Thermodynamic analysis of Saccharomyces cerevisiae growth: a theoretical-experimental approach

Abstract

The glucose molecule, commonly found in industrial and laboratorial cultivation media, is used by yeasts in two ways: as carbon and energy source. Saccharomyces cerevisiae, a microorganism employed in the biotechnology industry, has two possible energetic metabolisms: fermentation and respiration, which allows cells to conserve their Gibbs energy through glucose oxidation. This energy is temporarily stored in ATP (adenosine triphosphate molecule) chemical bonds, presenting a balance of 2 ATP/glucose in purely fermentative metabolism and around 30 ATP/glucose in exclusively respiratory metabolism. However, through thermodynamic analysis, it is possible to notice that a large part of Gibbs energy related to the different metabolic reactions of fermentation and respiration does not result in ATP synthesis. Furthermore, the conversion of glucose into biomass, which is typically around 0.1 grams of cell dry mass per gram of glucose (g CDM/g GLC) in fermentative metabolism, and about 0.5 g CDM/g GLC in respiratory metabolism, is not consistent with the respective energetic balances. Therefore, the question is whether all this energy is just dissipated or perhaps the cell stores it in other ways, for example in some compounds that typically compose the biomass. In this work, we intent to test the hypothesis that the cell stores at least part of this spare energy in the form of chemical bonds in compounds as proteins, lipids and/or other molecules that compose the cell itself. To do so, we will analyze the cellular composition of the yeast Saccharomyces cerevisiae in two different situations, one with cells in purely fermentative metabolism and another with cells in purely respiratory metabolism. Thus, we will verify whether there is a major energetic content in cells growing in respiratory metabolism, compared to cells growing in fermentative metabolism. (AU)