In an effort to discover answers to the planet’s environmental crisis, MIT Associate Professor Daniel Suess is examining Earth’s remote history.
At the beginnings of life’s evolution, cells acquired the capability to carry out reactions such as transferring electrons from one atom to another. These reactions, which assist cells in forming carbon-based or nitrogen-based compounds, depend on unique enzymes featuring clusters of metal atoms.
By deepening his understanding of how these enzymes function, Suess aims to eventually invent innovative methods for conducting essential chemical reactions that could aid in capturing carbon from the atmosphere or facilitate the creation of alternative fuels.
“We must discover a way to reconfigure society so that we do not solely depend on vast stocks of reduced carbon, fossil fuels, and their combustion using oxygen,” he states. “What we’re doing is looking backward, up to a billion years before the advent of oxygen and photosynthesis, to determine if we can pinpoint the chemical principles that govern processes that do not depend on burning carbon.”
His research may also illuminate other significant cellular reactions, such as the conversion of nitrogen gas to ammonia, which is a crucial step in synthesizing fertilizers.
Investigating chemistry
Suess, who was raised in Spokane, Washington, developed an early interest in mathematics but ultimately pursued majors in chemistry and English at Williams College, chosen for its appealing array of courses.
“I was drawn to institutions that emphasized the liberal arts model, with Williams being one of those. I believed they offered an ideal mix of intriguing courses and the freedom to select classes that interested you,” he recalls. “I entered not anticipating a major in chemistry, but I found that I truly enjoyed my chemistry classes and instructors.”
Throughout his coursework, he examined various aspects of chemistry and found them all captivating.
“I appreciated organic chemistry for its focus on creation, and I liked physical chemistry because it attempted at least a semiquantitative approach to comprehending the world. Physical chemistry outlines some of the most significant scientific advancements of the 20th century, such as quantum mechanics and its application to atoms and molecules,” he notes.
After completing his undergraduate studies, Suess joined MIT for graduate education and started collaborating with chemistry professor Jonas Peters, who had just relocated from Caltech. A few years later, Peters returned to Caltech, and Suess followed, continuing his PhD thesis on innovative methods to synthesize inorganic compounds.
His project centered on compounds that comprise a metal such as iron or cobalt linked to a nonmetallic group referred to as a ligand. In these compounds, the metal atom generally attracts electrons from the ligand. However, the molecules Suess investigated were engineered so that the metal would relinquish its electrons to the ligand. Such compounds can facilitate challenging reactions that necessitate breaking extremely strong bonds, like the nitrogen-nitrogen triple bond in N2.
During a postdoctoral fellowship at the University of California at Davis, Suess shifted his focus and began researching biomolecules — particularly, metalloproteins. These are protein enzymes that contain metals within their active sites, aiding in catalyzing reactions.
Suess examined how cells create the metal-containing active sites within these proteins, concentrating on an enzyme called iron-iron hydrogenase. This enzyme, primarily found in anaerobic bacteria, including some residing in the human digestive system, catalyzes reactions involving the transfer of protons and electrons. Specifically, it can combine two protons and two electrons to produce H2, or perform the reverse reaction, breaking H2 into protons and electrons.
“That enzyme is extremely significant because many cellular metabolic processes either produce excess electrons or require surplus electrons. If you create extra electrons, they need to be allocated somewhere, and one possibility is to associate them with protons to form H2,” Suess explains.
Reactions on a global scale
Since becoming part of the MIT faculty in 2017, Suess has continued his exploration of metalloproteins and the reactions they facilitate.
“We are focused on global-scale chemical reactions, meaning they are occurring at the microscopic level but manifesting on a grand scale,” he states. “They affect the planet and have defined the molecular makeup of the biosphere and its future state.”
Photosynthesis, which originated around 2.4 billion years ago, has significantly impacted the atmosphere by enriching it with oxygen, yet Suess emphasizes reactions that cells began utilizing even earlier, at a time when the atmosphere was devoid of oxygen and cellular metabolism could not be supported by respiration.
Many of these ancient reactions, still employed by cells today, involve a category of metalloproteins known as iron-sulfur proteins. These enzymes, present across all life kingdoms, play a vital role in catalyzing numerous challenging reactions that occur in cells, including forming carbon radicals and converting nitrogen to ammonia.
To investigate the metalloenzymes that drive these reactions, Suess’s lab adopts two distinct methodologies. In one approach, they create synthetic versions of proteins that may contain fewer metal atoms, allowing for enhanced control over the protein’s composition and structure, making them easier to analyze.
In another method, they utilize the natural version of the protein, substituting one of the metal atoms with an isotope that simplifies the application of spectroscopic techniques for analyzing the protein’s structure.
“This enables us to study both the bonding in the resting state of an enzyme as well as the bonding and structures of reaction intermediates that can only be characterized spectroscopically,” Suess explains.
Grasping how enzymes execute these reactions could assist researchers in discovering new approaches to extract carbon dioxide from the atmosphere by combining it with other molecules to forge larger compounds. Identifying alternative methods for converting nitrogen gas to ammonia could also significantly affect greenhouse gas emissions, as the current Haber Bosch process employed for fertilizer synthesis consumes considerable amounts of energy.
“Our main objective is to comprehend the natural world, but I believe that as we explore different methods to configure biological catalysts for efficient reactions impacting society, we need to understand how that configuration operates. And that is what we are attempting to unravel,” he concludes.