73C-5

The productivity of xanthan gum, an industrially important microbial exopolysaccharide produced by Xanthomonas campestris because of its unique rheological properties, has been greatly hindered by limited oxygen transfer due to high broth viscosity. With improved oxygen transfer capacity, higher xanthan productivity has been reached in the centrifugal, packed-bed reactor (CPBR). However, in depth analysis is needed to assess if the production mechanism matches the mass balance and the overall metabolic flux distribution. We hypothesize that, with defined metabolic rates and corresponding constraints, all the rates in metabolic network can be determined using metabolic flux analysis based on the pseudo-steady state hypothesis.

 

Our objective was to analyze the CPBR fermentation data in stationary phase using the metabolic network model.

 

StationaryStationary phase xanthan fermentation data in CPBR were employed and analyzed against xanthan biosynthesis and glucose catabolism pathway for X. campestris. The reactions and rates in xanthan biosynthesis and glucose catabolism pathway can be represented by nine rates to be determined and seven constraints on them, leaving the freedom of the system two.

With two measured rates (glucose consumption and xanthan formation respectively at 1.50 and 0.48 mmol L^-1 h^-1), the optimized solutions to the other seven rates involved in the metabolic network could be acquired by using a nonlinear least square method. The rates of G-6-P to xanthan, G-6-P to PEP, pyruvate to PEP, pyruvate to CO₂, and acetyl-ScoA to CO₂ were determined to be 0.38, 0.48, 0.47, 0.31, and 0.61 mmol L^-1 h^-1, respectively. The calculated metabolic rates followed the mass balance with relative errors < 0.05.

By using metabolic flux analysis, we can use the measured rates to determine all the metabolic rates in the metabolic network. The results provide insights towards the metabolism characteristics of xanthan formation in CPBR and are important to process scale up evaluations.