Thermodynamics ethanol, fermentation

The thermodynamics of methanol and ethanol production help to explain why methanol is not manufactured by fermentation processes whereas ethanol is. The overall stoichiometries for converting glucose (C6H12O6) to methanol and ethanol by alcoholic fermentation under physiological conditions (pH 7, 25°C, unit activity in aqueous solution) are... 

S. cerevisiae has been transformed with the P. stipitis genes XYLl and XYL2 coding for XR and XDH, respectively [46,47,141 ]. The choice of P. stipitis as the donor organism was based on its capability to utilize NADH in the xylose reduction step. Attempts to ferment xylose to ethanol with these recombinant S. cerevisiae producing XR/XDH have resulted in low ethanol yield and considerable xylitol by-product formation. This has been ascribed to the unfavorable thermodynamic properties of the reactions [140] and the fact that the first reaction preferably consumes NADPH, whereas the second reaction exclusively produces NADH. When less NADH is consumed in the XR reaction, then less NAD" is available for the XDH reaction. If the amount of NAD+ is insufficient, xylitol is produced and excreted [133]. 

Beer making relies on the fermentation of sugars (produced from grains such as wheat or barley). The fermentation process produces ethanol and is thermodynamically favorable (AG is negative). The direct conversion of sugar into ethanol has a very large activation energy and therefore does not occur by itself at an appreciable rate ... 

Another overexpression strategy was tried with the NAD -dependent malic enzyme of E. coli Thermodynamically, the reduction of pyruvate to malate is favored, but in nature this reaction does not occur. A double mutant of E. coli, NZNlll, which is blocked in both pyruvate formate lyase pjT) and lactate dehydrogenase (Idh), was used as the host. It is unable to grow anaerobically because its pyruvate metabolism is blocked by the fermentation end products acetate, formate, ethanol, and lactic acid. The mutant NZNl 11 with multiple copies of malic enzyme accumulated succinic acid as a major end product only when the cells were switched to anaerobic metabolism gradually by metabolic depletion of oxygen in a sealed tube (Clark et al. 1988). Mutant strains blocked in either pfl or Idh did not alter their distribution of fermentation products when overexpressing malic enzyme. 

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