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ARĞA, KAZIM YALÇIN

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KAZIM YALÇIN

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2013 - 20152
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Author: TOKSOY ÖNER, EBRU × Subject: EXOPOLYSACCHARIDES ×

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    The Stimulatory Effect of Mannitol on Levan Biosynthesis: Lessons from Metabolic Systems Analysis of Halomonas smyrnensis AAD6(T)
    (WILEY, 2013) TOKSOY ÖNER, EBRU; Ates, Ozlem; Arga, Kazim Y.; Oner, Ebru Toksoy
    Halomonas smyrnensis AAD(T) is a halophilic, gram-negative bacterium that can efficiently produce levan from sucrose as carbon source via levansucrase activity. However, systems-based approaches are required to further enhance its metabolic performance for industrial application. As an important step toward this goal, the genome-scale metabolic network of Chromohalobacter salexigens DSM3043, which is considered a model organism for halophilic bacteria, has been reconstructed based on its genome annotation, physiological information, and biochemical information. In the present work, the genome-scale metabolic network of C. salexigens was recruited, and refined via integration of the available biochemical, physiological, and phenotypic features of H. smyrnensis AAD6(T). The generic metabolic model, which comprises 1,393 metabolites and 1,108 reactions, was then systematically analyzed in silico using constraints-based simulations. To elucidate the relationship between levan biosynthesis and other metabolic processes, an enzyme-graph representation of the metabolic network and a graph decomposition technique were employed. Using the concept of control effective fluxes, significant links between several metabolic processes and levan biosynthesis were estimated. The major finding was the elucidation of the stimulatory effect of mannitol on levan biosynthesis, which was further verified experimentally via supplementation of mannitol to the fermentation medium. The optimal concentration of 30 g/L mannitol supplemented to the 50 g/L sucrose-based medium resulted in a twofold increase in levan production in parallel with increased sucrose hydrolysis rate, accumulated extracellular glucose, and decreased fructose uptake rate. (c) 2013 American Institute of Chemical Engineers Biotechnol. Prog., 29:1386-1397, 2013
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    Effective stimulating factors for microbial levan production by Halomonas smyrnensis AAD6(T)
    (SOC BIOSCIENCE BIOENGINEERING JAPAN, 2015) TOKSOY ÖNER, EBRU; Sarilmiser, Hande Kazak; Ates, Ozlem; Ozdemir, Gonca; Arga, Kazim Yalcin; Oner, Ebru Toksoy
    Levan is a bioactive fructan polymer that is mainly associated with high-value applications where exceptionally high purity requirements call for well-defined cultivation conditions. In this study, microbial levan production by the halophilic extremophile Halomonas smyrnensis AAD6(T) was investigated systematically. For this, different feeding strategies in fed-batch cultures were employed and fermentation profiles of both shaking and bioreactor cultures were analyzed. Initial carbon and nitrogen source concentrations, production pH, NaCl and nitrogen pulses, nitrogen and phosphorous limitations, trace elements and thiamine contents of the basal production medium were found to affect the levan yields at different extends. Boric acid was found to be the most effective stimulator of levan production by increasing the sucrose utilization three-fold and levan production up to five-fold. This significant improvement implied the important role of quorum sensing phenomenon and its regulatory impact on levan production mechanism. Levan produced by bioreactor cultures under conditions optimized within this study was found to retain its chemical structure. Moreover, its biocompatibility was assessed for a broad concentration range. Hence H. smyrnensis AAD6(T) has been firmly established as an industrially important resource microorganism for high-quality levan production. (C) 2014, The Society for Biotechnology, Japan. All rights reserved.