Just like mechanics cobble together old engine parts to build a new roadster, synthetic biologists have remade chloroplasts, the engine at the heart of photosynthesis. By combining the light-harvesting machinery of spinach plants with enzymes from nine different organisms, scientists report making an artificial chloroplast that operates outside of cells to harvest sunlight and use the resulting energy to convert carbon dioxide (CO2) into energy-rich molecules. The researchers hope their souped-up photosynthesis system might eventually convert CO2 directly into useful chemicals—or help genetically engineered plants absorb up to 10 times the atmospheric CO2 of regular ones.
“[This] is very ambitious,” says Frances Arnold, a chemical engineer at the California Institute of Technology who wasn’t involved in the research. She says the work’s effort to reprogram biology could improve attempts to convert CO2 into plant matter and also directly into useful chemicals.
Photosynthesis is a two-step process. In chloroplasts, chlorophyll molecules absorb sunlight and pass the extra energy to molecular partners that use it to generate the energy-storing chemicals adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH). A suite of other enzymes working in a complex cycle then use ATP and NADPH to convert CO2 from the air into glucose and other energy-rich organic molecules that the plant uses to grow.
CO2 conversion starts with an enzyme called RuBisCO, which prompts CO2 to react with a key organic compound, starting a chain of reactions needed to make vital metabolites in plants. As effective as photosynthesis is, it also has a problem, says Tobias Erb, a synthetic biologist at the Max Planck Institute for Terrestrial Microbiology. “RuBisCO is superslow,” he says. Each copy of the enzyme can grab and use just five to 10 CO2 molecules per second. That puts a speed limit on how fast plants can grow.
In 2016, Erb and his colleagues sought to ramp things up by designing a new set of chemical reactions. Instead of RuBisCO, they substituted a bacterial enzyme that can catch CO2 molecules and force them to react 10 times faster. In combination with 16 other enzymes from nine different organisms, this created a new CO2-to-organic-chemical cycle they dubbed the CETCH cycle.
That took care of the second step. But to get the whole process to run on sunlight—the first step—Erb and his colleagues turned to chloroplast components called thylakoid membranes, pouchlike assemblies that hold chlorophyll and other photosynthesizing enzymes. Other researchers had previously shown that thylakoid membranes can operate outside plant cells. So Erb and his colleagues plucked thylakoid membranes from spinach leaf cells and showed that their assemblies, too, could absorb light and transfer its energy to ATP and NADPH molecules. Pairing the light-harvesting thylakoids with their CETCH cycle system allowed the team to use light to continually convert CO2 to an organic metabolite called glycolate, they reported yesterday in Science.
In order to integrate the light-harvesting apparatus with the CETCH cycle, the researchers had to make a few tweaks, Erb notes, swapping in and out a few of the CETCH pathway’s enzymes. To optimize the full ensemble, Erb and his colleagues teamed up with Jean-Christophe Baret, a microfluidics expert at the Paul Pascal Research Center. Baret’s team designed a device that generates thousands of tiny water droplets in oil and injects each one with different amounts of thylakoid membrane assemblies and CETCH cycle enzymes. That allowed the researchers to home in on the most efficient recipe for producing glycolate. Further comparisons of all the possible combinations and concentrations of different elements could make the process even more efficient, Arnold comments. “This is a nice way to do it.”
Erb says he and his colleagues hope to modify their setup further to produce other organic compounds that are even more valuable than glycolate, such as drug molecules. They also hope to more efficiently convert captured CO2 into organic compounds that plants need to grow. That would open the door to engineering the genes for this novel photosynthesis pathway into crops to create novel varieties that grow much faster than current varieties—a boon for agriculture in a world with a booming population.