Genome-scale metabolic reconstruction of freshwater organism daphnia pulex

The aim of this study is the reconstruction of the genome-scale model of an ecological organism and investigation of its cellular functionality towards toxicity by using computational systems biology approaches. Daphnia pulex is one of the keystone species of the toxicity tests for the environment, and hence it is here chosen as a model organism. The study on D. pulex genome was first published in February 2011 and contributed to numerous scientific researches. In the present study, manually curated genome-scale metabolic model of D. pulex is reconstructed by using KEGG and UniProt databases along with the literature search. The completed model includes 1051 reactions (985 internal, 66 exchange) and 774 metabolites (222 mitochondrial, 485 in the cytosol, 67 externals) related to 697 genes. Based on the constraint-based modeling, biomass is chosen as the objective function to inspect the cellular fluxes through the growth of D. pulex. The robustness analysis is performed on selected compounds such as L-isoleucine, selenite and sulfate so as to monitor their effects on biomass. L-isoleucine has a positive impact on the growth of D. pulex. The applicability of the reconstructed model of D. pulex to toxicity studies is tested by investigating the relation between selenate and sulfate uptakes, where these fluxes are estimated by flux balance analysis. The computational results demonstrate that when sulfate uptake is getting close to zero flux, selenate uptake begins. This observation on selenate and sulfate uptake mechanism indicates that predicted results are in agreement with the literature. Consequently, the reconstructed metabolic model of D. pulex is able to predict the cellular behavior and can be used for metabolic toxicity studies.