Prospects for engineering Ralstonia eutropha and Zymomonas mobilis for the autotrophic production of 2,3-butanediol from CO2 and H2

Fructose metabolism in Entner-Doudoroff pathway-deficient Cupriavidus necator H16 depends on the Calvin shunt

As a facultative chemolithoautotrophic bacterium, Cupriavidus necator H16 uses the Entner-Doudoroff (ED) pathway for heterotrophic growth on carbohydrates such as fructose and the Calvin cycle for lithoautotrophic carbon dioxide fixation. In a previous study, we found that an ED pathway-deficient C. necator strain can survive on fructose, but the underlying metabolic pathway remained unclear. This study aimed to elucidate the metabolic mechanism of fructose metabolism in this ED pathway-deficient C. necator strain. First, the metabolic characteristics of fructose catabolism in the deficient strain were examined. Then, the roles of glycolysis/gluconeogenesis, the Calvin shunt, and the non-oxidative pentose phosphate pathway (non-OxPPP) in the metabolism of fructose were identified through comparative transcriptomic analysis combined with 13C tracer experiments. Further growth experiments using knockout strains of key genes involved in these pathways confirmed that the non-OxPPP compensates for the blocked ED pathway to metabolize fructose and provide a precursor for the Calvin shunt, thereby driving subsequent carbon fluxes. Additionally, phosphoglycolate salvage pathways, particularly the malate cycle, are crucial for recycling glycolate-2-phosphate produced during RuBisCO-catalyzed oxidation. This study revealed a novel fructose metabolism mechanism in C. necator and highlighted its metabolic flexibility, thereby deepening our understanding of its carbon utilization strategies and providing a theoretical basis for further metabolic engineering research.

Terephthalate Copolyesters Based on 2,3-Butanediol and Ethylene Glycol and Their Properties

This study explores the synthesis and performance of novel copolyesters containing 2,3-butanediol (2,3-BDO) as a biobased secondary diol. This presents an opportunity for improving their thermal properties and reducing crystallinity, while also being more sustainable. It is, however, a challenge to synthesize copolyesters of sufficient molecular weight that also have high 2,3-BDO content, due to the reduced reactivity of secondary diols compared to primary diols. Terephthalate-based polyesters were synthesized in combination with different ratios of 2,3-BDO and ethylene glycol (EG). With a 2,3-BDO to EG ratio of 28:72, an Mn of 31.5 kDa was reached with a Tg of 88 °C. The Mn dropped with increasing 2,3-BDO content to 18.1 kDa for a 2,3-BDO to EG ratio of 78:22 (Tg = 104 °C) and further to 9.8 kDa (Tg = 104 °C) for the homopolyester of 2,3-BDO and terephthalate. The water and oxygen permeability both increased significantly with increasing 2,3-BDO content and even the lowest content of 2,3-BDO (28% of total diol) performed significantly worse than PET. The incorporation of 2,3-BDO had little effect on the tensile properties of the polyesters, which were similar to PET. The results suggest that 2,3-BDO can be potentially applied for polyesters requiring higher Tg and lower crystallinity than existing materials (mainly PET).

Gas fermentation combined with water electrolysis for production of polyhydroxyalkanoate copolymer from carbon dioxide by engineered Ralstonia eutropha

A recycled-gas closed-circuit culture system was developed for safe autotrophic cultivation of a hydrogen-oxidizing, polyhydroxyalkanoate (PHA)-producing Ralstonia eutropha, using a non-combustible gas mixture with low-concentration of H2 supplied by water electrolysis. Automated feedback regulation of gas flow enabled input of H2, CO2, and O2 well balanced with the cellular demands, leading to constant gas composition throughout the cultivation. The engineered strain of R. eutropha produced 1.71 g/L of poly(3-hydroxybutyrate-co-12.5 mol% 3-hydroxyhexanoate) on a gas mixture of H2/CO2/O2/N2 = 4:12:7:77 vol% with a 69.2 wt% cellular content. Overexpression of can encoding cytosolic carbonic anhydrase increased the 3HHx fraction up to 19.6 mol%. The yields of biomass and PHA on input H2 were determined to be 72.9 % and 63.1 %, corresponding to 51.0 % and 44.2 % yield on electricity, respectively. The equivalent solar-to-biomass/PHA efficiencies were estimated to be 2.1-3.8 %, highlighting the high energy conversion capability of R. eutropha.