Physiology of yeasts in relation to biomass yields

Abstract

The stoichiometric limit to the biomass yield (maximal assimilation of the carbon source) is determined by the amount of CO 2 lost in anabolism and the amount of carbon source required for generation of NADPH. This stoichiometric limit may be reached when yeasts utilize formate as an additional energy source. Factors affecting the biomass yield on single substrates are discussed under the following headings: -Energy requirement for biomass formation (YATP)' YATP depends strongly on the nature of the carbon source. Cell composition. The macroscopic composition of the biomass, and in particular the protein content, has a considerable effect on the ATP requirement for biomass formation. Hence, determination of for instance the protein content of biomass is relevant in studies on bioenergetics. Transport of the carbon source. Active (i.e. energy-requiring) transport, which occurs for a number of sugars and polyols, may contribute significantly to the calculated theoretical ATP requirement for biomass formation. Pia-ratio. The efficiency of mitochondrial energy generation has a strong effect on the cell yield. The Pia-ratio is determined to a major extent by the number of proton-translocating sites in the mitochondrial respiratory chain. Maintenance and environmental factors. Factors such as osmotic stress, heavy metals, oxygen and carbon dioxide pressures, temperature and pH affect the yield of yeasts. Various mechanisms may be involved, often affecting the maintenance energy requirement. Metabolites such as ethanol and weak acids. Ethanol increases the permeability ofthe plasma membrane, whereas weak acids can act as proton conductors. Energy content of the growth substrate. It has often been attempted in the literature to predict the biomass yield by correlating the energy content of the carbon source (represented by the degree of reduction) to the biomass yield or the percentage assimilation of the carbon source. An analysis of biomass yields of Candida uti/is on a large number of carbon sources indicates that the biomass yield is mainly determined by the biochemical pathways leading to biomass formation, rather than by the energy content of the substrate.

Kinetics of growth and sugar consumption in yeasts

“…Also in aerobic chemostat cultures, the presence of non-metabolizable weak acids results in an enhancement of catabolism. As can be predicted from the model mentioned above, the uncoupling effect of benzoate on aerobic, glucose-limited chemostat cultures increased with decreasing culture pH (Verduyn 1991).…”

Dynamic regulation of mitochondrial respiratory chain efficiency in Saccharomyces cerevisiae

“…From these cultures, the overall steady state carbon fluxes were analysed (Table 2). We observed fully respiratory metabolism from 30 to 37 u C. Ethanol production was observed only at cultivation temperatures higher than 37 u C. Interestingly, the specific oxygen consumption rate was constant at all temperatures, even those above 37 u C. Since the biomass yield on ATP (Y ATP ) is suggested to decrease upon an increase in temperature (Verduyn, 1991;Wallace & Holms, 1986), we conducted a detailed analysis of catabolism to assess the contributions of respiration and fermentation to the energy required for growth. Next, we developed a method to determine the efficiency of the respiratory chain in vivo.…”

“…This value can be directly measured in isolated mitochondria. In yeast, in vivo P/O is often assumed to be constant since the ubiquinone reducing part of the electron transport chain (ETC) is essentially linear (Verduyn et al, 1991). From genome-scale mathematical models, P/O values can be calculated to be around 1.0 (Famili et al, 2003), but the data for such models often describe only one condition.…”

“…We assessed the effect of both changes in the environmental temperature and lowered oxygen availability. We chose to study the effect of various cultivation temperatures, because in both E. coli and yeast, increased temperature was shown to cause an increase in the maintenance energy (Mensonides, 2007;Verduyn, 1991;Wallace & Holms, 1986). In order to sustain growth and simultaneously deal with the increased maintenance, metabolism is expected to be optimized, which is in stark contrast with a temperature-induced switch to fermentation that we observed before (Postmus et al, 2008).…”

“…Microbes such as yeasts can rapidly adapt to challenging conditions by altering their genetic expression profile or tuning the activity of key enzymes to function in the variety of environments these organisms encounter (Bouwman et al, 2011;Gasch & Werner-Washburne, 2002;Postmus et al, 2008;Smits & Brul, 2005;Strassburg et al, 2010). Adaptive responses put a significant additional energetic burden on the cells, since the gene expression cascade requires a substantial expenditure of energy (Tempest & Neijssel, 1984;Verduyn, 1991;Warner, 1999). The fraction of the energy generated in catabolism that is used in processes other than biomass production is the so-called maintenance energy (Pirt, 1965).…”