title: Utilizing elementary mode analysis, pathway thermodynamics, and a genetic algorithm for metabolic flux determination and optimal metabolic network design.
abstract: BACKGROUND
  Microbial hosts offer a number of unique advantages when used as production systems for both native and heterologous small-molecules. These advantages include high selectivity and benign environmental impact; however, a principal drawback is low yield and/or productivity, which limits economic viability. Therefore a major challenge in developing a microbial production system is to maximize formation of a specific product while sustaining cell growth. Tools to rationally reconfigure microbial metabolism for these potentially conflicting objectives remain limited. Exhaustively exploring combinations of genetic modifications is both experimentally and computationally inefficient, and can become intractable when multiple gene deletions or insertions need to be considered. Alternatively, the search for desirable gene modifications may be solved heuristically as an evolutionary optimization problem. In this study, we combine a genetic algorithm and elementary mode analysis to develop an optimization framework for evolving metabolic networks with energetically favorable pathways for production of both biomass and a compound of interest.
  
  
  RESULTS
  Utilization of thermodynamically-weighted elementary modes for flux reconstruction of E. coli central metabolism revealed two clusters of EMs with respect to their Delta Gp degrees. For proof of principle testing, the algorithm was applied to ethanol and lycopene production in E. coli. The algorithm was used to optimize product formation, biomass formation, and product and biomass formation simultaneously. Predicted knockouts often matched those that have previously been implemented experimentally for improved product formation. The performance of a multi-objective genetic algorithm showed that it is better to couple the two objectives in a single objective genetic algorithm.
  
  
  CONCLUSIONS
  A computationally tractable framework is presented for the redesign of metabolic networks for maximal product formation combining elementary mode analysis (a form of convex analysis), pathway thermodynamics, and a genetic algorithm to optimize the production of two industrially-relevant products, ethanol and lycopene, from E. coli. The designed algorithm can be applied to any small-scale model of cellular metabolism theoretically utilizing any substrate and applied towards the production of any product.
authors.affiliation: Tufts University, Department of Chemical & Biological Engineering, Science & Technology Center, 4 Colby Street, Medford, MA 02155, USA.
authors.forename: Brett A
authors.initials: BA
authors.lastname: Boghigian
authors.forename: Hai
authors.initials: H
authors.lastname: Shi
authors.forename: Kyongbum
authors.initials: K
authors.lastname: Lee
authors.forename: Blaine A
authors.initials: BA
authors.lastname: Pfeifer
mesh_terms: D000465:Algorithms; D018533:Biomass; D002244:Carbon / Q000737:chemistry; D002338:Carotenoids / Q000737:chemistry / Q000378:metabolism; D019295:Computational Biology / Q000379:methods; D004926:Escherichia coli / Q000235:genetics* / Q000378:metabolism; D000431:Ethanol / Q000737:chemistry; D005798:Genes, Bacterial / Q000235:genetics*; D005838:Genotype; D000077276:Lycopene; D053858:Metabolic Networks and Pathways / Q000235:genetics*; D008954:Models, Biological; D008957:Models, Genetic; D012984:Software; D049490:Systems Biology; D013816:Thermodynamics
chemical_list: D002338:Carotenoids; D000431:Ethanol; D002244:Carbon; D000077276:Lycopene