Carbon dioxide can “change” glucose and fatty acids


Schematic diagram of the conversion of carbon dioxide and water into long-chain products through electrochemically coupled biological fermentation Source: Courtesy of the research team

Previously, Chinese scientists realized the de novo synthesis of carbon dioxide to starch for the first time in the world. So, in addition to “changing” starch, can carbon dioxide “change” other things?

The answer is yes!

On April 28, a new study published in Nature-Catalysis in the form of a cover article showed that electrocatalysis combined with biosynthesis can efficiently reduce carbon dioxide into high concentrations of acetic acid, further using microorganisms, and can synthesize glucose and oils.

“The work couples artificial electrocatalysis and biological enzyme catalysis, and develops a new way from water and carbon dioxide to small molecules of acetate containing energy chemistry, and then engineering modified yeast microorganisms to catalyze the synthesis of high value-added products such as glucose and free fatty acids, providing a new technology for artificial and semi-artificial synthesis of ‘grains’.” Li Can, academician of the Chinese Academy of Sciences and director of the Catalysis Professional Committee of the Chinese Chemical Society, commented.

This achievement was jointly completed by the Xia Chuan Research Group of the University of Electronic Science and Technology of China, the Yu Tao Research Group of the Shenzhen Institute of Advanced Technology of the Chinese Academy of Sciences, and the Zeng Jie Research Group of the University of Science and Technology of China.

Industrial exhaust gases become “vinegar” under mild conditions

So, how exactly does carbon dioxide become glucose and fat?

“First of all, we need to convert carbon dioxide into raw materials that can be used by microorganisms to facilitate microbial fermentation.” Zeng Jie introduced that clean and efficient electrocatalytic technology can work under normal temperature and pressure conditions, which is the ideal choice for this process, and they have developed many mature electrocatalyst systems.

As for what kind of “raw material” to convert, the researchers set their sights on acetic acid. Because it is not only the main ingredient in vinegar, but also an excellent biosynthetic carbon source that can be converted into other biological substances such as glucose.

“Direct electrolysis of carbon dioxide can obtain acetic acid, but the efficiency is not high, so we adopt a ‘two-step’ strategy – first efficiently obtain carbon monoxide, and then from carbon monoxide to acetic acid.” Zeng Jie said.

Even so, the current electrosynthetic efficiency (i.e., faraday acetate efficiency) and purity of carbon monoxide to acetic acid are still unsatisfactory. In this regard, the researchers found that the acetate formed by carbon monoxide catalyzed by the catalyst is specifically affected by the geometry of the catalyst surface, and the grain boundary copper formed by the pulsed electrochemical reduction process can catalyze the synthesis of faradi acetate with an efficiency of up to 52%.

“In actual production, increasing the current can increase the power, but may reduce the Faraday efficiency.” Xia Chuan said that it is like extending the daily working hours from 8 hours to 12 hours, although the working hours are longer, but the work efficiency will decline. “When we increased the maximum bias current density to 321mA/cm2 (mA per square centimeter), the faraday acetate efficiency remained at 46%, which was a good balance between ‘high current’ and ‘high faraday efficiency’.”

However, the acetic acid produced by conventional electrocatalytic units is mixed with many electrolyte salts and cannot be used directly for biological fermentation. Therefore, in order to “feed” microorganisms, it is not only necessary to improve the conversion efficiency and ensure the quantity of “food”, but also to obtain pure acetic acid without electrolyte salts to ensure the quality of “food”.


Solid-state electrolyte reaction Source: Courtesy of the research team

“We used the new solid electrolyte reaction device to use the solid electrolyte instead of the original electrolyte salt solution, and directly obtained a pure aqueous solution of acetic acid without further separation.” Xia Chuan introduced that using this device, it is possible to continuously prepare an aqueous acetic acid solution with a purity of 97% for more than 140 hours within a bias current density of 250mA/cm2.


Aqueous acetic acid solution and sodium acetate powder prepared by the research team through a solid electrolyte reactor Source: Courtesy of the research team

Microorganisms “eat vinegar” to produce glucose

After obtaining acetic acid, the researchers tried to use the microorganism Saccharomyces cerevisiae to synthesize glucose.

