# Redox poise in R. rubrum phototrophic growth drives large-scale changes in macromolecular pathways

## TL;DR

This study explores how photosynthetic bacteria like Rhodospirillum rubrum manage energy from photosynthesis by adjusting their production of cell components like lipids and proteins.

## Contribution

The paper introduces physics-based models to explain how redox poise influences macromolecular production in phototrophic bacteria.

## Key findings

- Redox poise in Rhodospirillum rubrum leads to large-scale changes in biosynthetic pathways and macromolecule levels.
- Reverse electron flow contributes minimally to reduced cofactor production compared to other oxidative processes.
- ATP levels drive reductive processes even when NADPH levels are low.

## Abstract

During photoheterotrophic growth on organic substrates, purple nonsulfur photosynthetic bacteria like Rhodospirillum rubrum can acquire electrons by multiple means, including oxidation of organic substrates, oxidation of inorganic electron donors (e.g., H2), and by reverse electron flow from the photosynthetic electron transport chain. These electrons are stored as reduced electron-carrying cofactors (e.g., NAD(P)H and ferredoxin). The overall ratio of oxidized to reduced cofactors (e.g., NAD(P)+:NAD(P)H), or ’redox poise’, is difficult to understand or predict, as are the cellular processes for dissipating these reducing equivalents. Using physics-based models that capture mass action kinetics consistent with the thermodynamics of reactions and pathways, a range of redox conditions for heterophototrophic growth are evaluated, from conditions in which the NADP+/NADPH levels approach thermodynamic equilibrium to conditions in which the NADP+/NADPH ratio is far above the typical physiological values. Modeling predictions together with experimental measurements indicate that the redox poise of the cell results in large-scale changes in the activity of biosynthetic pathways and, thus, changes in cell macromolecule levels (DNA, RNA, proteins, and fatty acids). Furthermore, modeling predictions indicate that during phototrophic growth, reverse electron flow from the quinone pool is a minor contributor to the production of reduced cofactors (e.g., NAD(P)H) compared to other oxidative processes (H2 and carbon substrate oxidation). Instead, the quinone pool primarily operates to aid ATP production. The high level of ATP, in turn, drives reduction processes even when NADPH levels are relatively low compared to NADP+ by coupling ATP hydrolysis to the reductive processes. The model, in agreement with experimental measurements of macromolecule ratios of cells growing on different carbon substrates, indicates that the dynamics of nucleotide versus lipid and protein production is likely a significant mechanism of balancing oxidation and reduction in the cell.

During photosynthesis, plants capture light and use it to produce oxygen from water. In doing so, the energy captured from sunlight is turned into chemical energy. The question that this report seeks to answer is how certain photosynthetic bacteria dissipate this energy. We are specifically interested in the energy associated with oxidation-reduction processes (the rusting of iron is an oxidation-reduction process). In photosynthesis, light energy is initially stored as high energy electrons, e−. These electrons are reduced in energy when reacted with positively charged chemical species such as protons, H+ to produce neutral hydrogen, e−+H+→H. For instance in the production of reduced carbon compounds, the electrons are captured in protons, e−+H+→H, while simultaneously carbon dioxide is reduced to a hydrogenated carbon compound, CO2 + 2 H → CH2O + 0.5 O2. A common example is when plants take in carbon dioxide and use it to produce sugar, C6H12O6. This requires that the nutrient, CO2 in this case, be more oxidized than the product, C6H12O6. More generally, the product is the cell (or biomass) itself, which typically has a molecular formula of approximately C4H7O2N. However, for some photosynthetic bacteria, the nutrient is already more reduced (contains more hydrogen) than the biomass. The question then is how do these bacteria dissipate the high potential electrons from photosynthesis into a reduced product if the starting material is already more reduced than the product? We find that the bacteria likely accomplish this by increasing their production of reduced biomass components such as lipids, proteins, and small acid-containing compounds.