Flux Balance Analysis of Photoautotrophic Metabolism

Abstract

Photosynthesis is the principal process responsible for fixation of inorganic carbon dioxide into organic molecules with sunlight as the energy source. Potentially, many chemicals could be inexpensively produced by photosynthetic organisms. Mathematical modeling of photoautotrophic metabolism is therefore important to evaluate maximum theoretical product yields and to deeply understand the interactions between biochemical energy, carbon fixation, and assimilation pathways. Flux balance analysis based on linear programming is applied to photoautotrophic metabolism. The stoichiometric network of a model photosynthetic prokaryote, Synechocystis sp. PCC 6803, has been reconstructed from genomic data and biochemical literature and coupled with a model of the photophosphorylation processes. Flux map topologies for the hetero-, auto-, and mixotrophic modes of metabolism under conditions of optimal growth were determined and compared. The roles of important metabolic reactions such as the glyoxylate shunt and the transhydrogenase reaction were analyzed. We also theoretically evaluated the effect of gene deletions or additions on biomass yield and metabolic flux distributions.

The Metabolic Network of Synechocystis sp. PCC 6803: Systemic Properties of Autotrophic Growth

“…We identify inconsistencies in the current gene annotation and suggest several genes with putative novel or different enzymatic activity for further experimental validation. Our work significantly improves upon earlier reconstructions, which were either restricted to the primary carbon metabolism (Shastri and Morgan, 2005;Hong and Lee, 2007;Navarro et al, 2009) or incomplete with respect to several metabolic routes (Fu, 2008;Kun et al, 2008). During the reconstruction process, we identified various obstacles that currently hamper the large-scale construction of metabolic models.…”

“…An estimation of the macromolecular biomass composition for Synechocystis sp. PCC 6803 under different growth conditions was given by Shastri and Morgan (2005) and afterward utilized in several studies. However, as the size of our reconstruction is considerably larger, the inclusion of additional components was necessary.…”

“…However, as the size of our reconstruction is considerably larger, the inclusion of additional components was necessary. To this end, we modified and extended the BOF given by Shastri and Morgan (2005) based on information on the macromolecular composition reported for the cyanobacterium Arthrospira (Spirulina) platensis (Cogne et al, 2003). The BOF is given in Table I and detailed in "Materials and Methods.…”

“…Specifically, FBA allows the prediction of optimal steady-state fluxes that maximize a given objective function, usually the synthesis of biomass or biomass precursors required for growth. Although certainly not without pitfalls, the predictions of FBA have proven to reasonably reflect the modes of cellular operation, with manifold applications ranging from microorganisms to algae and plant metabolism (Varma and Palsson, 1994;Shastri and Morgan 2005;Boyle andMorgan, 2009, Feist et al, 2009;Grafahrend-Belau et al, 2009;Oberhardt et al, 2009;Poolman et al, 2009).…”

Engineering a Cyanobacterial Cell Factory for Production of Lactic Acid

“…To put the maximal turnover capacity of the LDHs in the cell extract into a quantitative perspective, it is relevant to realize that for Synechocystis to grow at max , i.e., with an 8-h doubling time, a minimal flux of carbon through the Calvin-Benson cycle of 3.70 mmol carbon/g DW/h is necessary (based on an estimated cellular composition of the biomass of C 4 H 7 O 2 N [29,35]). Hence, the measured in vitro enzyme activity of the lactate dehydrogenases demonstrates that in order to achieve the goal of a true "cell factory" for lactic acid (a cell that is able to channel the large majority of the fixed carbon into lactic acid), at least 10-fold-higher LDH activity is required than was achieved in this study.…”