Exploring the Flexible Structure of Sulfite Reductase Enzyme

A picture is said to be worth a thousand words.

But to fully understand the complex chemistry of an enzyme involved in breaking down sulfur, found in fruits, vegetables, alcohol, and even gasoline, into the colorless gas known for its distinct smell, scientists needed millions of images.

Most people have experienced, or rather smelled, the work of a protein enzyme called sulfite reductase. This enzyme chemically reduces sulfite to hydrogen sulfide, the gas responsible for the rotten egg odor often associated with decomposing organic material, landfills, and sewage treatment facilities.

Until recently, the lack of a detailed visual representation of this enzyme's structure limited researchers' ability to understand exactly how it works. That gap has now been addressed by Elizabeth Stroupe, a professor of biological science at Florida State University, and Behrouz Ghazi Esfahani, a former Ph.D. student. Their findings were published in Nature Communications.

Artificial intelligence has gotten better at predicting protein structures, but at the end of the day, it is not data. This gives us the primary knowledge we need to better understand this kind of structure.

Elizabeth Stroupe, Professor, Biological Science, Florida State University

How Did Researchers Solve This?

Stroupe and Ghazi Esfahani used cryo-electron microscopy to visualize the three-dimensional structure of sulfite reductase. This advanced imaging technique captures snapshots of chemical reactions in progress, allowing researchers to reconstruct the enzyme’s structure in high detail.

While protein molecules may appear to be complex chains of chemicals, cryo-electron microscopy enables scientists to pinpoint the exact arrangement of atoms and trace how electrons are transferred during reactions.

“I think of it as an octopus with four yo-yos because the molecule is particularly flexible,” Stroupe remarked.

Why Does it Matter?

Supported by the National Science Foundation, this research provides important insights that could help scientists better control or modify chemical reactions, an approach widely used in pharmaceutical and industrial applications.

There are environmental implications, too. Some bacteria use sulfur as an energy source in the same way that humans or other living organisms use oxygen. This enables us to understand how certain bacteria thrive in anaerobic conditions.

Behrouz Ghazi Esfahani, Former Doctoral Student, Florida State University

What is Next?

While questions remain about how sulfite reductase functions as part of a larger protein complex, and how similar enzymes operate in other organisms, such as the tuberculosis pathogen, which relies on sulfur to survive in the human body, this study marks a meaningful step toward answering them. Stroupe’s lab continues to investigate other structural aspects of sulfur metabolism.