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  • Abou Hachem, Maher, et al. (författare)
  • Carbohydrate-binding modules from a thermostable Rhodothermus marinus xylanase : Cloning, expression and binding studies
  • 2000
  • Ingår i: Biochemical Journal. - : Portland Press. - 0264-6021. ; 345:1, s. 53-60
  • Tidskriftsartikel (refereegranskat)abstract
    • The two N-terminally repeated carbohydrate-binding modules (CBM4-1 and CBM4-2) encoded by xyn10A from Rhodothermus marinus were produced in Escherichia coli and purified by affinity chromatography. Binding assays to insoluble polysaccharides showed binding to insoluble xylan and to phosphoric-acid-swollen cellulose but not to Avicel or crystalline cellulose. Binding to insoluble substrates was significantly enhanced by the presence of Na+ and Ca2+ ions. The binding affinities for soluble polysaccharides were tested by affinity electrophoresis; strong binding occurred with different xylans and β-glucan. CBM4-2 displayed a somewhat higher binding affinity than CBM4-1 for both soluble and insoluble substrates but both had similar specificities. Binding to short oligosaccharides was measured by NMR; both modules bound with similar affinities. The binding of the modules was shown to be dominated by enthalpic forces. The binding modules did not contribute with any significant synergistic effects on xylan hydrolysis when incubated with a Xyn10A catalytic module. This is the first report of family 4 CBMs with affinity for both insoluble xylan and amorphous cellulose.
  • Aranburu, A., et al. (författare)
  • Transcription factor AP-4 is a ligand for immunoglobulin-κ promoter E-box elements
  • 2001
  • Ingår i: Biochemical Journal. - : Portland Press. - 0264-6021. ; 354:2, s. 431-438
  • Tidskriftsartikel (refereegranskat)abstract
    • Immunoglobulin (Ig)-κ promoters from humans and mice share conserved sequences. The octamer element is common to all Ig promoters and pivotal for their function. However, other conserved sequence motifs, that differ between lg variable gene families, are required for normal promoter function. These conserved motifs do not stimulate transcription in the absence of an octamer. One example is an E-box of the E47/E12 type (5′-CAGCTG-3′), which is found in all promoters of the human and murine Ig-κ gene subgroups/families, with the exception of subgroups II and VI and their related murine families. In the present study we show that the ubiquitously expressed transcription factor AP-4, and not E47, interacts specifically with the κ promoter E-boxes when tested in electrophoretic mobility-shift assays using nuclear extracts derived from human and murine B-cell lines. Furthermore, AP-4, unlike E47, did not act as a transactivator, which is in agreement with previous studies on intact κ promoters, showing that transcription is absent when the octamer element has been mutated. Based on these data, and the conservation of the 5′-CAGCTG-3′ motif among human and murine κ promoters, we propose that AP-4 is the major ligand for Ig-κ promoter E-boxes.
  • Davies, Julia R., et al. (författare)
  • Identification of MUC5B, MUC5AC and small amounts of MUC2 mucins in cystic fibrosis airway secretions
  • 1999
  • Ingår i: Biochemical Journal. - : Portland Press. - 0264-6021. ; 344:2, s. 321-330
  • Tidskriftsartikel (refereegranskat)abstract
    • To investigate the genetic identities of the mucins secreted in cystic fibrosis (CF) airways, sputum was collected from five individuals. Samples were separated into gel and sol phases by high-speed centrifugation and the gel phase was extracted in 6 M guanidinium chloride. The 'insoluble' residue remaining after extraction of the gel phase was brought into solution by reduction/alkylation. Density-gradient centrifugation in CsCl revealed polydisperse distributions of sialic acid-containing mucins in the gel phase, insoluble residue and sol phase fractions and the degree of variation between the different individuals was low. Antibodies recognizing MUC5AC and MUC5B identified these mucins in each of the fractions. MUC2, however, was present only as a component of the insoluble residue from the gel which accounted for less than 4% by mass of the total mucins, MUC5B and MUC5AC from the gel phase were large oligomeric species composed of disulphide-bond linked subunits and MUC5B was present as two populations with different charge densities which are likely to correspond to MUC5B 'glycoforms'. The sol phase contained, in addition to MUC5AC and MUC5B mainly smaller mucins which did not react with the antisera and which were probably degraded. MUC5AC appeared to be enriched in the sol, suggesting that this mucin may be more susceptible to proteolytic degradation than MUC5B. The mucins present in sputum remained broadly similar during acute exacerbation and following antibiotic treatment, although the relative amount of an acidic MUC5B glycoform was decreased during infection.
  • Becker, D., et al. (författare)
  • Engineering of a glycosidase Family 7 cellobiohydrolase to more alkaline pH optimum : the pH behaviour of Trichoderma reesei CeI7A and its E223S/A224H/L225V/T226A/D262G mutant
  • 2001
  • Ingår i: Biochemical Journal. - 0264-6021 .- 1470-8728. ; 356, s. 19-30
  • Tidskriftsartikel (refereegranskat)abstract
    • The crystal structures of Family 7 glycohydrolases suggest that a histidine residue near the acid/base catalyst could account for the higher pH optimum of the Humicola insolens endoglucanase Cel7B, than the corresponding Trichoderma reesei enzymes. Modelling studies indicated that introduction of histidine at the homologous position in T. reesei Cel7A (Ala(224)) required additional changes to accommodate the bulkier histidine side chain. X-ray crystallography of the catalytic domain of the E223S/A224H/L225V/T226A/D262G mutant reveals that major differences from the wild-type are confined to the mutations themselves, The introduced histidine residue is in plane with its counterpart in H. insolens Cel7B, but is 1.0 Angstrom (= 0.1 nm) closer to the acid/base Glu(217) residue, with a 3.1 Angstrom contact between N-2 and O'(1). The pH variation of k(cat)/K-m for 3,4-dinitrophenyl lactoside hydrolysis was accurately bell-shaped for both wildtype and mutant, with pK(1) shifting from 2.22+/-0.03 in the wild-type to 3.19+/-0.03 in the mutant, and pK(2) shifting from 5.99+/-0.02 to 6.78+/-0.02. With this poor substrate, the ionizations probably represent those of the free enzyme. The relative k(cat) for 2-chloro-4-nitrophenyl lactoside showed similar behaviour. The shift in the mutant pH optimum was associated with lower k(cat)/K-m values for both lactosides and cellobiosides, and a marginally lower stability. However, k(cat) values for cellobiosides are higher for the mutant. This we attribute to reduced nonproductive binding in the +1 and +2 subsites; inhibition by cellobiose is certainly relieved in the mutant. The weaker binding of cellobiose is due to the loss of two water-mediated hydrogen bonds.
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