Abstract
Photosynthesis is the biological process that converts solar energy to biomass, bio-products, and biofuel. It is the only major natural solar energy storage mechanism on Earth. To satisfy the increased demand for sustainable energy sources and identify the mechanism of photosynthetic carbon assimilation, which is one of the bottlenecks in photosynthesis, it is essential to understand the process of solar energy storage and associated carbon metabolism in photosynthetic organisms. Researchers have employed physiological studies, microbiological chemistry, enzyme assays, genome sequencing, transcriptomics, and 13C-based metabolomics/fluxomics to investigate central carbon metabolism and enzymes that operate in phototrophs. In this report, we review diverse CO2 assimilation pathways, acetate assimilation, carbohydrate catabolism, the tricarboxylic acid cycle and some key, and/or unconventional enzymes in central carbon metabolism of phototrophic microorganisms. We also discuss the reducing equivalent flow during photoautotrophic and photoheterotrophic growth, evolutionary links in the central carbon metabolic network, and correlations between photosynthetic and non-photosynthetic organisms. Considering the metabolic versatility in these fascinating and diverse photosynthetic bacteria, many essential questions in their central carbon metabolism still remain to be addressed.
Highlights
Phototrophic bacteria use light as the energy source to produce phosphate bond energy (ATP) and reductants [e.g., NAD(P)H and reduced ferredoxin] through photosynthetic electron transport
We review diverse CO2 assimilation pathways, acetate assimilation, carbohydrate catabolism, the tricarboxylic acid cycle and some key, and/or unconventional enzymes in central carbon metabolism of phototrophic microorganisms
We review the studies of key central carbon metabolic pathways, including CO2 and acetate assimilations, carbohydrate catabolism and the tricarboxylic acid (TCA) cycle, and highlight several essential and/or unconventional enzymes contributing to the critical metabolic pathways in phototrophic bacteria
Summary
Phototrophic bacteria use light as the energy source to produce phosphate bond energy (ATP) and reductants [e.g., NAD(P)H and reduced ferredoxin] through photosynthetic electron transport. We review the studies of key central carbon metabolic pathways, including CO2 and acetate assimilations, carbohydrate catabolism and the tricarboxylic acid (TCA) cycle, and highlight several essential and/or unconventional enzymes contributing to the critical metabolic pathways in phototrophic bacteria. Acetate assimilation Many AAPs, AnAPs, FAPs, GSBs, and heliobacteria have been reported to grow heterotrophically or mixotrophically (in the presence of CO2) on acetate with three acetate assimilation mechanisms: the (oxidative) glyoxylate cycle (Figure 3A), pyruvate synthase (Figure 3B), and the ethylmalonyl-CoA pathway (Figure 3C).
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