abstract
- Xylitol bioprocess production has been largely reported from xylose assimilating microorganisms like Candida parapsilosis among others. In such bioprocesses, the dissolved oxygen supply is a main factor controlling xylitol yields, which results from an imbalance of the key enzymes xylose reductase and xylitol dehydrogenase. However, when dissolved oxygen is regarded as a variable that induces cellular redox phenomena, such as modifying redox cofactors ratios NADH/NAD+ and NADPH2/NADP+ a clearer picture is possible in terms of the thermodynamic feasibility concept. Thermodynamic feasibility implies having negative Gibbs free energy in the biological transformation of xylose to xylitol, the free energy is estimated from redox potential of the involved compounds, namely xylose, xylitol, redox cofactors and oxygen. A mathematical representation of the system has been developed through a cybernetic approach that permits the estimation of the redox cofactor ratios controlling the enzymatic activity of the key enzymes. The estimated redox cofactor ratios allowed the estimation of the Gibbs free energy for the biotransformation, and the analysis revealed that free energy of key coupled reactions explains the xylitol production rather than dissolved oxygen availability in the bioreaction system.