The influence of crowding conditions on the thermodynamic feasibility of metabolic pathways

Intracellular reactions are carried out in a crowded medium where the macromolecules occupy ~40% of the total vol- ume. This decrease in the available volume affects the activity of the reactants. Scaled particle theory is used for the estimation of the activity coefficients of the metabolites, and thereby for the assessment of the impact of the presence of background molecules, on the estimation of the Gibbs free energy change (DrG) of the reactions. The lactic acid pathway and the central carbon metabolism of Actinobacillus succinogenes for the production of succinic acid from glycerol have been used as illustrative case studies. Results suggest the importance of maintaining intracellular crowded regions to favor the feasibility of a pathway that in other circum- stances would be infeasible. Moreover, the crowding conditions may change the directionality of reactions and can modify the feasible range of fluxes estimated for a metabolic system compared with those obtained at standard biological conditions.