Macrocyclic systems containing 2,6,9-trioxabicyclo[3.3.1]-nona-3,7-dienes as chiral spacer groups: Synthesis, stereochemical features and preliminary complexation properties

Novel 2:2-macrocycles bearing bridged concave 2,6,9-trioxabicyclo[3.3.1]nona-3,7-dienes as chiral spacer units were obtained by cyclocondensation reaction of the chiral bisacid chloride and the corresponding diols, while use of methylene diamines instead of diols afforded 1:1 macrocycles only. Applying the same, but now template-assisted, experimental procedure to the reaction of the bisacid chloride with triethylene glycol brought about a significant increase in yield as well as a suitable simplification of the work-up during preparation and separation of the corresponding 1:1 as well as 2:2 macrocycles, when compared to results reported previously. HPLC separation on chiral columns revealed the presence of diastereoisomers [RR(S,S)- and RS-(meso)-forms] for all 2:2 macrocycles, which was further evidenced by the CD spectrum of one of those species as an example. Preliminary ESI-MS experiments indicated strong complexation abilities of the sulphur-containing ligand towards Ag(I), Cu(II) and Au(III) ions.

Model for growth hormone receptor activation based on subunit rotation within a receptor dimer

Growth hormone is believed to activate the growth hormone receptor (GHR) by dimerizing two identical receptor subunits, leading to activation of JAK2 kinase associated with the cytoplasmic domain. However, we have reported previously that dimerization alone is insufficient to activate full-length GHR. By comparing the crystal structure of the liganded and unliganded human GHR extracellular domain, we show here that there is no substantial change in its conformation on ligand binding. However, the receptor can be activated by rotation without ligand by inserting a defined number of alanine residues within the transmembrane domain. Fluorescence resonance energy transfer ( FRET), bioluminescence resonance energy transfer (BRET) and coimmunoprecipitation studies suggest that receptor subunits undergo specific transmembrane interactions independent of hormone binding. We propose an activation mechanism involving a relative rotation of subunits within a dimeric receptor as a result of asymmetric placement of the receptor-binding sites on the ligand.

The STARD statement for reporting studies of diagnostic accuracy: Explanation and elaboration

The quality of reporting of studies of diagnostic accuracy is less than optimal. Complete and accurate reporting is necessary to enable readers to assess the potential for bias in the study and to evaluate the generalisability of the results. A group of scientists and editors has developed the STARD (Standards for Reporting of Diagnostic Accuracy) statement to improve the reporting the quality of reporting of studies of diagnostic accuracy. The statement consists of a checklist of 25 items and flow diagram that authors can use to ensure that all relevant information is present. This explanatory document aims to facilitate the use, understanding and dissemination of the checklist. The document contains a clarification of the meaning, rationale and optimal use of each item on the checklist, as well as a short summary of the available evidence on bias and applicability. The STARD statement, checklist, flowchart and this explanation and elaboration document should be useful resources to improve reporting of diagnostic accuracy studies. Complete and informative reporting can only lead to better decisions in healthcare.

Inhibition of protein interactions with the beta(2) sliding clamp of Escherichia coli DNA polymerase III by peptides from beta(2)-binding proteins

The sliding clamp of the Escherichia coli replisome is now understood to interact with many proteins involved in DNA synthesis and repair. A universal interaction motif is proposed to be one mechanism by which those proteins bind the E. coli sliding clamp, a homodimer of the beta subunit, at a single site on the dimer. The numerous beta(2)-binding proteins have various versions of the consensus interaction motif, including a related hexameric sequence. To determine if the variants of the motif could contribute to the competition of the beta-binding proteins for the beta(2) site, synthetic peptides derived from the putative beta(2)-binding motifs were assessed for their abilities to inhibit protein-beta(2) interactions, to bind directly to beta(2), and to inhibit DNA synthesis in vitro. A hierarchy emerged, which was consistent with sequence similarity to the pentameric consensus motif, QL(S/D)LF, and peptides containing proposed hexameric motifs were shown to have activities comparable to those containing the consensus sequence. The hierarchy of peptide binding may be indicative of a competitive hierarchy for the binding of proteins to beta(2) in various stages or circumstances of DNA replication and repair.

Rugulotrosins A and B: Two new antibacterial metabolites from an Australian isolate of a penicillium sp.

Two new antibacterial agents, rugulotrosin A (1) and B (2), were obtained from cultures of a Penicillium sp. isolated from soil samples acquired near Sussex Inlet, New South Wales, Australia. Rugulotrosin A (1) is a chiral symmetric dimer, and its relative stereostructure was determined by spectroscopic and X-ray crystallographic analysis. Rugulotrosin B (2) is a chiral asymmetric dimer isomeric with 1. Its structure was determined by spectroscopic analysis with comparison to the co-metabolite 1 and previously reported fungal metabolites. Both rugulotrosins A and B displayed significant antibacterial activity against Bacillus subtilis, while rugulotrosin A was also strongly active against Enterococcus faecalis and B. cereus.

