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Oligomer Formation (oligomer + formation)
Selected AbstractsHomo-oligomer formation by basigin, an immunoglobulin superfamily member, via its N-terminal immunoglobulin domainFEBS JOURNAL, Issue 14 2000Seiya Yoshida Basigin (Bsg) is a highly glycosylated transmembrane protein with two immunoglobulin (Ig)-like domains. A number of studies, including gene targeting, have demonstrated that Bsg plays pivotal roles in spermatogenesis, implantation, neural network formation and tumor progression. In the present study, to understand the mechanism of action of Bsg, we determined its expression status on the plasma membrane. Cotransfection of Bsg expression vectors with two different tags clarified that Bsg forms homo-oligomers in a cis -dependent manner on the plasma membrane. If the disulfide bond of the more N-terminally located Ig-like domain was destroyed by mutations, Bsg could not form oligomers. In contrast, the mutations of the C-terminal Ig-like domain or N-glycosylation sites did not affect the association. The association of mouse and human Bsgs, which exhibit high homology in the transmembrane and intracellular domains but low homology in the extracellular domain, was very weak as compared with that within the same species, suggesting the importance of the extracellular domain in the association. If the extracellular domain of the human Ret protein was replaced with the N-terminal Ig-like domain of Bsg, the resulting chimera protein was associated with intact wild-type Bsg, but not if the C-terminal Ig-like domain, instead of the N-terminal one, of Bsg was used. No oligomer formation took place between the intact wild-type Ret and Bsg proteins. In conclusion, these data indicate that the N-terminal Ig-like domain is necessary and sufficient for oligomer formation by Bsg on the plasma membrane. [source] Mechanistic understanding of degradation in bioerodible polymers for drug deliveryAICHE JOURNAL, Issue 12 2002Domenico Larobina A new model was developed to understand the mechanism of erosion in bioerodible polymers, which is essential to accurately predict drug release and precisely design controlled release devices. This model takes into account the phenomenon of microphase separation observed for polyanhydrides of certain copolymer compositions, and assumes that erosion is dominated by degradation and, thus, in a system with a fast eroding and a slow eroding species, two rate constants,one for each species,essentially control the evolution of the polymer microstructure. Expressions were derived for the fraction of each monomer released, as well as for the porosity in the system. A partition coefficient accounts for thermodynamic partitioning of a drug into the microdomains. The solutions of the model equations were fitted to experimental data on monomer release kinetics from two polyanhydride systems to obtain the erosion rate constants. Drug release kinetics experiments are compared to the model solution for drug release, and the partition coefficient of the drug is obtained from the fits. The comparisons to the data are promising, while pointing out the limitations of the model. The model does not account for oligomer formation prior to monomer release or for the dependence of the rate constants on parameters such as the degree of crystallinity, the local pH, and the polymer molecular weight. [source] Effects of external donors and hydrogen concentration on oligomer formation and chain end distribution in propylene polymerization with Ziegler-Natta catalystsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 2 2010Torvald Vestberg Abstract The effect of type and concentration of external donor and hydrogen concentration on oligomer formation and chain end distribution were studied. Bulk polymerization of propylene was carried out with two different Ziegler-Natta catalysts at 70 °C, one a novel self-supported catalyst (A) and the other a conventional MgCl2 -supported catalyst (B) with triethyl aluminum as cocatalyst. The external donors used were dicyclopentyl dimethoxy silane (DCP) and cyclohexylmethyl dimethoxy silane (CHM). The oligomer amount was shown to be strongly dependent on the molecular weight of the polymer. Catalyst A gave approximately 50 % lower oligomer content than catalyst B due to narrower molecular weight distribution in case of catalyst A. More n -Bu-terminated chain ends were found for catalyst A indicating more frequent 2,1 insertions. Catalyst A also gave more vinylidene-terminated oligomers, suggesting that chain transfer to monomer, responsible for the vinylidene chain ends, was a more important chain termination mechanism for this catalyst, especially at low hydrogen concentration. Low site selectivity, due to low external donor concentration or use of a weak external donor (CHM), was also found to increase formation of vinylidene-terminated oligomers. