Bioprocess Technology- Fundamentals and Applications Chapt 4_Metabolic basis(7)

Ethanol production is a means to dispose of the surplus carbon when the metabolismdownstream the pyruvate can not keep up with the high rate of pyruvate formation forced onthe cell by a high glucose concentration. Concomitantly some of the pyruvate (about 10% ofethanol amount) is secreted as acetaldehyde and acetate. The additional glycolysiscontributed by the overflow metabolism provides extra ATP in addition to that obtained fromglucose going through to respiration. This extra energy supply is used for increasing thegrowth rate. A mathematical description of these reactions are given in the section of fed-batch technique ( chapter 7).

The overflow metabolism of yeast is sometimes referred to as the glucose effect. However, itmust be distinguished from the original meaning of the glucose effect, that was used as asynonym to the Crabtree effect, which is catabolite repression (i.e. on enzyme synthesislevel) of the respiration, that takes place during long-term exposure of the cell to high glucoseconcentration. The overflow metabolism is observed within seconds after exposure of the cellto high glucose concentration and it is therefore sometimes also referred to as the short-termglucose effect.

In E. coli the overflow metabolism is observed as conversion of pyruvate by pyruvatedehydrogenase to acetylCoA and further to actetate that is secreted to the medium (see Fig4.6, though observe that bacteria do not have mitochondria !). A maximum respiratory rate,that is reached before the maximum glucose uptake is reached, is characteristic also for E.coli. When the rate of glycolysis is low, acetate is resorbed by the cells, if present in themedium. The regulatory explanation to acetic acid formation by E. coli is even less clear thanthe ethanol formation by yeast. It may, in fact, have several causes. Firstly, there appears tobe a lack of regulation of the glucose uptake rate at high glucose concentrations. Enz IIglc(Fig 4.4) is believed not to respond to the normal regulatory signal (the membrane potential)when saturated with glucose. Secondly, at high glucose concentrations the α-ketoglutaratedehydrogenase activity is decreased, as it is under anaerobic conditions, thus interrupting theTCA-cycle. In analogy with the situation in yeast, acetic acid production may, in terms ofenergy production, be beneficial to the bacterium. Though, different strains of E. coli vary intheir ability to form acetic acid.4.3.5 Summary.

Catabolism is regulated at the point of entrance of various substrates into the centralmetabolic pathways. These pathways are made up of constitutive enzymes whose activitiesare regulated in relation to the energy demand and the properties of the substrate. Infacultative aerobic organisms overflow metabolism results in production of partially oxidisedproducts like ethanol and acetic acid. Understanding the regulation of catabolism and energyformation is especially important in biotechnical processes because these activities furnishthe basis for the rest of the metabolism (Figs. 4.1 and 4.3).

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