Aromatic compounds are ubiquitous in nature. Three of the 20 common amino acids have aromatic structures and a large fraction of all plant biomass is composed of aromatic compounds. Aromatic compounds are also common environmental pollutants that are derived from petroleum byproducts such as gasoline. For a long time it was thought that biodegradation of aromatic compounds required the presence of free oxygen. However, an increasing number of microorganisms have been discovered that consume aromatic hydrocarbons under strictly anoxic conditions. Metabolically versatile bacterium Rhodopseudomonas palustris (RP) can metabolize aromatic compounds aerobically or phototrophically under anaerobic conditions. Aside from its role as a model organism for studying metabolism of aromatic compounds, RP has also been extensively studied for its ability to produce hydrogen gas (H2), an attractive renewable energy source. To gain a better understanding of the biochemical underpinnings of the metabolism of aromatic compounds, as well as production of H2 by purple non-sulfur bacteria, we developed a curated genome-scale model of metabolism in RP. We have used the model to investigate RP’s : 1) Metabolic capabilities, 2) genetic robustness to various perturbation, 3) metabolic needs and limitations when using aromatic compounds as carbon source, 4) metabolism when producing H2. In case of photoheterotrophic metabolism, results of our analyses agree with previous observations regarding the connection between the oxidizing potential of the imported carbon source and the amount of carbon that is “wasted” by the system as excreted carbon dioxide (CO2). Anaerobic metabolism of most aromatic compounds proceeds through benzoyl-coA, the product of the biological Birch type reduction of benzoate ring. Examining the trade-off between productions of H2, CO2 and biomass, we find that the production of the first two gasses severely reduces the rate of cellular growth. Competition for use of limited quantity of electrons explains these observations. We find that depending on the ratio of carbon to protons and carbon to oxygen atoms in the imported carbon source, RP might import or export small quantities of protons from/to the surrounding system. Comparing the model predictions with measured metabolic byproducts and rates of growth verifies this finding. Eliminating these proton transfer processes will reduce the rate of cellular growth.

System-level analysis of phototrophic metabolism of aromatic compounds in Rhodopseudomonas palustris

251st National Meeting (2016)

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