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 351,358, 2010 [source] Cobalt(II) octanoate and cobalt(II) perfluorooctanoate catalyzed atom transfer radical polymerization of styrene in toluene and fluorous media,A versatile route to catalyst recycling and oligomer formationJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 17 2005Marc-Stephan Weiser Abstract Cobalt(II) perfluorooctanoate-catalyzed atom transfer radical polymerization (ATRP) and reverse ATRP were developed to prepare oligostyrenes (Mn < 2500) with low polydispersities Mw/Mn < 1.5. Fluorous biphase catalysis was applied for effective recycling of catalyst and fluorous solvent. The homogeneous polymerization reaction was performed at 90 °C in toluene/cyclohexane/perfluorodecalin mixture (1:1:1) and fluorine-free solvents. Temperature-induced phase separation of this fluorous solvent mixture occurred at room temperature and proved to be the key for the very effective separation of the cobalt(II) perfluorooctanoate from the oligostyrene and fluorine-free solvents. Both the fluorine-tagged cobalt catalysts and the fluorous media were recycled and reused up to three times without encountering catalyst activity losses. The roles of cobalt catalysts, fluorous media, and monomer/initiator ratio were examined with respect to the polymerization kinetics. Fluorine-containing and fluorine-free cobalt(II) octanoate catalyzed controlled styrene oligomerization according to the ATRP mechanism. The molar mass control range was limited in fluorous biphase catalysis most likely because of precipitation of high molar mass polystyrenes in the fluorous reaction medium. To the best of our knowledge, this is the first time temperature-induced phase separation of fluorous and fluorine-free solvents has been successfully applied to polymerization processing. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3804,3813, 2005 [source] Tissue transglutaminase modulates ,-synuclein oligomerizationPROTEIN SCIENCE, Issue 8 2008Ine M.J. Segers-Nolten Abstract We have studied the interaction of the enzyme tissue transglutaminase (tTG), catalyzing cross-link formation between protein-bound glutamine residues and primary amines, with Parkinson's disease-associated ,-synuclein protein variants at physiologically relevant concentrations. We have, for the first time, determined binding affinities of tTG for wild-type and mutant ,-synucleins using surface plasmon resonance approaches, revealing high-affinity nanomolar equilibrium dissociation constants. Nanomolar tTG concentrations were sufficient for complete inhibition of fibrillization by effective ,-synuclein cross-linking, resulting predominantly in intramolecularly cross-linked monomers accompanied by an oligomeric fraction. Since oligomeric species have a pathophysiological relevance we further investigated the properties of the tTG/,-synuclein oligomers. Atomic force microscopy revealed morphologically similar structures for oligomers from all ,-synuclein variants; the extent of oligomer formation was found to correlate with tTG concentration. Unlike normal ,-synuclein oligomers the resultant structures were extremely stable and resistant to GdnHCl and SDS. In contrast to normal ,-sheet-containing oligomers, the tTG/,-synuclein oligomers appear to be unstructured and are unable to disrupt phospholipid vesicles. These data suggest that tTG binds equally effective to wild-type and disease mutant ,-synuclein variants. We propose that tTG cross-linking imposes structural constraints on ,-synuclein, preventing the assembly of structured oligomers required for disruption of membranes and for progression into fibrils. In general, cross-linking of amyloid forming proteins by tTG may prevent the progression into pathogenic species. [source] Rational improvement of simvastatin synthase solubility in Escherichia coli leads to higher whole-cell biocatalytic activityBIOTECHNOLOGY & BIOENGINEERING, Issue 1 2009Xinkai Xie Abstract Simvastatin is the active pharmaceutical ingredient of the blockbuster cholesterol lowering drug Zocor. We have previously developed an Escherichia coli based whole-cell biocatalytic platform towards the synthesis of simvastatin sodium salt (SS) starting from the precursor monacolin J sodium salt (MJSS). The centerpiece of the biocatalytic approach is the simvastatin synthase LovD, which is highly prone to misfolding and aggregation when overexpressed from E. coli. Increasing the solubility of LovD without decreasing its catalytic activity can therefore elevate the performance of the whole-cell biocatalyst. Using a combination of homology structural prediction and site-directed mutagenesis, we identified two cysteine residues in LovD that are responsible for nonspecific intermolecular crosslinking, which leads to oligomer formation and protein aggregation. Replacement of Cys40 and Cys60 with alanine residues resulted in marked gain in both protein solubility and whole-cell biocatalytic activities. Further mutagenesis experiments converting these two residues to small or polar natural amino acids showed that C40A and C60N are the most beneficial, affording 27% and 26% increase in whole cell activities, respectively. The double mutant C40A/C60N combines the individual improvements and displayed ,50% increase in protein solubility and whole-cell activity. Optimized fed-batch high-cell-density fermentation of the double mutant in an E. coli strain engineered for simvastatin production quantitatively (>99%) converted 45 mM MJSS to SS within 18 h, which represents a significant improvement over the performance of wild-type LovD under identical conditions. The high efficiency of the improved whole-cell platform renders the biocatalytic synthesis of SS an attractive substitute over the existing semisynthetic routes. Biotechnol. Bioeng. 2009;102: 20,28. © 2008 Wiley Periodicals, Inc. [source] | ||||||||||