Phosphoenolpyruvate carboxylase from Acetobacter aceti

The key to acetate: metabolic fluxes of acetic acid bacteria under cocoa pulp fermentation-simulating conditions

Acetic acid bacteria (AAB) play an important role during cocoa fermentation, as their main product, acetate, is a major driver for the development of the desired cocoa flavors. Here, we investigated the specialized metabolism of these bacteria under cocoa pulp fermentation-simulating conditions. A carefully designed combination of parallel 13C isotope labeling experiments allowed the elucidation of intracellular fluxes in the complex environment of cocoa pulp, when lactate and ethanol were included as primary substrates among undefined ingredients. We demonstrate that AAB exhibit a functionally separated metabolism during coconsumption of two-carbon and three-carbon substrates. Acetate is almost exclusively derived from ethanol, while lactate serves for the formation of acetoin and biomass building blocks. Although this is suboptimal for cellular energetics, this allows maximized growth and conversion rates. The functional separation results from a lack of phosphoenolpyruvate carboxykinase and malic enzymes, typically present in bacteria to interconnect metabolism. In fact, gluconeogenesis is driven by pyruvate phosphate dikinase. Consequently, a balanced ratio of lactate and ethanol is important for the optimum performance of AAB. As lactate and ethanol are individually supplied by lactic acid bacteria and yeasts during the initial phase of cocoa fermentation, respectively, this underlines the importance of a well-balanced microbial consortium for a successful fermentation process. Indeed, AAB performed the best and produced the largest amounts of acetate in mixed culture experiments when lactic acid bacteria and yeasts were both present.

Purification and characterization of the phosphoenolpyruvate carboxylase from the facultative chemolithotroph Thiobacillus novellus (ATCC 8093)

Phosphoenolpyruvate carboxylase (EC4.1.1.31), which catalyzes the carboxylation of phosphoenolpyruvate to produce oxaloacetate was purified 465-fold from extracts of organotrophically grown Thiobacillus novellus. Nondenaturing polyacrylamide gel electrophoresis (PAGE) of the purified enzyme revealed the presence of two bands after staining with Buffalo Black. Gels stained with Fast Violet B after incubation with PEP, HCO3-, Mg2+ and acetyl CoA also showed two bands of activity with the faster moving the more active of the two. Sodium dodecylsulfate (SDS)-PAGE of the enzyme heated at 100 degrees C for 5 min revealed the presence of three intensely stained bands of M(r) 95 K, 51 K, and 28 K. However, electrophoresis of the enzyme heated for 2 min showed a single band of about 100 K, indicating that the preparation was likely homogeneous. The 51 K and 28 K subunits are thus products of the 95 K subunit. Gel filtration studies of the native enzyme yielded a M(r) of 360 K. Therefore, the enzyme is a tetramer. The optimum pH in Tris buffer was 8.0, with Km for PEP 0.64 mM, HCO3- 0.11 mM, and acetyl CoA a potent activator, 1.3 microM. A divalent cation best served by Mg2+ gave sigmoidal initial velocity plots. Hill plots of the data gave coefficients (nH) of 2.6. None of the metabolites tested, nucleotide triophosphates excepted, significantly affected enzyme activity. Binding studies with 14C-labelled PEP revealed the binding of about 20 moles PEP per mole (360,000 g) of PEPC. Initial velocity studies suggest that the reaction is catalyzed by a random Bi Bi mechanism. Despite the lack of inhibition by certain metabolites, the enzyme's function is probably anaplerotic.