The design of new generation bio-processing plants is increasingly dependent on the design of process-compatible microorganisms. The latter, whether through genetic or physiological manipulations can be greatly assisted by metabolic engineering. An emerging powerful tool in metabolic engineering research is computer assisted cell design using mathematical programming. In this work the problem of optimizing cellular metabolic networks has been formulated as a Mixed Integer Non-Linear Programming (MINLP) model. The model can assist genetic engineers to identify which cellular enzymes should be modified and the new levels of activity required to produce an optimal network. Results are presented from the Tricarboxylic Acid Cycle in Dictyostelium discoideum.