Phosphoenolpyruvate carboxykinase function

Cocoa bean fermentation is a mixed-culture process, consisting initially of fermentations by yeast and lactic acid bacteria followed by oxidation of the fermentation products ethanol and lactic acid into acetic acid and acetoin by several Acetohacter strains, of which /I. pasteurianus is the prominent one (Moens et al. 2014). A C-based carbon flux analysis of Acetohacter during cocoa pulp fermentation-simulating conditions revealed a functionally separated metabolism during co-consumption of ethanol and lactate. Acetate was almost exclusively derived from ethanol, whereas lactate served for formation of acetoin and biomass building blocks. This switch was attributed to the lack of phosphoenolpyruvate carboxykinase and malic enzyme activities, which prevents conversion of oxalo-acetate and malate formed by acetate metabolism in the TCA cycle to PEP and pyruvate and subsequently to acetoin (Adler et al. 2014). Lactate, on the other hand, can be converted to pyruvate, which is then used for acetoin formation or, after conversion to PEP by pymvate phosphate dikinase, for gluconeogenesis. The inability of conversion of TCA cycle intermediates to PEP resembles the situation in G. oxydans, where in addition no enzyme for conversion of pyruvate to PEP is present. 

Figure 6. Variations on phosphoenolpyruvate (PEP) and pyruvate (PYR) metabolism in animals. In mammalian liver, pyruvate kinase (PK) and PEP carboxykinase (PEPCK) function in opposite directions to support glycolysis versus gluconeogenesis. In anoxia-tolerant mollusks, PEP is routed via PK when oxygen is present and via PEPCK in anoxia. Note that PEPCK is adapted for physiological function in opposite directions in the two situations.

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