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Sökning: WFRF:(Sahlin Fredrik)

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  • Oliw, Ernst H., 1948-, et al. (författare)
  • Manganese lipoxygenase oxidizes bis-allylic hydroperoxides and octadecenoic acids by different mechanisms
  • 2011
  • Ingår i: Biochimica et Biophysica Acta - Molecular and Cell Biology of Lipids. - 1388-1981 .- 1879-2618. ; 1811:3, s. 138-147
  • Tidskriftsartikel (refereegranskat)abstract
    • Manganese lipoxygenase (MnLOX) oxidizes (11R)-hydroperoxylinolenic acid (11R-HpOTrE) to a peroxyl radical. Our aim was to compare the enzymatic oxidation of 11R-HpOTrE and octadecenoic acids with LOO-H and allylic C-H bond dissociation enthalpies of ~88 and ~87kcal/mol. Mn(III)LOX oxidized (11Z)-, (12Z)-, and (13Z)-18:1 to hydroperoxides with R configuration, but this occurred at insignificant rates (<1%) compared to 11R-HpOTrE. We next examined whether transitional metals could mimic this oxidation. Ce(4+) and Mn(3+) transformed 11R-HpOTrE to hydroperoxides at C-9 and C-13 via oxidation to a peroxyl radical at C-11, whereas Fe(3+) was a poor catalyst. Our results suggest that MnLOX oxidizes bis-allylic hydroperoxides to peroxyl radicals in analogy with Ce(4+) and Mn(3+). The enzymatic oxidation likely occurs by proton-coupled electron transfer of the electron from the hydroperoxide anion to Mn(III) and H(+) to the catalytic base, Mn(III)OH(-). Hydroperoxides abolish the kinetic lag times of MnLOX and FeLOX by oxidation of their metal centers, but 11R-HpOTrE was isomerized by MnLOX to (13R)-hydroperoxy-(9Z,11E,15Z)-octadecatrienoic acid (13R-HpOTrE) with a kinetic lag time. This lag time could be explained by two competing transformations, dehydration of 11R-HpOTrE to 11-ketolinolenic acid and oxidation of 11R-HpOTrE to peroxyl radical; the reaction rate then increases as 13R-HpOTrE oxidizes MnLOX with subsequent formation of two epoxyalcohols. We conclude that oxidation of octadecenoic acids and bis-allylic hydroperoxides occurs by different mechanisms, which likely reflect the nature of the hydrogen bonds, steric factors, and the redox potential of the Mn(III) center.
  • Almqvist, Andreas, et al. (författare)
  • An Abbott curve based rough surface contact mechanics approach
  • 2005
  • Ingår i: Proceedings of the World Tribology Congress III - 2005 : presented at ..., September 12 - 16, 2005, Washington, DC. - New York : American Society of Mechanical Engineers. ; s. 397-398
  • Konferensbidrag (refereegranskat)abstract
    • In this way all the height information of the surface profile is preserved and not only a few parameters, like Ra, Rq, Rz, Rsk, etc. The aim of this work is to investigate how classes of surfaces based on a single Abbott curve perform in terms of contact mechanical parameters like the real area of contact. The result shows that surfaces taken from a class of random surfaces generated from a specific Abbott curve behaves similar in a contact mechanics simulation. That is, the distribution of for example the real area of contact within such a class is compact, having a small deviation from its mean.This implies that it is possible to simulate classes of surfaces based on Abbott curves and to use the results to predict contact mechanical properties of real surface topographies.
  • Almqvist, Andreas, et al. (författare)
  • Development of a lubrication simulation model
  • 2009
  • Ingår i: Svenska mekanikdagarna : Södertälje 2009. - Stockholm : Svenska nationalkommittén för mekanik. ; s. 74
  • Konferensbidrag (övrigt vetenskapligt)
  • Almqvist, Andreas, et al. (författare)
  • On the dry elasto-plastic contact of nominally flat surfaces
  • 2007
  • Ingår i: Tribology International. - 0301-679X. ; 40:4, s. 574-579
  • Tidskriftsartikel (refereegranskat)abstract
    • A model to be used for numerical simulation of the contact of linear elastic perfectly plastic rough surfaces was developed. Energy dissipation due to plastic deformation is taken into account. Spectral theory and an FFT-techique are used to facilitate the numerical solution process. Results of simulations using four two-dimensional profiles with different topographies in contact with a rigid plane for a number loads are reported. From the results it is clear that the real area of contact (Ar) changes almost linearly with load and is only slightly affected by the difference in topography. A plasticity index is defined as the ratio of plastically deformed area (Ap) and Ar. Plastic deformation occurs even at low loads and there is a significant difference in plasticity index between the surface profiles considered. An investigation on how the spectral content of the surface profile influences the results presented is also performed. This is to ensure that the metrological limitations of the optical profiler used to measure the surfaces do not have a significant influence. It is concluded that the highest frequencies of the measured profile have a negligible influence on the real area of contact.
