Neutral Versus Acidic Conditions for Product Formation

3 NEUTRAL VERSUS ACIDIC CONDITIONS FOR PRODUCT FORMATION 

Several microbial hosts were shown to be capable of efficient succinate production, as presented in the previous sections and summarized in Table 16.1. The metabolic 

Another issue associated with the pH is carbon dioxide fixation. As outlined above, probably all succinate production pathways are critically dependent on the net fixation of carbon dioxide by C3 carboxylation reactions. Enzymes responsible for this conversion are PEP carboxylase, PEPCK, pyruvate carboxylase, and malic enzyme. Studies on the active species of CO2 used by these enzymes indicated that PEP carboxylase and pyruvate carboxylase use bicarbonate as substrate, whereas PEP carboxykinase and malic enzyme use CO2 (for references see Bott and Thauer, 1989). Under acidic conditions, the equilibrium of the reaction CO2 (aq) -I- H2O 5 HCO3 -l-H+(p a = 6. 33 at 30°C) is strongly shifted to carbon dioxide, which may positively or negatively impact carbon dioxide fixation, depending on the carboxylating enzymes present. 

Besides efficient producer strains and optimal fermentation conditions, a suitable purification strategy is a critical issue in the establishment of an economically viable 

FIGURE 16 A Display of involved downstream processing steps for purification of succinic acid produced under low and neutral pH scenario. 

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Neutral Versus Acidic Conditions for Product Formation... 

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