Yeast metabolism

3 Yeast Metabolism
Metabolism refers to the biochemical assimilation (in anabolic pathways) and dissimilation (in catabolic pathways) of nutrients by a cell. Like in other organisms, in yeast these processes are mediated by enzymic reactions, and regulation of the underlying pathways have been studied in great detail in yeast. Anabolic pathways include reductive processes leading to the production of new cellular material, while catabolic pathways are oxidative processes which remove electrons from substrates or intermediates that are used to generate energy. Preferably, these processes use NADP or NAD, respectively, as co-factors. Although all yeasts are microorganisms that derive their chemical energy, in the from of ATP, from the breakdown of organic compounds, there is metabolic diversity in how these organisms generate and consume energy from these substrates. Knowledge of the underlying regulatory mechanisms is not only valuable in the understanding of general principles of regulation but also of great importance in biotechnology, if new metabolic capabilities of particular yeasts have to be exploited. It is now well established that most yeasts employ sugars as their main carbon and hence energy source, but there are particular yeasts which can utilize non-conventional carbon sources. With regard to nitrogen metabolism, most yeasts are capable of assimilating simple nitrogenous sources to biosynthesize amino acids and proteins (Table 3-1). Aspects of phosphorus and sulphur metabolism as well as aspects of metabolism of other inorganic compounds have been studied in some detail, predominantly in the yeast, Saccharomyces cerevisiae. Table 3-1: Nutrients for growth of yeast (S. cerevisiae) cells.
Substrate Saccharose Maltose Melibiose Glucose Ethanol Intermediates Enzymes Invertase Maltase Melibiase Alcohol-Dehydrogenase Products Glucose + Fructose Glucose Glucose + Galactose Products of Glycolysis Glucose by gluconeogenesis

Lactate Glycerol