  • Almqvist, Andreas, et al. (författare)
  • On the dry elasto-plastic contact of nominally flat surfaces
  • 2004
  • Ingår i: Proceedings of the 11th Nordic symposium on tribology : NORDTRIB 2004 : Tromsø, Harstad, Hurtigruten, [Bodø], Norway, June [1 - 5], 2004. ; s. 753-762
  • Konferensbidrag (refereegranskat)
  • Aurelius, Oskar, et al. (författare)
  • The Crystal Structure of Thermotoga maritima Class III Ribonucleotide Reductase Lacks a Radical Cysteine Pre-Positioned in the Active Site
  • 2015
  • Ingår i: PLoS ONE. - Public Library of Science. - 1932-6203. ; 10:7
  • Tidskriftsartikel (refereegranskat)abstract
    • Ribonucleotide reductases (RNRs) catalyze the reduction of ribonucleotides to deoxyribonucleotides, the building blocks for DNA synthesis, and are found in all but a few organisms. RNRs use radical chemistry to catalyze the reduction reaction. Despite RNR having evolved several mechanisms for generation of different kinds of essential radicals across a large evolutionary time frame, this initial radical is normally always channelled to a strictly conserved cysteine residue directly adjacent to the substrate for initiation of substrate reduction, and this cysteine has been found in the structures of all RNRs solved to date. We present the crystal structure of an anaerobic RNR from the extreme thermophile Thermotoga maritima (tmNrdD), alone and in several complexes, including with the allosteric effector dATP and its cognate substrate CTP. In the crystal structure of the enzyme as purified, tmNrdD lacks a cysteine for radical transfer to the substrate pre-positioned in the active site. Nevertheless activity assays using anaerobic cell extracts from T. maritima demonstrate that the class III RNR is enzymatically active. Other genetic and microbiological evidence is summarized indicating that the enzyme is important for T. maritima. Mutation of either of two cysteine residues in a disordered loop far from the active site results in inactive enzyme. We discuss the possible mechanisms for radical initiation of substrate reduction given the collected evidence from the crystal structure, our activity assays and other published work. Taken together, the results suggest either that initiation of substrate reduction may involve unprecedented conformational changes in the enzyme to bring one of these cysteine residues to the expected position, or that alternative routes for initiation of the RNR reduction reaction may exist. Finally, we present a phylogenetic analysis showing that the structure of tmNrdD is representative of a new RNR subclass IIIh, present in all Thermotoga species plus a wider group of bacteria from the distantly related phyla Firmicutes, Bacteroidetes and Proteobacteria.
  • Crona, Mikael, et al. (författare)
  • A Rare Combination of Ribonucleotide Reductases in the Social Amoeba Dictyostelium discoideum
  • 2013
  • Ingår i: Journal of Biological Chemistry. - 0021-9258. ; 288:12, s. 8198-8208
  • Tidskriftsartikel (refereegranskat)abstract
    • Ribonucleotide reductases (RNRs) catalyze the only pathway for de novo synthesis of deoxyribonucleotides needed for DNA replication and repair. The vast majority of eukaryotes encodes only a class I RNR, but interestingly some eukaryotes, including the social amoeba Dictyostelium discoideum, encode both a class I and a class II RNR. The amino acid sequence of the D. discoideum class I RNR is similar to other eukaryotic RNRs, whereas that of its class II RNR is most similar to the monomeric class II RNRs found in Lactobacillus spp. and a few other bacteria. Here we report the first study of RNRs in a eukaryotic organism that encodes class I and class II RNRs. Both classes of RNR genes were expressed in D. discoideum cells, although the class I transcripts were more abundant and strongly enriched during mid-development compared with the class II transcript. The quaternary structure, allosteric regulation, and properties of the diiron-oxo/radical cofactor of D. discoideum class I RNR are similar to those of the mammalian RNRs. Inhibition of D. discoideum class I RNR by hydroxyurea resulted in a 90% reduction in spore formation and decreased the germination viability of the surviving spores by 75%. Class II RNR could not compensate for class I inhibition during development, and an excess of vitamin B12 coenzyme, which is essential for class II activity, did not improve spore formation. We suggest that class I is the principal RNR during D. discoideum development and growth and is important for spore formation, possibly by providing dNTPs for mitochondrial replication.
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