Substrate inhibition yeast

The principal advantages of this type of cyclic system with transient operating techniques are apparent in bioprocesses whose maximum productivity is in a transient region. The products of secondary metabolism (Pirt, 1974) are a typical example of this group of processes. Another group consists of processes whose optimal operation requires an optimal substrate concentration— biomass production with bakers yeast, for example (Aiba et al., 1976)—or where the process is subject to substrate inhibition. An important area of application for this is in biological waste water purification. These periodic modes of operation generally show increased productivity. More systematic and detailed study is needed in this area. 

Bioethanol can be produced from the fermentable sugars obtained in the hydrolysis step by bacteria, yeast, or filamentous fungi. In order to prevent the substrate inhibition effects enzymatic hydrolysis and fermentation steps can be combined in a bioprocess called simultaneous saccharification and fermentation (SSF) (Hahn-Hagerdal et al., 2006). At the end of fermentation, ethanol can be purified by distillation and molecular sieves or other separation techniques, which will be ready to be used as a fuel, either neat or blended with gasoline (Hahn-Hagerdal et al., 2006). 

Octonol is an intermediate for the production of several optically active pharmaceuticals, such as steroids and vitamins. The asymmetric reduction of 2-octanone to (5)-2-octonol by baker s yeast was inhibited severely by substrate and product concentration of 10 him and 6 mM respectively. Whole-cell biotransformation of 2-octanone in a water-ra-dodecane biphasic system yielded a high product concentration of 106him with 89% ee in 96h [37],... 

The specificity of levansucrase98 is dependent not only on the d-fructoside but also on the aldoside residue of the substrate. Neither inulin nor methyl D-fructofuranoside was hydrolyzed by levansucrase, and even when these two substrates were hydrolyzed by inulase (prepared from inulin-fermenting Torula yeast) or by yeast invertase respectively, no levan formation occurred with levansucrase. However, neither methyl D-fructofuranoside nor inulin inhibited levan formation from sucrose by levansucrase. No levan was formed from potassium D-glucose... 

In addition to bistability and hysteresis, the minimal model of glycolysis also allows nonstationary solutions. Indeed, as noted above, one of the main rationales for the construction of kinetic models of yeast glycolysis is to account for metabolic oscillations observed experimentally for several decades [297, 305] and probably the model system for metabolic rhythms. In the minimal model considered here, oscillations arise due to the inhibition of the first reaction by its substrate ATP (a negative feedback). Figure 24 shows the time courses of oscillatory solutions for the minimal model of glycolysis. Note that for a large... 

Figure 30. A medium complexity model of yeast glycolysis [342], The model consists of nine metabolites and nine reactions. The main regulatory step is the phosphofructokinase (PFK), combined with the hexokinase (HK) reaction into a single reaction vi. As in the minimal model, we only consider the inhibition by its substrate ATP, although PFK is known to have several effectors. External glucose (Glc ) and ethanol (EtOH) are assumed to be constant. Additional abbreviations Glucose (Glc), fructose 1,6 biphosphate (FBP), pool of triosephosphates (TP), 1,3 biphosphogly cerate (BPG), and the pool of pyruvate and acetaldehyde (Pyr).

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