Disulfide-linked dimers of human adrenaline synthesizing enzyme PNMT are catalytically active

The crystal structure of human phenylethanolamine N-methyltransferase (hPNMT) reveals a disulfide- linked dimer, despite the presence of reducing agent in the crystallisation conditions. By removing the reducing agent, hPNMT crystals grow more rapidly and at lower protein concentrations. However, it was unclear whether the disulfide bonds are only present in the crystal form or whether these affect enzyme activity. The solution oligomeric state of hPNMT was investigated using biochemical techniques and activity assays. We found that in the absence of reducing agent, hPNMT forms dimers in solution. Furthermore, the solution dimer of hPNMT incorporates disulfide bonds, since this form is sensitive to reducing agent. The C48A and C139A mutants of hPNMT, which are incapable of forming the disulfide bond observed in the crystal structure, have a decreased propensity to form dimer in solution. Those dimers that do form are also sensitive to reducing agent. Further, the C48A/C139A double mutant shows only monomeric behaviour. Both dimeric and monomeric hPNMT, as well as mutants have wildtype enzyme activity. These results show that a variety of disulfides, including those observed in the crystal structure, can form in solution. In addition, disulfide-linked dimers are as active as the monomeric enzyme indicating that the crystal structure of the protein is a valid target for inhibitor design. Crown Copyright (c) 2005 Published by Elsevier B.V. All rights reserved.

The 2.1 angstrom crystal structure of the far-red fluorescent protein HcRed: Inherent conformational flexibility of the chromophore

We have determined the crystal structure of HcRed, a far-red fluorescent protein isolated from Heteractis crispa, to 2.1 resolution. HcRed was observed to form a dimer, in contrast to the monomeric form of green fluorescent protein (GFP) or the tetrameric forms of the GFP-like proteins (eqFP611, Rtms5 and DsRed). Unlike the well-defined chromophore conformation observed in GFP and the GFP-like proteins, the HcRed chromophore was observed to be considerably mobile. Within the HcRed structure, the cyclic tripeptide chromophore, Glu64-Tyr65-Gly66, was observed to adopt both a cis coplanar and a tran. non-coplanar conformation. As a result of these two con formations, the hydroxyphenyl moiety of the chromophore makes distinct interactions within the interior of the b-can. These data together with a quantum chemical model of the chromophore, suggest the cis coplanar conformation to be consistent with the fluorescent properties of HcRed, and the trans non-coplanar conformation to be consistent with non-fluorescent properties of hcCP, the chromoprotein parent of HcRed. Moreover, within the GFP-like family, it appears that where conformational freedom is permissible then flexibility in the chromophore conformation is possible. 2005 Elsevier Ltd. All rights reserved.

Silencing of integrated human papillomavirus type 18 oncogene transcription in cells expressing SerpinB2

The serine protease inhibitor SerpinB2 (PAI-2), a major product of differentiating squamous epithelial cells, has recently been shown to bind and protect the retinoblastoma protein (Rb) from degradation. In human papillomavirus type 18 (HPV-18) -transformed epithelial cells the expression of the E6 and E7 oncoproteins is controlled by the HPV-18 upstream regulatory region (URR). Here we illustrate that PAI-2 expression in the HPV-18-transformed cervical carcinoma line HeLa resulted in the restoration of Rb expression, which led to the functional silencing of transcription from the HPV-18 URR. This caused loss of E7 protein expression and restoration of multiple E6- and E7-targeted host proteins, including p53, c-Myc, and c-Jun. Rb expression emerged as sufficient for the transcriptional repression of the URR, with repression mediated via the C/EB beta-YY1 binding site (URR 7709 to 7719). In contrast to HeLa cells, where the C/EBP beta-YY1 dimer binds this site, in PAI-2- and/or Rb-expressing cells the site was occupied by the dominant-negative C/EBP beta isoform liver-enriched transcriptional inhibitory protein (LIP). PAI-2 expression thus has a potent suppressive effect on HPV-18 oncogene transcription mediated by Rb and LIP, a finding with potential implications for prognosis and treatment of HPV-transformed lesions.

Mechanisms of acetohydroxyacid synthases

Acetohydroxyacid synthases are thiamin diphosphate- (ThDP-) dependent biosynthetic enzymes found in all autotrophic organisms. Over the past 4-5 years, their mechanisms have been clarified and illuminated by protein crystallography, engineered mutagenesis and detailed single-step kinetic analysis. Pairs of catalytic subunits form an intimate dimer containing two active sites, each of which lies across a dimer interface and involves both monomers. The ThDP adducts of pyruvate, acetaldehyde and the product acetohydroxyacids can be detected quantitatively after rapid quenching. Determination of the distribution of intermediates by NMR then makes it possible to calculate individual forward unimolecular rate constants. The enzyme is the target of several herbicides and structures of inhibitor-enzyme complexes explain the herbicide-enzyme interaction.

