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Scientists Uncover Ancient Mechanism of Cellular Respiration

Researchers affiliated with Goethe University Frankfurt, the University of Marburg, and Stockholm University have unraveled a historic cellular respiration mechanism. Their study focused on bacteria that utilize carbon dioxide and hydrogen, converting these gases into acetic acid—a metabolic process believed to have originated early in the evolutionary timeline. This international collaboration has shed light on how these microbes harness this process for energy production. Notably, these microorganisms also demonstrate the potential for mitigating climate change by removing CO2 from their surroundings.

In the natural world, animals, plants, and various organisms typically rely on oxygen to oxidize substances like sugars into carbon dioxide and water, during which adenosine triphosphate (ATP)—an essential energy carrier for cellular functions—is generated. However, during the early epochs of Earth’s history, the atmosphere lacked free oxygen. Research on ancient bacteria that still thrive in anaerobic environments, such as deep-sea hydrothermal springs, suggests that alternative respiratory mechanisms must have existed during this era.

These specific bacteria effectively “breathe” carbon dioxide and hydrogen, converting them into acetic acid. While the metabolic pathway for this process has been known, the mechanism through which it generates ATP was previously a mystery. The current study finally clarifies this process. “We discovered that the production of acetic acid triggers a complex mechanism that involves the expulsion of sodium ions from the bacterial cell,” explains Professor Volker Müller, Chair of Molecular Microbiology and Bioenergetics at Goethe University Frankfurt. “This reduction in sodium concentration inside the cell leads to a scenario where the cell membrane functions as a barrier for the ions. When this barrier is breached, sodium ions flow back inside, generating ATP through a molecular turbine effect.”

Recent Advances in Cellular Respiration Enzymes

The process hinges on the Rnf complex, a group of proteins that span the bacterial membrane. “This complex is extraordinarily delicate, and we were only able to isolate it a few years ago,” emphasizes Müller. In this reaction, carbon dioxide and hydrogen interact to form acetic acid, transferring electrons from hydrogen to carbon through various intermediate stages, with the Rnf complex facilitating electron transfer.

The team’s investigation has delineated the intricacies of this process. Anuj Kumar, a structural biologist and PhD candidate aligned with both Müller’s and Dr. Jan Schuller’s groups, employed cryo-electron microscopy to capture and analyze the Rnf complex from Acetobacterium woodii. The complex was rapidly frozen and applied onto a carrier surface, creating a thin ice film that preserved millions of Rnf complexes for electron microscope observation. This method allows varying perspectives of the complexes to be analyzed, enabling the construction of a three-dimensional representation that offers insight into its structure, particularly the crucial components responsible for electron transport.

“From our three-dimensional imaging, we observed that the Rnf complex is not static; its components exhibit dynamic movements that facilitate the transfer of electrons over longer distances,” Kumar elaborates. This fundamental understanding leads to the next question: How does this electron flow stimulate sodium ion efflux? A molecular dynamics simulation conducted by Prof. Dr. Ville Kaila’s group at Stockholm University suggested an explanation. A central cluster of iron and sulfur atoms in the membrane, upon receiving an electron, becomes negatively charged, attracting positively charged sodium ions from the internal cellular environment. “This attraction encourages protein shifts around the iron-sulfur cluster, much like a switch, creating openings through which sodium ions exit the membrane,” Roth describes.

Roth validated this mechanism through targeted genetic modifications to the Rnf proteins, providing strong evidence for this novel process. The collaborative effort among the three institutions has uncovered significant insights not only into the fundamental aspects of this bacterial energy generation but also into practical applications. The ability of these microorganisms to sequester CO2 during acetic acid production could hold promise for addressing greenhouse gas emissions from industrial sources. “Understanding how these bacteria produce energy might allow us to refine this process to yield higher-quality products,” hopes Müller. Furthermore, the findings may inspire the development of new therapeutic strategies against pathogens utilizing similar respiratory mechanisms.

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