AbstractThe yeast, Saccharomyces cerevisiae requires certain elements for the growth and development of healthy cultures. The divalent cation, zinc is of paramount importance to this yeast, as zinc is a structurally and functionally essential metal that cannot be replaced by any other element. Zinc accumulation by S. cerevisiae is a biphasic response, consisting of a rapid metabolism independent and a metabolism dependent phase. Metabolism-independent metal ion accumulation is a physical process, whereby the ions are associated with the cell wall. This stage of uptake is often referred to as biosorption and zinc uptake is influenced by temperature, pH, biomass concentration and the presence of competing ions. The second phase of zinc uptake (metabolism-dependent metal ion accumulation) concerns the intracellular accumulation of the ions. This biological accumulation, often abbreviated to bioaccumulation, is slower than biosorption as the zinc ions are transported into the cell, via the plasma membrane by the energy consuming process, active transport. The presence and type of metabolisable energy source, metabolic inhibitors, as well as the factors that affect biosorption also affect bioaccumulation. The genetics governing zinc accumulation by S. cerevisiae has recently been unravelled (Zhao and Eide, 1996a & b). Research has shown that a high (ZRT1) and a low (ZRT2) affinity transporter proteins exists, which act in zinc limiting and zinc replete conditions, respectively. Once the transporters aid zinc uptake into the cell, this important divalent cation is either utilised immediately or compartmentalised in the vacuole until required. Zinc accumulation is influenced by yeast cell physiology. Upon examination of zinc uptake with respect to cell growth, in various metabolisable energy sources, the results demonstrate that zinc is influential in the growth of industrial relevant strains of S. cerevisiae, and that zinc accumulation is affected by the presence and type of metabolisable energy source e.g. glucose, fructose, maltose and sucrose. Optimal growth was achieved when the lager yeast and wine yeast was grown in a minimal media containing sucrose as the metabolisable energy source after a 24 hour period with distillers yeast and bakers yeast growth was maximum when grown for 24 hours in a fructose supplemented media. The industrial strains of yeast studied appeared to sequester maximum zinc when the YPDM was supplemented with monosaccharides, as opposed to disaccharide, after a 24 hour examination period.
The accumulation of zinc by S. cerevisiae lager yeast is a cyclical event with uptake occurring during lag and early exponential phase of growth, with zinc appearing to convey a protective effect on cells which have been subjected to a chemical (15% ethanol) and a physical (heat shock- 45°C) stress. The influence of zinc accumulation on yeast cell physiology was studied with respect to specific enzyme (Alcohol Dehydrogenase) and metabolite (ethanol) production. The results demonstrate a general trend, with more ADH produced when the cells have sequestered more zinc, this in turn had a positive effect on the overall ethanol production of a strain of lager yeast.
|Date of Award||Nov 2001|
|Sponsors||European Social Fund (ESF)|
|Supervisor||Graeme Walker (Supervisor)|