two best studied DNA targets might contribute to promoter-specific transcription, this structure is compared with the previously determined complex crystal structure containing NF-kappaB bound to the Ig kappa light chain gene enhancer as well as to a second NF-kappaB.Ig kappa light chain gene enhancer complex also reported in this paper. The global binding modes of all NF-kappaB.DNA complex structures are similar, although crystal-packing interactions lead to differences between identical complexes of the same crystallographic asymmetric unit. An extensive network of stacked amino acid side chains that contribute to base-specific DNA contacts is conserved among the three complexes. Consistent with earlier reports, however, the IFNbeta-kappaB DNA is bent significantly less by NF-kappaB than is the Ig kappa light chain gene enhancer. This and other small structural changes may play a role in explaining why NF-kappaB-directed transcription is sensitive to the context of specific promoters. The precise molecular mechanism behind the involvement of the high mobility group protein I(Y) in interferon beta enhanceosome formation remains elusive.

Crystallography, X-Ray↗
PubMed1992

Refined crystal structure of a recombinant immunoglobulin domain and a complementarity-determining region 1-grafted mutant.

We report the solution of the crystal structure of a mutant of the immunoglobulin VL domain of the antibody McPC603, in which the complementarity-determining region 1 segment is replaced with that of a different antibody. The wild-type and mutant crystal structures have been refined to a crystallographic R-factor of 14.9% at a nominal resolution of 1.97 A. A detailed description of the structures is given. Crystal packing results in a dimeric association of domains, in a fashion closely resembling that of an Fv fragment. The comparison of this VL domain with the same domain in the Fab fragment of McPC603 shows that the structure of an immunoglobulin VL domain is largely independent of its mode of association, even in places where the inter-subunit contacts are not conserved between VL and VH. In all three complementarity-determining regions we observe conformations that would not have been predicted by the canonical structure hypothesis. Significant differences between the VL domain dimer and the Fab fragment in the third complementarity-determining region show that knowledge of the structure of the dimerization partner and its exact mode of association may be needed to predict the precise conformation of antigen-binding loops.

Amino Acid Sequence↗
PubMed1994

Solid-state phase transition in the crystal structure of ribulose 1,5-bisphosphate carboxylase/oxygenase.

The crystal structure is described of ribulose 1,5-bisphosphate carboxylase/oxygenase in a new crystal form. This new form (form V) was obtained from a previously known crystal form (form III) through a solid-state phase transition. The solid-state phase transition was brought about by transferring the crystal from a high-salt low-pH mother liquor to a low-salt high-pH synthetic mother liquor. The interplay of electrostatic repulsion and osmotic pressure induced a unit-cell shrinkage of 24 A along the c axis and expansion of 4 A along the a and b axes. The space group also changed from I422 to I4. The new crystal form was shown to be more resistant to X-ray radiation damage, which suggests the effect of crystal stabilization by non-penetrating molecules. The structure of ribulose 1,5-bisphosphate carboxylase/oxygenase in the new crystal form is compared with that of the old crystal form.

Journal Article↗
PubMed2004

Crystal structure of F-93 from Sulfolobus spindle-shaped virus 1, a winged-helix DNA binding protein.

Sulfolobus spindle-shaped viruses (SSVs), or Fuselloviridae, are ubiquitous crenarchaeal viruses found in high-temperature acidic hot springs around the world (pH </=4.0; temperature of >/=70 degrees C). Because they are relatively easy to isolate, they represent the best studied of the crenarchaeal viruses. This is particularly true for the type virus, SSV1, which contains a double-stranded DNA genome of 15.5 kilobases, encoding 34 putative open reading frames. Interestingly, the genome shows little sequence similarity to organisms other than its SSV homologues. Together, sequence similarity and biochemical analyses have suggested functions for only 6 of the 34 open reading frames. Thus, even though SSV1 is the best-studied crenarchaeal virus, functions for most (28) of its open reading frames remain unknown. We have undertaken biochemical and structural studies for the gene product of open reading frame F-93. We find that F-93 exists as a homodimer in solution and that a tight dimer is also present in the 2.7-A crystal structure. Further, the crystal structure reveals a fold that is homologous to the SlyA and MarR subfamilies of winged-helix DNA binding proteins. This strongly suggests that F-93 functions as a transcription factor that recognizes a (pseudo-)palindromic DNA target sequence.

Amino Acid Sequence↗
PubMed2000

Interaction of the factor XIII activation peptide with alpha -thrombin. Crystal structure of its enzyme-substrate analog complex.

