Formation of Aliphatic and Aromatic α-Hydroxy Ketones by Zygosaccharomyces bisporus

The wild-type yeast strain Zygosaccharomyces bisporus CBS 702 produced a-hydroxy-ketones (acyloins) from amino acid precursors after transamination to the corresponding 2-oxo acids. The key enzyme of the subsequent decarboxylation and C-C bond forming reaction, pyruvate decarboxylase (PDC), was examined for its substrate- and stereo-specific-ity. A wide variety of saturated and unsaturated aliphatic and aromatic aldehydes was successfully converted to acyloins. 19 of the biotransformation products identified had not been reported as natural substances before. Product yields were strongly affected by substrate structure. © 2000, Verlag der Zeitschrift für Naturforschung. All rights reserved

Purification, characterisation and cDNA sequencing of pyruvate decarboxylase from Zygosaccharomyces bisporus

Cells of the wild-type yeast strain Zygosaccharomyces bisporus CBS 702 form alpha-hydroxy ketones from aromatic amino acid precursors during fermentation, Pyruvate decarboxylase (PDC, E.C. 4.1.1.1), the key enzyme of this biotransformation catalysing the nonoxidative decarboxylation of pyruvate and other 2-oxo-acids, was purified and characterised. The active enzyme is homotetrameric (alpha(4)) with a molecular mass of about 244 kDa, Activation of PDC by its substrate pyruvate results in a sigmoidal dependence of the reaction rate from substrate concentration (apparent K-m value 1.73 mM; Hill coefficient 2.10). A cDNA library was screened using a PCR-based procedure, and a 1856 bp cDNA of PDC was identified and sequenced. The cDNA encodes a polypeptide of 563 amino acid residues (monomeric unit), Sequence alignments demonstrate high homologies (> 80%) to PDC genes from Saccharomyces cerevisiae, Kluyveromyces lactis and Kluyveromyces marxianus.DF

Adaptive Mesh Refinement for Singular Current Sheets in Incompressible Magnetohydrodynamic Flows

The formation of current sheets in ideal incompressible magnetohydrodynamic flows in two dimensions is studied numerically using the technique of adaptive mesh refinement. The growth of current density is in agreement with simple scaling assumptions. As expected, adaptive mesh refinement shows to be very efficient for studying singular structures compared to non-adaptive treatments.Comment: 8 pages RevTeX, 13 Postscript figure

Model simulations of chemical effects of sprites in relation with observed HO2 enhancements over sprite-producing thunderstorms

Recently, measurements by the Superconducting Submillimeter-Wave Limb Emission Sounder (SMILES) satellite instrument have been presented which indicate an increase in mesospheric HO2 above sprite-producing thunderstorms. The aim of this paper is to compare these observations to model simulations of chemical sprite effects. A plasma chemistry model in combination with a vertical transport module was used to simulate the impact of a streamer discharge in the altitude range 70–80 km, corresponding to one of the observed sprite events. Additionally, a horizontal transport and dispersion model was used to simulate advection and expansion of the sprite air masses. The model simulations predict a production of hydrogen radicals mainly due to reactions of proton hydrates formed after the electrical discharge. The net effect is a conversion of water molecules into H+OH. This leads to increasing HO2 concentrations a few hours after the electric breakdown. Due to the modelled long-lasting increase in HO2 after a sprite discharge, an accumulation of HO2 produced by several sprites appears possible. However, the number of sprites needed to explain the observed HO2 enhancements is unrealistically large. At least for the lower measurement tangent heights, the production mechanism of HO2 predicted by the model might contribute to the observed enhancements

Lebensmittelchemie 2012

Neben der Identifizierung von Aromastoffen durch die molekulare Sensorik sind die Aufklärung ihrer ‧Biosynthese sowie ihre biotechnische Produktion ins Zentrum der Aromastoffforschung gerückt. Geruch und Geschmack vermitteln die Qualität eines Lebensmittels. Den Geruch bestimmen flüchtige Aromastoffe, die heute vielen industriell hergestellten Lebensmitteln zugesetzt werden, um herstellungsbedingte Verluste auszugleichen oder das Aroma abzurunden. Der Verbraucher akzeptiert dabei eher natürliche Aromen. Der Begriff „natürlich“ darf nach EU-Recht allerdings nur dann zur Bezeichnung eines Aromas verwendet werden, wenn es ausschließlich Aromaextrakte oder natürliche Aromastoffe enthält.1) Dazu zählen biotechnisch erzeugte Aromastoffe, die in puncto Nachhaltigkeit für Aromenhersteller eine Alternative sind zu den klassischen natürlichen Aromen aus pflanzlichen Rohstoffen wie ätherischen Ölen. Biotechnische Verfahren dienen vermehrt dazu, strukturell komplexe Aromastoffe billiger und in größeren Mengen herzustellen, als dies mit chemischen Synthesen oder durch Pflanzenextraktion möglich ist.2) Fortschritte in Enzymtechnik, Gentechnik, Bioprozess-Monitoring und Produktisolierung bieten viel versprechende Möglichkeiten für biotechnische Aromastoffproduktion.3) Spezifische Strategien sind hierfür aber nur dann weiterzuentwickeln, wenn Enzymologie und Regulation der Aromastoffbiosynthese auf molekularer Ebene verstanden sind. Daher ist die biochemische Forschung zur Aufklärung von Biosynthesewegen in Pflanzen ein wichtiger Bestandteil der Aromabiotechnik

A comparison of cell wall disruption techniques for the isolation of intracellular metabolites from Pleurotus and Lepista sp.

Different techniques were compared for their effectiveness in the disruption of the rigid cell walls of Basidiomycetes, Grinding under liquid nitrogen, stirred glass bead milling and enzymatic cell lysis were applied to the mycelia of Pleurotus sapidus and Lepista irina grown submerged. Each of the disruption procedures was evaluated by testing the quantity and quality of released intracellular metabolites: DNA, RNA, enzymes, and secondary metabolites. The most suitable method for nucleic acid isolation was grinding under liquid nitrogen, while bead mill homogenization was the superior technique for isolation of active enzymes. A new effective method is proposed for isolation of secondary metabolites with the aid of bead milling of fungal mycelia. © 2006 Verlag der Zeitschrift für Naturforschung