“Saccharomyces cerevisiae is mainly used to ferment cheese, steamed buns, wine, etc., and because of its excellent industrial properties, it is often used as a model organism for microbial manufacturing and cell biology research.” Yu Tao said that the process of synthesizing glucose through acetic acid by Saccharomyces cerevisiae is like microorganisms “eating vinegar”, and Saccharomyces cerevisiae synthesizes glucose by constantly “eating vinegar”. “However, in this process, Saccharomyces cerevisiae itself also metabolizes a part of the glucose, so the yield is not high.”

In response, the research team abolished the ability of Saccharomyces cerevisiae to metabolize glucose by knocking out three key enzyme elements in Saccharomyces cerevisiae that metabolize glucose — Glk1, Hxk1 and Hxk2. After knockout, the engineered yeast strains in the experiment produced 1.7 g/L of glucose under the condition of shaker fermentation.

“The model organism Saccharomyces cerevisiae synthesized glucose ‘from scratch’ at the gram level, which represents the high production level and development potential of this strategy.” Yu Tao said that in order to further increase the production of synthetic glucose, it is necessary not only to abolish the ability of Saccharomyces cerevisiae to reuse glucose endogenously, but also to strengthen its own ability to accumulate glucose.

So the researchers knocked out two enzyme elements (YLLR446W, EMI2) suspected of having the ability to metabolize glucose, and inserted glucose phosphatase elements (AGPP, YIHX) from Ubiquitous and E. coli.

Yu Tao said that these two enzymes can “find another way” to convert phosphoric acid molecules in other pathways in yeast into glucose, increasing the yeast’s ability to accumulate glucose. The engineered yeast strain produced 2.2 g/L glucose yield and increased yield by 30%.


The ferment broth (brown solution) of the yeast strain used to prepare glucose after the transformation and the prepared glucose (white solution) Source: Courtesy of the research team

New catalytic methods facilitate the production of high value-added compounds

The process of efficient carbon dioxide electroremediation to prepare high value-added chemicals and fuels is considered by the academic community to be one of the important research directions to achieve the transformation of “zero carbon emission” substances in the future.

At present, the research on carbon dioxide electroretrovirus technology is mostly limited to small molecular products such as one carbon and two carbon, and how to efficiently and sustainably convert carbon dioxide into energy-rich carbon-based long-chain molecules is still a huge challenge.

“In order to avoid the limitations of carbon dioxide electroreverted products, it can be considered to couple the carbon dioxide electroretrige process with the biological process, with the electrocatalytic product as an electronic carrier for microbes to subsequently ferment and synthesize long carbon chain chemical products for production and life.” Xia Chuan said.

The right electron carrier is essential for microbial fermentation. Because the vapor phase products of carbon dioxide electroretrid are difficult to dissolve in water and the bioavailability efficiency is low, the liquid phase products of carbon dioxide electroremediation are often preferred as the electronic carrier of biological fermentation. However, the liquid products obtained in ordinary electrochemical reactors are mixtures mixed with electrolyte salts and cannot be used directly for biological fermentation. In view of this, the development of solid-state electrolyte reactors effectively solves the problem of separation of carbon dioxide electroreverted liquid products, and can continuously and stably provide liquid electronic carriers for microbial fermentation.

As a living cell factory, the advantage of microorganisms is that the product diversity is very high, and it can synthesize many compounds that cannot be artificially produced or have low artificial production efficiency, and it is a very rich “material synthesis toolbox”. For example, in the common processing of food and drugs such as liquor, steamed buns, and antibiotics, microorganisms play an important role.

Zeng Jie said, “Through the new catalytic method of electrocatalysis combined with biosynthesis, the added value of carbon can be effectively improved. Next, we will further investigate the homocompatibility and compatibility of the two platforms of electrocatalysis and bio-fermentation. “In the future, to synthesize starch, manufacture pigments, produce drugs, etc., while maintaining the original electrocatalytic facilities, it is only necessary to replace the microorganisms used in fermentation.

“This work opens up new strategies for the catalytic preparation of glucose and other food products by electrochemically combining living cells, provides a new paradigm for the further development of new electricity-driven agriculture and bio-manufacturing, and is an important development direction in carbon dioxide utilization.” Deng Zixin, academician of the Chinese Academy of Sciences and director of the State Key Laboratory of Microbial Metabolism at Shanghai Jiao Tong University, commented. (Source: China Science Daily Diao Wenhui)

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