Specificity of a UDP-GalNAc pyranose-furanose mutase: a potential therapeutic target for Campylobacter jejuni infections.

Pyranose-furanose mutases are essential enzymes in the life cycle of a number of microorganisms, but are absent in mammalian systems, and hence represent novel targets for drug development. To date, all such mutases show preferential recognition of a single substrate (e.g., UDP-Gal). We report here the detailed structural characterization of the first bifunctional pyranose-furanose mutase, which recognizes both UDP-Gal and UDP-GalNAc. The enzyme under investigation (cjUNGM) is involved in the biosynthesis of capsular polysaccharides (CPSs) in Campylobacter jejuni 11168. These CPSs are known virulence factors that are required for adhesion and invasion of human epithelial cells. Using a combination of UV/visible spectroscopy, X-ray crystallography, saturation transfer difference NMR spectroscopy, molecular dynamics and CORCEMA-ST calculations, we have characterized the binding of the enzyme to both UDP-Galp and UDP-GalpNAc, and compared these interactions with those of a homologous monofunctional mutase enzyme from E. coli (ecUGM). These studies reveal that two arginines in cjUNGM, Arg59 and Arg168, play critical roles in the catalytic mechanism of the enzyme and in controlling its specificity to ultimately lead to a GalfNAc-containing CPS. In ecUGM, these arginines are replaced with histidine and lysine, respectively, and this results in an enzyme that is selective for UDP-Gal. We propose that these changes in amino acids allow C. jejuni 11168 to produce suitable quantities of the sugar nucleotide substrate required for the assembly of a CPS containing GalfNAc, which is essential for viability.

Elucidation of substrate specificity in Aspergillus nidulans UDP-galactose-4-epimerase.

The frequency of invasive fungal infections has rapidly increased in recent years. Current clinical treatments are experiencing decreased potency due to severe host toxicity and the emergence of fungal drug resistance. As such, new targets and their corresponding synthetic pathways need to be explored for drug development purposes. In this context, galactofuranose residues, which are employed in fungal cell wall construction, but are notably absent in animals, represent an appealing target. Herein we present the structural and biochemical characterization of UDP-galactose-4-epimerase from Aspergillus nidulans which produces the precursor UDP-galactopyranose required for galactofuranose synthesis. Examination of the structural model revealed both NAD(+) and UDP-glucopyranose were bound within the active site cleft in a near identical fashion to that found in the Human epimerase. Mutational studies on the conserved catalytic motif support a similar mechanism to that established for the Human counterpart is likely operational within the A. nidulans epimerase. While the K m and k cat for the enzyme were determined to be 0.11 mM and 12.8 s(-1), respectively, a single point mutation, namely L320C, activated the enzyme towards larger N-acetylated substrates. Docking studies designed to probe active site affinity corroborate the experimentally determined activity profiles and support the kinetic inhibition results.

Preliminary X-ray crystallographic studies of UDP-glucose-4-epimerase from Aspergillus nidulans.

UDP-glucose-4-epimerase (GALE) from Aspergillus nidulans was overexpressed in Escherichia coli, purified via His-tag affinity chromatography and cocrystallized with UDP-galactose using the microbatch method. The crystals diffracted to 2.4 Šresolution using synchrotron radiation on the Canadian Light Source 08ID-1 beamline. Examination of the data with d*TREK revealed nonmerohedral twinning, from which a single lattice was ultimately extracted for processing. The final space group was found to be C2, with unit-cell parameters a = 66.13, b = 119.15, c = 161.42 Å, ß = 98.48°. An initial structure solution has been obtained via molecular replacement employing human GALE (PDB entry 1hzj) as a template model.

Crystallization and preliminary X-ray studies of the N-domain of the Wilson disease associated protein.

Wilson disease associated protein (ATP7B) is essential for copper transport in human cells. Mutations that affect ATP7B function result in Wilson's disease, a chronic copper toxicosis. Disease-causing mutations within the N-domain of ATP7B (WND) are known to disrupt ATP binding, but a high-resolution X-ray structure of the ATP-binding site has not been reported. The N-domain was modified to delete the disordered loop comprising residues His1115-Asp1138 (WNDDelta(1115-1138)). Unlike the wild-type N-domain, WNDDelta(1115-1138) formed good-quality crystals. Synchrotron diffraction data have been collected from WNDDelta(1115-1138) at the Canadian Light Source. A native WNDDelta(1115-1138) crystal diffracted to 1.7 A resolution and belonged to space group P4(2)2(1)2, with unit-cell parameters a = 39.2, b = 39.2, c = 168.9 A. MAD data were collected to 2.7 A resolution from a SeMet-derivative crystal with unit-cell parameters a = 38.4, b = 38.4, c = 166.7 A. The WNDDelta(1115-1138) structure is likely to be solved by phasing from multiwavelength anomalous diffraction (MAD) experiments.

Selective guest inclusion in a non-porous H-bonded host.

A hydrogen-bonded host solid demonstrates reversible and selective guest inclusion, despite not having a porous interlayer.

Structural constraints in the design of silver sulfonate coordination networks: three new polysulfonate open frameworks.

