Exergy and exergy cost analysis of biochemical networks in living systems far from equilibrium

Whilst humanity has reached a high level of technological development, finding efficient substitutes to petroleum energy is a challenging task. In this context, metabolically engineered microorganisms are used in biomass production. Considering the availability of data in genomic and metabolic fronts, Escherichia.Coli is one of the primary options for biofuel production, which could be later exploited as a ‘solo’ energy source, or coupled with nowadays available fuels. To survive, an organism must provide an amount of exergy greater than the exergy required to process equilibrium operations. Therefore, extra exergy amounts are needed for a living system to accomplish production, growth and evolution in time, as the above mentioned process is highly irreversible. This paper reviews the available studies on exergy analysis and exergy-cost theory-ECT application, along with the use of flux balance analysis-FBA and flux variability analysis-FVA, as a tool for gaining biological insights. The paper is structured as the following; first, a brief description of exergy analysis and the exergy-cost theory is presented. Second, the exergy analysis application on living cells is discussed through introducing exergy analysis of metabolic networks. Thirdly, the application on E.Coli is explained, highlighting its potential role in biofuel production. Finally, an approach, applied within a current PhD research project regarding the application of the exergy analysis to a generic metabolic network is introduced. In this approach, the exergy costs associated with all the flows present in the targeted network are calculated, according to the ECT. The perspective is to use the exergy cost information for defining additional constraints in the FBA of the metabolic network. Which could provide better insight about organisms and identify directions for the optimization of biomass production, and the enhancement of biofuel use.