Browsing by Subject "pectin"

New alternative feedstocks are needed for biofuel production to fulfil the growing demand in the coming years. The industry is moving away from second-generation biofuels, produced from food and feed crops, to using waste streams from industrial processes. An abundant, cheap and attractive waste stream for processing in Europe is the pectin-rich pulp from sugar beet processing and fruit juice industry. Sugar beet pulp is particularly rich in D-galacturonic acid and arabinose, but neither are naturally used by the yeast Saccharomyces cerevisiae, which would be an interesting candidate for the microbial fermentation of the biomass. S. cerevisiae is one of the most used organisms in the industrial biotechnology, and methods for the genetic engineering of the organism are highly developed. To overcome the natural limitations of the yeast for D-galacturonic acid fermentation, the metabolic pathways present in other organisms could be integrated in the yeast genome. Two bacterial and one fungal pathway are known to convert D-galacturonic acid into metabolites of the yeast glycolytic and ethanol fermentation pathways, and are thus considered promising for engineering in yeast. A major engineering challenge in integrating the fungal pathway in yeast is the redox imbalance caused by the two NADPH-specific reducing enzymes. The aim of this thesis was to review the potential of different D-galacturonic acid pathways for yeast fermentation. S. cerevisiae is a well-characterised organism for heterologous protein expression, but at times functional expression of foreign proteins is not achieved. One approach to study the pathways was to clone and express enzymes of the bacterial isomerase and dehydratase pathways in S. cerevisiae, and to test their activity in culture lysates. In addition, to overcome the redox imbalance in the eukaryotic pathway, two approaches were used to obtain an NADH-spesific D-galacturonic acid reductase. First, a mutant library of the Trichoderma reesei gar1 reductase was designed with the structure-guided cofactor specificity reversal tool CSR-SALAD. An automated high-throughput screening method for expression in Escherichia coli was developed, and the library was screened for enzymatic activity. The second approach was to try to identify the sequence for the characterised NADH-utilising reductase from the single-cell algae Euglena gracilis. A cDNA library of the algae was made and screened with PCR and in vivo methods. The reductase uxaB of the isomerase pathway and dehydrogenase kduD of the dehydratase pathway were functionally expressed in S. cerevisiae, with specific activities of 1.1 µmol min-1 mg-1 and 0.22 µmol min-1 mg-1 , respectively. The enzymes dehydratase uxaA and isomerase kduI did not exhibit activity in activity assays. The galurD of the dehydratase pathway was expressed in E. coli, and the purified enzyme was successfully used to convert D-galacturonate to 5-keto-4-deoxy galacturonate. The approaches to change the cofactor specificity of the NADPH-specific reductase of the eukaryotic pathway did not lead to a discovery of a NADH-specific enzyme. More research is needed for engineering active enzymes for S. cerevisiae expression and constructing a fully functional D-galacturonic acid pathway for feasible D-galacturonic acid fermentation.