To help find solutions to the planet's climate crisis, MIT Associate Professor Daniel Suess is looking to Earth's ancient past.
Early in the evolution of life, cells gained the ability to perform reactions such as transferring electrons from one atom to another. These reactions, which help cells to build carbon-containing or nitrogen-containing compounds, rely on specialized enzymes with clusters of metal atoms.
By learning more about how those enzymes work, Suess hopes to eventually devise new ways to perform fundamental chemical reactions that could help capture carbon from the atmosphere or enable the development of alternative fuels.
"We have to find some way of rewiring society so that we are not just relying on vast reserves of reduced carbon, fossil fuels, and burning them using oxygen," he says. "What we're doing is we're looking backward, up to a billion years before oxygen and photosynthesis came along, to see if we can identify the chemical principles that underlie processes that aren't reliant on burning carbon."
His work could also shed light on other important cellular reactions such as the conversion of nitrogen gas to ammonia, which is also the key step in the production of synthetic fertilizer.
Exploring chemistry
Suess, who grew up in Spokane, Washington, became interested in math at a young age, but ended up majoring in chemistry and English at Williams College, which he chose based on its appealing selection of courses.
"I was interested in schools that were more focused on the liberal arts model, Williams being one of those. And I just thought they had the right combination of really interesting courses and freedom to take classes that you wanted," he says. "I went in not expecting to major in chemistry, but then I really enjoyed my chemistry classes and chemistry teachers."
In his classes, he explored all aspects of chemistry and found them all appealing.
"I liked organic chemistry, because there's an emphasis on making things. And I liked physical chemistry because there was an attempt to have at least a semiquantitative way of understanding the world. Physical chemistry describes some of the most important developments in science in the 20th century, including quantum mechanics and its application to atoms and molecules," he says.
After college, Suess came to MIT for graduate school and began working with chemistry professor Jonas Peters, who had recently arrived from Caltech. A couple of years later, Peters ended up moving back to Caltech, and Suess followed, continuing his PhD thesis research on new ways to synthesize inorganic molecules.
His project focused on molecules that consist of a metal such as iron or cobalt bound to a nonmetallic group known as a ligand. Within these molecules, the metal atom typically pulls in electrons from the ligand. However, the molecules Suess worked on were designed so that the metal would give up its own electrons to the ligand. Such molecules can be used to speed up difficult reactions that require breaking very strong bonds, like the nitrogen-nitrogen triple bond in N2.
During a postdoc at the University of California at Davis, Suess switched gears and began working on biomolecules - specifically, metalloproteins. These are protein enzymes that have metals tucked into their active sites, where they help to catalyze reactions.
Suess studied how cells synthesize the metal-containing active sites in these proteins, focusing on an enzyme called iron-iron hydrogenase. This enzyme, found mainly in anaerobic bacteria, including some that live in the human digestive tract, catalyzes reactions involving the transfer of protons and electrons. Specifically, it can combine two protons and two electrons to make H2, or can perform the reverse reaction, breaking H2 into protons and electrons.
"That enzyme is really important because a lot of cellular metabolic processes either generate excess electrons or require excess electrons. If you generate excess electrons, they have to go somewhere, and one solution is to put them on protons to make H2," Suess says.
Global scale reactions
Since joining the MIT faculty in 2017, Suess has continued his investigations of metalloproteins and the reactions that they catalyze.
"We're interested in global-scale chemical reactions, meaning they're occurring on the microscopic scale but happening on a huge scale," he says. "They impact the planet and have determined what the molecular composition of the biosphere is and what it's going to be."
Photosynthesis, which emerged around 2.4 billion years ago, has had the biggest impact on the atmosphere, filling it with oxygen, but Suess focuses on reactions that cells began using even earlier, when the atmosphere lacked oxygen and cell metabolism could not be driven by respiration.
Many of these ancient reactions, which are still used by cells today, involve a class of metalloproteins called iron-sulfur proteins. These enzymes, which are found in all kingdoms of life, are involved in catalyzing many of the most difficult reactions that occur in cells, such as forming carbon radicals and converting nitrogen to ammonia.
To study the metalloenzymes that catalyze these reactions, Suess's lab takes two different approaches. In one, they create synthetic versions of the proteins that may contain fewer metal atoms, which allows for greater control over the composition and shape of the protein, making them easier to study.
In another approach, they use the natural version of the protein but substitute one of the metal atoms with an isotope that makes it easier to use spectroscopic techniques to analyze the protein's structure.
"That allows us to study both the bonding in the resting state of an enzyme, as well as the bonding and structures of reaction intermediates that you can only characterize spectroscopically," Suess says.
Understanding how enzymes perform these reactions could help researchers find new ways to remove carbon dioxide from the atmosphere by combining it with other molecules to create larger compounds. Finding alternative ways to convert nitrogen gas to ammonia could also have a big impact on greenhouse gas emissions, as the Haber Bosch process now used to synthesize fertilizer produces requires huge amounts of energy.
"Our primary focus is on understanding the natural world, but I think that as we're looking at different ways to wire biological catalysts to do efficient reactions that impact society, we need to know how that wiring works. And so that is what we're trying to figure out," he says.
