Yeast: Metabolism and Fructose Glucose

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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

Lactate
Glycerol

Intermediates

Enzymes
Invertase
Maltase
Melibiase

Acetaldehayde >
Acetyl-CoA>
Oxaloacetate>
Pyruvate>
Glycerol-3phosphate>
Dihydroxyacetonphosphate

Alcohol-Dehydrogenase

Lactate-Dehydrogenase

Products
Glucose + Fructose
Glucose
Glucose + Galactose
Products of Glycolysis
Glucose by gluconeogenesis

Glucose by gluconeogenesis
Glucose by gluconeogenesis

Amino acids
Glutamate
Ammonium

3.1 Sugar Catabolism in Yeast
3.1.1 Principal Pathways
The major source for energy production in the yeast, Saccharomyces cerevisiae, is glucose and glycolysis is the general pathway for conversion of glucose to pyruvate, whereby production of energy in form of ATP is coupled to the generation of intermediates and reducing power in form of NADH for biosynthetic pathways.

Two principal modes of the use of pyruvate in further energy production can be distinguished: respiration and fermentation (Figure 3-1). In the presence of oxygen and absence of repression,

pyruvate enters the mitochondrial matrix where it is oxidatively decarboxylated to acetyl CoA by the pyruvate dehydrogenase multi enzyme complex. This reaction links glycolysis to the citric acid cycle, in which the acetyl CoA is completely oxidized to give two molecules of CO2 and reductive equivalents in form of NADH and FADH2. However, the citric acid cycle is an amphibolic pathway, since it combines both catabolic and anabolic functions. The latter results, for example, from the production of intermediates for the synthesis of amino acids and nucleotides. Replenishment of compounds necessary to drive the citric acid cycle, such as oxaloacetate and α-ketoglutarate, are (i) the fixation of CO2 to pyruvate by the actions of the enzymes pyruvate carboxylase (ATP-dependent) and phosphoenolpyruvate carboxykinase and (ii) the glyoxalate cycle (a shortcut across the citric acid cycle), which is important when yeasts are grown on two-carbon sources, such as acetate or ethanol.

Figure 3-1: Metabolism in yeast under aerobic and anaerobic conditions. During alcoholic fermentation of sugars, yeasts re-oxidize NADH to NAD in a two-step reaction from pyruvate, which is first...
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