The serine protease thrombin proteolytically activates blood coagulation factor XIII by cleavage at residue Arg(37); factor XIII in turn cross-links fibrin molecules and gives mechanical stability to the blood clot. The 2.0-A resolution x-ray crystal structure of human alpha-thrombin bound to the factor XIII-(28-37) decapeptide has been determined. This structure reveals the detailed atomic level interactions between the factor XIII activation peptide and thrombin and provides the first high resolution view of this functionally important part of the factor XIII molecule. A comparison of this structure with the crystal structure of fibrinopeptide A complexed with thrombin highlights several important determinants of thrombin substrate interaction. First, the P1 and P2 residues must be compatible with the geometry and chemistry of the S1 and S2 specificity sites in thrombin. Second, a glycine in the P5 position is necessary for the conserved substrate conformation seen in both factor XIII-(28-37) and fibrinopeptide A. Finally, the hydrophobic residues, which occupy the aryl binding site of thrombin determine the substrate conformation further away from the catalytic residues. In the case of factor XIII-(28-37), the aryl binding site is shared by hydrophobic residues P4 (Val(34)) and P9 (Val(29)). A bulkier residue in either of these sites may alter the substrate peptide conformation.

Amino Acid Sequence↗
PubMed2004

Design and crystal structure of bacteriophage T4 mini-fibritin NCCF.

Fibritin is a fibrous protein that forms "whiskers" attached to the neck of bacteriophage T4. Whiskers interact with the long tail fibers regulating the assembly and infectivity of the virus. The fibritin trimer includes the N-terminal domain responsible for attachment to the phage particle and for the collar formation, the central domain forming a 500 A long segmented coiled-coil structure, and the C-terminal "foldon" domain. We have designed a "mini" fibritin with most of the coiled-coil domain deleted, and solved its crystal structure. The non-helical N-terminal part represents a new protein fold that tightly interacts with the coiled-coil segment forming a single domain, as revealed by calorimetry. The analysis of the crystal structure and earlier electron microscopy data on the collar-whisker complex suggests the necessity of other proteins to participate in the collar formation. Crystal structure determination of the N-terminal domain of fibritin is the first step towards elucidating the detailed structure and assembly mechanism of the collar-whisker complex.

Amino Acid Motifs↗
PubMed2005

Crystal structures and magnetic properties of mixed iridium-ruthenium triple perovskites. 2. Ba3MRuIrO9 (M=Li, Na, Mg, Ni, Zn, Bi, In).

Crystal structures and magnetic properties of polycrystalline Ba3MRuIrO9 (M1+=Li, Na; M2+=Mg, Ni, Zn; M3+=Bi, In) were investigated. Rietveld refinements of the crystal structures using powder diffraction data indicate that, with the exception of Ba3BiRuIrO9, all compounds crystallize in the 6H-BaTiO3 structure type in space group P63/mmc; Ba3BiRuIrO9 crystallizes in space group C2/c. The 6H-BaTiO3, or triple-perovskite, structure is composed of hexagonal and cubic stacking of [AO3] layers and contains face- and corner-sharing octahedra. The structures in this study contain a disordered mixture of Ir and Ru in the face-sharing octahedra dimers, which are connected via corner-shared MO6 octahedra. Magnetic susceptibility measurements as a function of temperature were carried out on each compound. Effective magnetic moments were smaller than values estimated using spin-only moments, which indicate the presence of spin-orbit coupling and strong interactions in the face-sharing octahedra that contain a disordered mixture of Ru and Ir on a single crystallographic site. Over a broad temperature range, a divergence of the zero-field-cooled and field-cooled data were observed for Ba3MgRuIrO9, Ba3NiRuIrO9, Ba3ZnRuIrO9, and Ba3LiRuIrO9.

Journal Article↗
PubMed2001

Crystal structure of human peroxiredoxin 5, a novel type of mammalian peroxiredoxin at 1.5 A resolution.

The peroxiredoxins define an emerging family of peroxidases able to reduce hydrogen peroxide and alkyl hydroperoxides with the use of reducing equivalents derived from thiol-containing donor molecules such as thioredoxin, glutathione, trypanothione and AhpF. Peroxiredoxins have been identified in prokaryotes as well as in eukaryotes. Peroxiredoxin 5 (PRDX5) is a novel type of mammalian thioredoxin peroxidase widely expressed in tissues and located cellularly to mitochondria, peroxisomes and cytosol. Functionally, PRDX5 has been implicated in antioxidant protective mechanisms as well as in signal transduction in cells. We report here the 1.5 A resolution crystal structure of human PRDX5 in its reduced form. The crystal structure reveals that PRDX5 presents a thioredoxin-like domain. Interestingly, the crystal structure shows also that PRDX5 does not form a dimer like other mammalian members of the peroxiredoxin family. In the reduced form of PRDX5, Cys47 and Cys151 are distant of 13.8 A although these two cysteine residues are thought to be involved in peroxide reductase activity by forming an intramolecular disulfide intermediate in the oxidized enzyme. These data suggest that the enzyme would necessitate a conformational change to form a disulfide bond between catalytic Cys47 and Cys151 upon oxidation according to proposed peroxide reduction mechanisms. Moreover, the presence of a benzoate ion, a hydroxyl radical scavenger, was noted close to the active-site pocket. The possible role of benzoate in the antioxidant activity of PRDX5 is discussed.

Amino Acid Sequence↗