Scientists engineer synthetic chloroplasts

Photosynthesis is an ingenious catalytic system, allowing plants and algae to use the energy from sunlight to convert carbon dioxide into the carbohydrates they need for their cells’ metabolism—a process called carbon fixation. Humans have now hijacked this natural process, crafting semi-synthetic, cell-size versions of it that harness the light-capturing properties of spinach membranes to power a 16-enzyme catalytic cycle that turns CO₂ into a small organic molecule (Science 2020, DOI: 10.1126/science.aaz6802).

“The blueprint is the chloroplast. Our design aimed at building something similar,” says Tobias Erb, a synthetic biologist at the Max Planck Institute of Terrestrial Microbiology, who co-led the work. It's “a miniatured machinery that does one of the most important reactions on the planet—driving CO₂ fixation with light.”

In 2016, Erb and his colleagues engineered a catalytic cycle they called the CETCH cycle, which combines enzymes from nine different organisms to transform CO₂ into the two-carbon molecule glyoxylate. In the current work, the researchers used photosynthetic membranes from chloroplasts in spinach to convert light into energy for running these reactions.

The researchers first showed that exposing the membranes to light could catalyze the reduction of NADP+ to make NADPH and the phosphorylation of ADP to make ATP. NADPH and ATP are two energy-packed biomolecules that drive carbon fixation in both natural photosynthesis and the CETCH cycle. Then the scientists mixed the membranes into water-in-oil droplets about 90 µm in diameter that contained enzymes needed for two simple ATP- and NADPH-powered carbon-fixing reactions and found that exposing the droplets to light successfully catalyzed the reactions. They then did the same with enzymes in the CETCH cycle to generate continuous CO₂ fixation.

Getting the CETCH cycle running efficiently was much more challenging, Erb says, because the individual reactions produce reactive oxygen species that can gum up the works. “In the natural system there are automatic repair systems,” he says. For example, proteins that aren’t working right get degraded and replaced. “In our system that is not possible yet,” Erb explains. So they used a microfluidic system to create multiple droplets in which they tested different reaction parameters, such as the ratio of membranes to enzymes, to find the conditions that worked best.

Ultimately, the team created droplets that, when exposed to light, could drive CO₂ fixation via the CETCH cycle for 2 h, with a production rate 100 times faster than previous synthetic efforts. “We are at least getting into the realm of biological reality, which is very exciting for us,” Erb says.

The small compartments inside cells in the study are “an outstanding achievement that demonstrates the creation of a functional hybrid synthetic organelle,” says Neal Devaraj, a synthetic biologist at the University of California San Diego. “The next step,” Deveraj says, “will be to engineer systems that mimic organelles from wholly synthetic components.”