A novel approach to design of overexpression strategy for metabolic engineering. Application to the carbohydrate metabolism in the citric acid producing mould Aspergillus niger

A metabolic model for the production of citric acid in Aspergillus niger is optimized with respect to the production rate of citrate. The model makes use of the S-system representation of biochemical systems, which renders it possible to use linear programming to optimize the process. While originally nonlinear, the optimization problem is reduced in such a way that methods of linear programming can be used. The optimizations lead to profiles of enzyme activities that are compatible with the physiology of the cells, which guarantees their viability and fitness, and yield higher rates of the desired final end products than the original systems. It was found that maintaining the metabolite pools within narrow physiological limits (20% around the basal steady state level) and allowing the enzyme concentrations to vary within the range of 0.1 to 50 times their basal values it is possible to triple the glycolytic flux while maintaining 100% yield of substrate transformation. To achieve these improvements it is necessary to modulate seven or more enzymes simultaneously.