Starch is polysaccharide produced by plants for energy storage during photosynthesis. Its natural synthesis is a complex biochemical process involving over 60 steps and hundreds of enzymes. Until 2021, the only way for human to obtain starch is traditional agriculture that has persisted for over 10,000 years. However, plants have adapted to low carbon dioxide concentrations and low light energy densities during evolution, so their photosynthesis destined to be inefficient. Only 2% sunlight energy is fixed in starch theoretically, while the actual efficiency is often less than 0.7%. Crop yields are also affected by weather, diseases, and pests. Sometimes, months or one year hard work will be wiped out by typhoons or droughts. Additionally, agriculture is not environmentally friendly, as numerous land and freshwater are consumed, and our ecosystem are destroyed by pesticides and fertilizers.
How does Cell-Free Method for Man-Made Starch design?
There are also many unclear principles, so it is infeasible to mimic photosynthesis directly. Computer was used by Chinese scientists to combined existing biochemical reactions for a new synthetic pathway based on minimal energy dissipation, the fewest reaction steps and the least carbon loss. Ultimately, a 9-step artificial process was selected from 6,568 chemical reactions.
However, simply tossing these ingredients into a container won't yield any starch we desire. Chinese scientists carefully examined each step and found that some enzymes were inhibited by products and some reactions were thermodynamically unfavorable. Therefore, the process was divided into four modules placed in different containers. These modules include C1 module for formaldehyde , C3 module for glyceraldehyde phosphate, C6 module for glucose, and Cn module for starch synthesis. Enzymes from 31 organisms were also modified by researchers. When artificial pathway was expanded to 11 steps for addressing these problem, the cell-free starch synthesis was finally achievable. Researchers find native and artificial starch are completely same in their structures, as they were examined by nuclear magnetic resonance. This pathway is named ASAP 1.0 that is abbreviation of artificial starch anabolic pathway.
Its efficiency is too low to reach a fraction of the natural process, let alone meet industrial production. The primary reason is low enzyme activity, inhibition from ATP and ADP, and competition for ATP among enzymes. Chinese scientists chemically modified one of the enzymes and found two other better-performing ones in mutant strains to create ASAP 2.0. This version was 7.6 times faster than ASAP 1.0. It has a speed comparable to natural process in corn.
At this point, humans had successfully making starch from methanol in laboratory. The next step was to couple ASAP 2.0 with chemical reactions that produce methanol from carbon dioxide and sunlight. Its challenge lies in the mismatched rates between chemical catalysis and biosynthesis, so the rapid accumulated intermediates always weaken enzyme activity. Therefore, research team placed them in different containers and reaction parameters were optimized. It is astonishing that 9% solar energy is absorbed by ASAP 3.0 theoretically. Its actual efficiency is 7%, even losses were taken into account such as gas compression. ASAP 3.0 synthesizes artificial starch 8.5 times faster than corn. Now our humans declare proudly we really turn the air into bread.