513 research outputs found
Characterization of 4,6-α-glucanotransferase enzymes and their functional role in Lactobacillus reuteri
Full text linkStarch, in rice, potato or corn, has been and is the main dietary carbohydrate of human beings. However, the abundant and quick release of high calorie glucose from starch after food intake by human beings is currently considered to be unhealthy and may cause chronic diseases, such as obesitas, diabetes or cardiovascular disease. Therefore, a reduction of the digestibility of starch via various modifications is highly demanded. In view of cost, sustainability and substrate availability, physical treatment of starch to produce resistant or slowly digestible starch, is most practical. However, the physically processed starch suffers losses upon boiling, baking or other food processing, which limits its application in many food products. Thus, structural modification through enzymatic treatment of starch has received increasing attention. For this purpose, the 4,6-α-glucanotransferase enzymes are explored and deeply studied in the Microbial Physiology group, headed by Prof. Lubbert Dijkhuizen, in the University of Groningen. These enzymes are capable of converting (α1→4) linkages of a starch substrate into (α1→6) linkages, forming a modified type of starch which is shown to be a soluble dietary fiber. It shows the potential of the 4,6-α-glucanotransferases to be of commercial value. However, industrial application is still a challenge because these enzymes are only available in relatively minor amounts. There is also a general lack of reliable enzyme activity assays needed to study such enzymes. These topics are addressed in this PhD thesis, also focusing on elucidation of the 4,6-α-glucanotransferase reaction mechanism by 3D structural protein analysis and mutagenesis
The evolution of cyclodextrin glucanotransferase product specificity
Full text linkCyclodextrin glucanotransferases (CGTases) have attracted major interest from industry due to their unique capacity of forming large quantities of cyclic α-(1,4)-linked oligosaccharides (cyclodextrins) from starch. CGTases produce a mixture of cyclodextrins from starch consisting of 6 (α), 7 (β) and 8 (γ) glucose units. In an effort to identify the structural factors contributing to the evolutionary diversification of product specificity amongst this group of enzymes, we selected nine CGTases from both mesophilic, thermophilic and hyperthermophilic organisms for comparative product analysis. These enzymes displayed considerable variation regarding thermostability, initial rates, percentage of substrate conversion and ratio of α-, β- and γ-cyclodextrins formed from starch. Sequence comparison of these CGTases revealed that specific incorporation and/or substitution of amino acids at the substrate binding sites, during the evolutionary progression of these enzymes, resulted in diversification of cyclodextrin product specificity.
Microbial demethylation of dimethylsulfoniopropionate and methylthiopropionate
Full text linkAs discussed in chapter 1 , there is an increased interest in the production of certain natural sulfur-containing flavor compounds or flavor precursors. Production of natural flavors is becoming increasingly important, because consumerts end to prefer natural compounds for health reasons. With the aid of extraction techniques it is possible to obtain flavors directly from plant material, but these methods are time consuming and expensive, because the most interesting flavors are present in only very low concentrations. A more recent method to produce flavors is based on a biotechnological approach where natural precursors, isolated mainly from plant material, can be convertedt o the desired flavor in a bioreactor with the aid of enzymes and/or microorganisms.
Functional analysis of the integrative plasmid PMEA300 of the actinomycete Amycolatopsis Methanolica
Full text linkWe are interested in the regulation of primary metabolism in A. methanolica, especially of the pathways for biosynthesis of aromatic amino acids and catabolism of glucose and methanol (De Boeretal., 1989; Dijkhuizen et al,. 1993). Attempts to construct amino acids overproducting strains were hampered by a general lack of methods and tools for genetic studies with A. methanolica. Aim of this work therefore was the construction of A. methanoloca-E. coli shuttle vectors allowing the cloning and molecular characterization of A. methanolica genes encoding enzymes involved in L-phenylalanine biosynthesis.
Zie: Summary and concluding remarks
Regulation of autothrophic and heterotrophic metabolism in pseudomonas oxalaticus OX1
Full text linkThe present study established the involvement of a cytoplasmatic membrane-boud formate dehydrogenase in the metabolism of both oxalate and formate. In studies with cytoplasmic membrane vesicles it was shown (chapter 9) that this enzyme feeds the electrons derived from formate oxidation into the respiratory chain at the level of cytochrome b. Since the affinity constant (Km) for formate of the cytoplasmic NAD-dependent formate dehydrogenase and the membrane-bound enzyme are of the same order of magnitude, it has to be expected that both enzymes compete for formate inside the cell. Physiologically, the two important processes of generation of reducing power for biosynthesis and membrane energization for ATP synthesis and other energy requiring purposes may thus be accomplished by the two seperate enzymes. ...
Zie: Summary
Microbial ecology of oxygen-sulfide interfaces in marine benthic ecosystems.
Full text linkTransport of dissolved compounds in sediments is mainly by diffusion. Because this is a relatively slow process, the availability of oxygen as electronacceptor to bacteria in these soils is limited. Most often oxygen therefore does not penetrate deeper than a few millimeters in the soil. ...
Zie: Summary
Process studies of dissolved organic carbon and bacterioplankton in the ocean.
Full text linkThe oceans play an important role in the global climate system by mediating as buffer of heat and greenhouse gasses (e.g. CO). Oceanic water constirutes one of the largest
carbon (C) reservoirs of the earth and the fluxes of carbon in the marine system affect the global biogeochemical cycle of carbon. Oceanic uptake of atmosphetic CO2 results mainly
from photosynthesis by phytoplankton (primary production). A portion of this newly fixed organic matter is eventually transformed into a dissolved phase. The export of particulate and dissolved organic carbon, from the surface layers to deep water and eventually burial of carbon on the sea floor, results in a net reduction of the atmospheric CO2, concentration. To quantifit and understand the physical and biological controls on these fluxes is the primary objective of the Joint Global Ocean Fiux Study (GOFS). The research encompassed in this thesis was carried out within the JGOFS framework or related projects. ...
Zie: Summary.
Phytoplankton cell lysis and its ecological implocations.
Full text linkDe populatiedynamiek van eencellige algen (fytoplankton, 0.001 mm tot I mm in diameter) in zee wordt gereguleerdd oor factorend ie de groei beinvloeden en door processen die voor een reductie van de algenbiomassa zorgen. Deze laatste worden dan ook verliesfactoren genoemd. Twee algemeen erkende processen die verantwoordelijk zijn voor een afname in
de concentratie van algencellen in een bepaald gebied zijn begrazing van fytoplankton door eencellige diertjes (zodplankton) en sedimentatie van algencellen uit de
waterkolom naar de bodem- ...
Zie: Samenvatting
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