German and American scientists have devised a biosynthetic pathway to achieve more efficient carbon fixation in plants by drawing on the expertise of DNA synthesis.
This new pathway is based on a new CO2-binding enzyme that is almost 20 times faster than the most prevalent enzyme in nature responsible for capturing CO2 in plants using sunlight for energy. The research has been published in Science.
"We have seen how efforts to directly assemble synthetic pathways for CO2 fixation in a living organism have been unsuccessful so far," says study director Tobias Erb of the Máx Planck Institute for Terrestrial Microbiology. So we took an approach radically different, reductionist, assembling the main synthetic components upstream in a test tube. "
Despite the great diversity of organisms on the planet that express enzymes to convert carbon dioxide into organic compounds such as sugars - as plants do through photosynthesis - efforts to take advantage of these capacities to transform CO2 into high-value products as biofuels and chemicals they have had limited success. Although the increasing concentration of CO2 in the atmosphere poses a challenge, researchers also see it as an opportunity.
The team started with several theoretical CO2 fixation routes that could lead to continuous carbon cycling, but they didn't stop there. "We did not restrict our design efforts to known enzymes, but we considered all the reactions that seemed biochemically feasible," says Erb.
Unlike DNA sequencing, where the language of life is read from an organism's genome, DNA synthesis first involves identifying a particular genetic element - such as an enzyme to fix carbon in the atmosphere - and write and express that code in a new system.
In the end, they obtained, through sequencing and synthesis, 17 different enzymes from nine different organisms across the three kingdoms of life and arranged these parts to achieve a working principle of CO2 fixation that exceeds what is known. can be found in nature. Erb calls this the "CETCH cycle" (crotonyl-CoA / ethylmalonyl-12 CoA / hydroxybutyryl-CoA).
IMPORTANT PIECES OF THE "CLIMATE PUZZLE"
By deploying the concept of metabolic "backsynthesis," dismantling the reaction step by step down to the smallest precursors, the team juggled thermodynamic conditions and developed a strategy that produced more promising results that favorably competed with natural metabolic pathways. So, they searched public databases for the enzymes that underpin their model and selected several dozen to test.
"First, we reconstituted its gradual CO2-binding reaction sequence, providing the ingredients to catalyze all the desired reactions, and then following the CO2 flow we discovered which particular key reaction was rate-limiting," Erb says. This turned out to be Methylsuccinyl-CoA dehydrogenase (Mcd), part of a family of enzymes involved in respiration, the metabolic reaction in the cells of organisms to convert nutrients like carbon into units of energy.
"To overcome this limitation, we designed the Mcd to use oxygen as an electron acceptor, to amplify the function, but this was not enough - Erb describes -. We had to replace the original pathway design with alternative reaction sequences, using more enzymatic engineering to minimize indiscriminate enzyme side reactions and introducing revision enzymes to correct the formation of dead-end metabolites. "
In support of the efforts of the MPI team, the United States Joint Genome Institute synthesized hundreds of Enoyl-CoA Carboxylase / Reductase (ECR) enzyme variants through its Community Science Program. This allowed the MPI team to zero ECR with the highest CO2 binding activity to successfully construct a more efficient artificial CO2 binding pathway in a test tube.
"ECRs are supercharged enzymes that are capable of fixing CO2 at a rate almost 20 times faster than the most widespread CO2-binding enzyme in nature, RuBisCo, which leads the strong increase involved in photosynthesis," explains Erb.
This chemical process takes advantage of sunlight to convert carbon dioxide into sugars that cells can use as energy along with other natural processes on the planet and represents the transformation of about 350 million tons of CO2 annually.