Abstract
Rhodopseudomonas palustris CGA009 is a purple non-sulfur bacterium that can fix carbon dioxide (CO2) and nitrogen or break down organic compounds for its carbon and nitrogen requirements. Light, inorganic, and organic compounds can all be used for its source of energy. Excess electrons produced during its metabolic processes can be exploited to produce hydrogen gas or biodegradable polyesters. A genome-scale metabolic model of the bacterium was reconstructed to study the interactions between photosynthesis, CO2 fixation, and the redox state of the quinone pool. A comparison of model-predicted flux values with available Metabolic Flux Analysis (MFA) fluxes yielded predicted errors of 5–19% across four different growth substrates. The model predicted the presence of an unidentified sink responsible for the oxidation of excess quinols generated by the TCA cycle. Furthermore, light-dependent energy production was found to be highly dependent on the quinol oxidation rate. Finally, the extent of CO2 fixation was predicted to be dependent on the amount of ATP generated through the electron transport chain, with excess ATP going toward the energy-demanding Calvin-Benson-Bassham (CBB) pathway. Based on this analysis, it is hypothesized that the quinone redox state acts as a feed-forward controller of the CBB pathway, signaling the amount of ATP available.
Highlights
Several studies conducted on R. palustris showed that in addition to the Calvin-Benson-Bassham (CBB) cycle’s role of carbon assimilation during autotrophic growth, the pathway plays a major role in maintaining redox balance under heterotrophic conditions[10,12,13,14]
A limited number of small-scale metabolic reconstructions have been developed for PNSB, examining either the central carbon metabolism[27] or the electron transport chain[28]
After the model indicated the presence of an unidentified quinol sink, in silico simulations were combined with published in vivo flux measurements[13,14] to study the effect of the quinone redox state on cellular growth, electron transport rate, and CO2 fixation
Summary
Several studies conducted on R. palustris showed that in addition to the Calvin-Benson-Bassham (CBB) cycle’s role of carbon assimilation during autotrophic growth, the pathway plays a major role in maintaining redox balance under heterotrophic conditions[10,12,13,14]. A limited number of small-scale metabolic reconstructions have been developed for PNSB, examining either the central carbon metabolism[27] or the electron transport chain[28] These models are limited in scope, as they consider less than 4% of the organism’s metabolic functionality and are incapable of capturing system-wide interactions between different metabolic modules. The model could be improved further by integrating recently annotated metabolic pathways for lignin monomer degradation[30], as well as making use of experimental data on gene essentiality[31] and metabolic flux analysis for growth under different carbon sources[13,14] to validate and refine the network. An understanding of the metabolic control points of this interconnected system constitutes the first step towards engineering strains capable of more efficiently harnessing photosynthetic energy and rerouting this energy towards bio-production and lignin valorization
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