The crystal structure of a bacterial Class II ketol-acid reductolsomerase: Domain conservation and evolution

Ketol-acid reductoisomerase (KARI; EC 1.1.1.86) catalyzes two steps in the biosynthesis of branched-chain amino acids. Amino acid sequence comparisons across species reveal that there are two types of this enzyme: a short form (Class 1) found in fungi and most bacteria, and a long form (Class 11) typical of plants. Crystal structures of each have been reported previously. However, some bacteria such as Escherichia coli possess a long form, where the amino acid sequence differs appreciably from that found in plants. Here, we report the crystal structure of the E. coli enzyme at 2.6 A resolution, the first three-dimensional structure of any bacterial Class 11 KARI. The enzyme consists of two domains, one with mixed alpha/beta structure, which is similar to that found in other pyridine nucleotide-dependent dehydrogenases. The second domain is mainly alpha-helical and shows strong evidence of internal duplication. Comparison of the active sites between KARI of E. coli, Pseudomonas aeruginosa, and spinach shows that most residues occupy conserved positions in the active site. E. coli KARI was crystallized as a tetramer, the likely biologically active unit. This contrasts with P. aeruginosa KARI, which forms a dodecamer, and spinach KARI, a dimer. In the E. coli KARI tetramer, a novel subunit-to-subunit interacting surface is formed by a symmetrical pair of bulbous protrusions.

Synthesis, characterization and X-ray crystal structures of dinuclear nickel(II) complexes of a novel potentially hexadentate but functionally tetradentate binucleating ligand

A binucleating potentially hexadentate chelating agent containing oxygen, nitrogen and sulfur as potential donor atoms (H2ONNO) has been synthesized by condensing alpha,alpha-xylenebis(N-methyldithiocarbazate) with 2,4-pentanedione. An X-ray crystallographic structure determination shows that the Schiff base remains in its ketoimine tautomeric form with the protons attached to the imine nitrogen atoms. The reaction of the Schiff base with nickel(II) acetate in a 1:1 stoichiometry leads to the formation of a dinuclear nickel(II) complex [Ni(ONNO)](2) (ONNO2- = dianionic form of the Schiff base) containing N,O-chelated tetradentate ligands, the sulfur donors remaining uncoordinated. A single crystal X-ray structure determination of this dimer reveals that each ligand binds two low spin nickel(II) ions, bridged by a xylyl group. The nickel(II) atoms adopt a distorted square-planar geometry in a trans-N2O2 donor environment. Reaction of the Schiff base with nickel(II) acetate in the presence of excess pyridine leads to the formation of a similar dinuclear complex, [Ni(ONNO)(py)](2), but in this case comprises five coordinate high-spin Ni(II) ions with pyridine ligands occupying the axial coordination sites as revealed by X-ray crystallographic analysis. (c) 2005 Published by Elsevier B.V.

Effect of osmolytes on the interaction of flavin adenine dinucleotide with muscle glycogen phosphorylase b

The effect of three osmolytes, trimethylamine N-oxide (TMAO), betaine and proline, on the interaction of muscle glycogen phosphorylase b with allosteric inhibitor FAD has been examined. In the absence of osmolyte, the interaction is described by a single intrinsic dissociation constant (17.8 muM) for two equivalent and independent binding sites on the dimeric enzyme. However, the addition of osmolytes gives rise to sigmoidal dependencies of fractional enzyme-site saturation upon free inhibitor concentration. The source of this cooperativity has been shown by difference sedimentation velocity to be an osmolyte-mediated isomerization of phosphorylase b to a smaller dimeric state with decreased affinity for FAD. These results thus have substantiated a previous inference that the tendency for osmolyte-enhanced self-association of dimeric glycogen phosphorylase b in the presence of AMP was being countered by the corresponding effect of molecular crowding on an isomerization of dimer to a smaller, nonassociating state. (C) 2004 Elsevier Ltd. Inc. All rights reserved.

Are insect immune suppressors driving cellular uptake reactions?

Many insect parasitoids that deposit their eggs inside immature stages of other insect species inactivate the cellular host defence to protect the growing embryo from encapsulation. Suppression of encapsulation by polydnavirus-encoded immune-suppressors correlates with specific alterations in hemocytes, mainly cytoskeletal rearrangements and actin-cytoskeleton breakdown. We have previously shown that the Cotesia rubecula polydnavirus gene product CrV1 causes immune suppression when injected into the host hemocoel. CrV1 is taken up by hemocytes although no receptors have been found to bind the protein. Instead CrV1 uptake depends on dimer formation, which is required for interacting with lipophorin, suggesting a CrV1-lipophorin complex internalisation by hemocytes. Since treatment of hemocytes with oligomeric lectins and cytochalasin D can mimic the effects of CrV1, we propose that some dimeric and oligomeric adhesion molecules are able to cross-link receptors on the cell surface and depolymerise actin by leverage-mediated clearance reactions in the hemolymph.