Three new silver sulfonate metal-organic frameworks are presented along with a design strategy for future generations. [[Ag6(mesitylenetrisulfonate)2(H2O)5].2H2O]infinity (1), [Ag4(durenetetrasulfonate)(H2O)2](infinity) (2), and [[Ag4(1,3,5,7-tetrakis(4,4'-sulfophenyl)adamantane)(H2O)2].1.3H2O]infinity (3) represent a series of open-framework silver sulfonate solids where the organic linker plays a key role in determining the overall structure. Compound 1 forms a pillared layered structure, while compounds 2 and 3 form 3-D nets derived from cross-linking of 1-D columns of silver sulfonates. All three solids incorporate water molecules, which can be removed to yield a solid stable to in excess of 300 degrees C. Powder X-ray diffraction studies and vapor sorption experiments show, for 1 and 2, that the solids retain their structure when guests are removed and, for all three, that water vapor is resorbed stoichiometrically by the solids. An idealized silver sulfonate framework is proposed, and upon comparison to the reported structures, guidelines are proposed for structural constraints in the design of future generations of 1-D and possibly 0-D aggregate structures.

3,3'-disubstituted BINAP ligands: synthesis, resolution, and applications in asymmetric hydrogenation.

A novel family of BINAP ligands were prepared with alkoxy- and acetoxy-derived substituents in the 3,3'-positions. They were prepared through a convergent synthesis starting from readily available 4-bromo-2-naphthol. These ligands afforded excellent enantioselectivities in the asymmetric hydrogenation of substituted olefins. The presence of the 3,3'-substituents was shown to be beneficial by a direct comparison with the parent unsubstituted BINAP. [reaction: see text]

Intra- and intermolecular second-sphere coordination chemistry: formation of capsules, half-capsules, and extended structures with hexaaquo- and hexaamminemetal ions.

In the design of novel extended solids, particularly those based on weaker interactions, reliable "synthons" are a valuable commodity. This work concerns the hydrogen-bonded assemblies which result from the second-sphere coordination interactions between a highly preorganized trisulfonate ligand and hexaaquo metal ions. Significantly, supramolecular structural variation, which may be rationalized on the basis of the features of the molecular building blocks, is observed. The results are formation of second-sphere capsules with trivalent ions (Fe(3+), Cr(3+), Al(3+)), and half-capsules with divalent ions (Mg(2+), Zn(2+)). The divalent systems further assemble into extensively hydrogen-bonded hexagonal nets. Effects of geometrical variation of the building blocks are also observed when a Jahn-Teller-distorted divalent ion (Cu(2+)) is substituted for the perfectly octahedral species. The second-sphere effects on the stabilization of the primary coordination sphere are illustrated by TGA experiments. In these assemblies, the potential of a new supramolecular synthon is illustrated, that being the complementary cis-aquo sulfonate interaction. These complexes illustrate the general utility of second-sphere effects, both as an assembly tool and to stabilize metal complexes in the solid state. Finally, as a comparison, a hydrogen-bonded assembly with a hexaammine complex of a trivalent metal (Co(3+)) is presented, which forms an extended network with a completely altered hydrogen bonding array.

Anion exchange in the channels of a robust alkaline earth sulfonate coordination network.

The barium sulfonate network presented herein, [[Ba2(L)(H2O)5]Cl]infinity (1), represents the first metal sulfonate compound to possess a cationic framework. The network is layered with channels between pillaring ligands in which chloride ions reside. Compound 1 contracts slightly upon dehydration but retains its overall structural motif to 420 degrees C. Significantly, the chloride ions of the structure can be exchanged in 80% yield for fluoride ions in a facile manner. This exchange is quantified by elemental analyses, gravimetric determination, and 19F NMR spectroscopy. Confirmation of retention of structure is provided by standardized powder X-ray diffraction experiments. This last point is notable as the F-analogue of the structure is not attainable by a direct synthesis. These results illustrate one of the hallmark features of supramolecular chemistry, that a robust and functional framework can result through cooperative interactions between more weakly interacting units.

An open channel coordination framework sustained by cooperative primary and secondary sphere interactions.

A three-dimensional coordination solid is presented in which every metal ion is stabilized by both primary and secondary sphere interactions to form an open channel structure.

Intercalation of alcohols in Ag sulfonates: topotactic behavior despite flexible layers.

This article presents the inaugural intercalation study of a layered metal sulfonate network. Silver triflate forms intercalation complexes with straight chain primary alcohols from ethanol (C(2)H(5)OH) to eicosanol (C(20)H(41)OH). Single-crystal data for the EtOH adduct, 1, are presented which show that the intercalation is coordinative to Ag. In contrast to many other layered hosts, no preheating of Ag triflate is required to liberate a coordination site for intercalation to take place, owing to the ability of the triflate ion to reorient. Crystal structure parameters for 1: C(4)H(6)F(6)S(2)O(7)Ag(2), a = 5.345(7) A, b = 11.310(2) A, c = 12.004(2) A, alpha = 116.87(1) degrees, beta = 90.46(1) degrees, gamma = 99.59(1) degrees, triclinic, space group P, Z = 2. Intercalate 1 presents the triflate ion in an unprecedented mu(5)-coordination mode. PXRD data on the family of complexes show that the intercalation is topotactic, as verified by the linear increase in d-spacing and calculated c-axis lengths for the intercalates, with increasing chain length. The data also show that the alcohol intercalates adopt an interdigitated rather than bilayer arrangement.