GAL gene system belong to a collection of structural and regulatory genes that allows cells to make use of galactose as a carbon supply in Saccharomyces cerevisiae. Phosphorylation of intracellular galactose can be catalyzed by galactokinase (determined by GAL1 gene). To look into the impact of GAL1 interference on glucose metabolism, strains need to be developed in aerobic group cultivations on YPD medium. Deletion of GAL1 results in reduction of the growth rate. This is most likely due to an increase in the rate of ethanol production (Klein et al.
, 1999). More often than not, sugar concentrations higher than 20% (W/V) are not used under industrial circumstances because increasing the concentration of ethanol delays the growth of the yeast (Alfenore et al. , 2002). Biochemical pathways in yeasts may perhaps be regulated at various levels. These include: enzyme blend i. e. induction, repression and derepression of gene expression), enzyme activity (allotter activation, inhibition or introversions of iso-enzymes) and cellular compartmentalization e.
g. mitochondrial localization of respiratory enzymes (Johnston and Carlson, 1992). Yeast, in particular S. cerevisiae, is used in many different biotechnological processes (Geoffrey and Cregg, 1999). These can be broadly divided into two categories depending on the metabolic state of the yeast cell. Production of ethanol occurs under fermentative growth conditions. Under respiratory growth conditions, yeast biomass is produced commercially in large scale fermentation processes. S.
cerevisiae cultures are increasingly used for the production of recombinant proteins or other modern biotechnological substances. Production of sufficient amounts of S. cerevisiae biomass stands also central to, and often initiates or is prerequisite for all fermentative yeast processes and also for making enough yeast available to satisfactorily start a fermentative process. The two different process categories depend on the presence of a fermentative and a respiratory route for the metabolism of fermentable substrate for energy production.
Yeast is able to adapt its cellular composition towards each mode of growth, making it a versatile, but sometimes intractable organism. (These modes differ considerably in the yield of biomass on substrate). For the application of yeast often metabolic characteristics are required, that are normally obtained during fermentative growth conditions. Both modes need to be dealt with satisfactorily to reach to optimized production (Geoffrey and Cregg, 1999).
A fermentative mode of growth is obtained under anaerobic conditions, in which the metabolic balance dose not allows the complete oxidation of the substrate without the supply of oxygen (Verduyn et al. , 1990). Also, many yeast strains show at aerobic conditions fermentative activity parallel to respiratory metabolism. Several authors have observed that yeast extract, ammonium, magnesium, calcium have a protective effect either on growth, fermentation, or viability, which overall stimulate the rate of ethanol production (Alfenore et al.
, 2002; Flores et al. , 2000; Johnston and Carlson, 1992). In S. cerevisiae, by modifying nutritional conditions, it is possible to increase ethanol production by traditional fed-batch (Panchal and Stewart, 1981). High ethanol production in fermentation using Saccharomyces cerevisiae are determined by several factors such as medium composition and operating parameters including substrate and vitamin feeding strategy, oxygen level and temperature (Aldiguier et al. , 2004).
Higher ethanol production was reported after disruption of cytochrome gene in yeast (Shi et al. , 1999). The impact of ethanol and temperature on the dynamic behaviour of S. cerevisiae in ethanol biofuel production was studied using isothermal fed-batch process at five different levels 27, 30, 33, 36 and 39°C (Aldiguier et al. , 2004). The ethanol yield increased significantly in cultivations of GAL1 mutant compared with wild type while the growth and biomass production were reduced.
This was probably due to formation of some byproducts (Alfenore et al. , 2002). Executive Summary and Conclusion Deletion of the GAL1 gene has a key impact on biomass and ethanol production. The GAL1 mutant will normally show signs of a decline in growth rate, but with improved ethanol production. In addition to this, glucose utilization by GAL1 mutants does not support biomass formation, but rather cause excessive respiratory based fermentation metabolism with linear increase in ethanol production. References
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