The study is now online in the Journal of the American Chemical Society.
DOE/Lawrence Berkeley National Laboratory
Enzymes inside natural BMCs can convert carbon dioxide into organic compounds that can be used by the bacteria, isolate toxic or volatile compounds from the surrounding cell, and carry out other chemical reactions that provide energy for the cell.
In this study, researchers introduced the iron-sulfur cluster into the tiny pores in the shell building block. This engineered protein serves as an electron relay across the shell, which is key to controlling the chemical reactivity of substances inside the shell.
Clement Aussignargues, the lead author of the study and postdoctoral researcher in the MSU-DOE Plant Research Laboratory in Michigan, said, "The beauty of our system is that we now have all the tools, notably the crystallographic structure of the engineered protein, to modify the redox potential of the system--its ability to take in electrons (reduction) or give off electrons (oxidation).
"If we can control this, we can expand the range of chemical reactions we can encapsulate in the shell. The limit of these applications will be what we put inside the shells, not the shells themselves."
He added, "Creating a new microcompartment from scratch would be very, very complicated. That's why we're taking what nature put before us and trying to add to what nature can do."
To design the metal binding site, Kerfeld's group first had to solve the structures of the building blocks of the nanocompartment to use as the template for design. These building blocks self-assemble into synthetic shells, which measure just 40 nanometers, or billionths of a meter, in diameter. The natural form of the shells can be up to 12 times larger.
The iron-sulfur cofactor of the engineered protein, which was produced in E. coli bacteria, was very stable even when put through several redox cycles--a characteristic essential for future applications, Aussignargues said. "The engineered protein was also more stable than its natural counterpart, which was a big surprise," he said. "You can treat it with things that normally make proteins fall apart and unwind."
A major challenge in the study was to prepare the engineered protein in an oxygen-free environment to form tiny crystals that best preserve their structure and their cofactor for X-ray imaging, Kerfeld said. The crystals were prepared in an air-sealed glovebox at MSU, frozen, and then shipped out for X-ray studies at Berkeley Lab's ALS and SLAC National Accelerator Laboratory's Stanford Synchrotron Radiation Lightsource (SSRL).
In follow-up work, the research team is exploring how to incorporate different metal centers into BMC shells to access a different range of chemical reactivity, she said.
"I'm working on incorporating a completely different metal center, which has a very positive reduction potential compared to the iron-sulfur cluster," said Jeff Plegaria, a postdoctoral researcher at the MSU-DOE Plant Research Laboratory who contributed to the latest study. "But it is the same sort of idea: To drive electrons in or out of the compartment."
He added, "The next step is to encapsulate proteins that can accept electrons into the shells, and to use that as a probe to watch the electron transfer from the outside of the compartment to the inside." That will bring researchers closer to creating specific types of pharmaceuticals or other chemicals.