Anaerobic fermentation, thermodynamics

The described sequence would be expected if reactions tended to occur in order of their thermodynamic possibility. The reductant (CH2O) will supply electrons to the lowest unoccupied electron level (O2) with more electrons available, successive levels—NOf, NO, Mn02(s) and so on—will be filled up. The described succession of reactions is mainly reflected in the vertical distribution of components in a nutrient-enriched (eutrophied) lake and in general also in the temporal succession in a closed system containing excess organic matter, such as a batch digester (anaerobic fermentation unit). 

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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