Cell-free Synthesis of Starch from CO2 with 8.5 fold higher Efficiency than Nature



October 1, 2021. By Kolemann Lutz


Scientists in China developed the first method for the artificial, cell-free synthesis of starch (C6H10O5) from CO2 with a rate of 22 nanomoles of CO2 per minute per milligram of total catalyst, which is an ~8.5-fold higher rate than starch synthesis from maize. Results are published in in the Synthetic Biology 2021 Report of Science.


The chemoenzymatic system and starch anabolic pathway method with 11 core reactions is many times more efficient than the process that takes place naturally in plants and saves 90%+ of cultivated land and freshwater resources.


Starch is commonly used in the production of foods (grain) as thickeners, textiles, brewing, pharmaceuticals and many other goods. At present, it is mainly produced by crops such as maize by fixing CO2 through photosynthesis. This process involves about 60 biochemical reactions as well as complex physiological regulation. Many plants turn glucose from photosynthesis into polymers that form insoluble starch granules ideal for long-term energy storage in roots and seeds. The theoretical energy conversion efficiency of this process is only about 2%. Strategies for the sustainable supply of starch and use of CO2 are urgently needed to overcome major challenges of mankind, such as the food crisis and climate change.


To address this, scientists at the Tianjin Institute of Industrial Biotechnology (TIB) of the Chinese Academy of Sciences (CAS) designed a chemoenzymatic system as well as an artificial starch anabolic route consisting of only 11 core reactions to convert CO2 into starch.


By focusing on artificial starch biosynthesis and CO2 utilization since 2015, TIB bioengineered a system that integrated chemical and biological catalytic modules to utilize high-density energy and high-concentration CO2.


In a chemoenzymatic system with spatial and temporal segregation, their artificial starch anabolic pathway (ASAP) driven by hydrogen, converts CO2 to starch at a rate of 22 nanomoles of CO2 per minute per milligram of total catalyst, an ~8.5-fold higher rate than starch synthesis in maize. This approach opens the way toward future chemo-biohybrid starch synthesis from CO2.


This artificial starch anabolic pathway relies on engineered recombinant enzymes from many different source organisms and can be tuned to produce two main components of starch: amylose (C₆H₁₀O₅, a preferred starch for storage in plants) or amylopectin (polysaccharide soluble polymer) at excellent rates.


"According to the current technical parameters, the annual production of starch in a one-cubic-meter bioreactor theoretically equates with the starch annual yield from growing 1/3 hectare of maize without considering the energy input," said Cai Tao, lead author of the study.


This work would open a window for industrial manufacturing of starch from CO2.

"If the overall cost of the process can be reduced to a level economically comparable with agricultural planting in the future, it is expected to save more than 90% of cultivated land and freshwater resources," said MA Yanhe, corresponding author of the study.


In addition, it would also help to avoid the negative environmental impact of using pesticides and fertilizers, improve human food security, facilitate a carbon-neutral bioeconomy, and eventually promote the formation of a sustainable bio-based society.


The synthetic biology, protein engineering, and chemoenzymatic starch system method holds the potential to manufacture starch from the air on CO2 abundant plants such as Earth, Mars, and Venus and to recovering significant amounts of upmass, costs, land, plants, and resources

 

Cai Tao et al, Cell-free chemoenzymatic starch synthesis from carbon dioxide, Synthetic Biology, Science (2021). DOI: 10.1126/science.abh4049