Maltotriose transport in yeast

[摘要] ENGLISH ABSTRACT: The conversion of sugar into ethanol and carbon dioxide is a process that has beenintertwined with human culture and long as civilized man has existed. This fermentationprocess has been dominated by the micro-organism Saccharomyces cerevisiae and fromproviding ancient seafaring explorers of a non perishable beverage to equipping bakerswith a raising agent to turn flour into bread; this organism with its fermentative potential,has formed an essential part of most societies.In more recent times, many industries still rely on this basic principle. Thecomplexities and efficiencies of the conversion of sugar into its various fermentative byproductshave been studied and optimised extensively to meet the specific demands ofindustries. Depending on the raw material used as starting point, the major beneficiaries ofthe useful characteristics have been alcoholic beverage producers (wine, beer, andwhiskey amongst others), bakers (bread leavening) and biofuel producers.One of the obstacles in fermentation optimisation is the sugar consumptionpreferences displayed by the organism used. S. cerevisiae can consume a wide variety ofsugars. Depending on the complexities of its structures, it shows a preference for thesimpler saccharides. The fermentation of certain more complex sugars is delayed and runsthe risk of being left residually after fermentation. Many of the crops utilised infermentation-based products contain large amounts of starch. During the starchdegradation process many different forms of sugars are made available for fermentation.Improved fermentation of starch and its dextrin products would benefit the brewing,whiskey, and biofuel industries. Most strains of Saccharomyces ferment glucose andmaltose, and partially ferment maltotriose, but are unable to utilise the larger dextrinproducts of starch. This utilisation pattern is partly attributed to the ability of yeast cells totransport the aforementioned mono-, di- and trisaccharides into the cytosol. Theinefficiency of maltotriose transport has been identified as the main cause for residualmaltotriose. The maltotriose transporting efficiency also varies between differentSaccharomyces strains.By advancing the understanding of maltotriose transport in yeast, efforts can bemade to minimise incomplete fermentation. This aim can be reached by investigating theexisting transporters in the yeast cell membrane that show affinity for maltotriose. Thisstudy focuses on optimising maltotriose transport through the comparison of the alphaglucoside transporter obtained from different strains of Saccharomyces. Through specificgenetic manipulations the areas important for maltotriose transport could be identified andcharacterised.This study offers prospects for the development of yeast strains with improved maltoseand maltotriose uptake capabilities that, in turn, could increase the overall fermentationefficiencies in the beer, whiskey, and biofuel industries.

[发布机构] Stellenbosch University