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Talk by Thomas R. M. Barends

Title: "Living on hydrazine: Metabolic protein complexes from an anaerobic ammonium oxidizer"
Occasion: SFB Seminar
Start: 14.11.2024 4:15 pm
Location: CellNanOs, 38/201

About the speaker: Dr. Thomas R. M. Barends conducts research at the Max Planck Institute for Medical Research in Heidelberg.

The discovery of anammox bacteria in the 1990's has dramatically changed our understanding of the global nitrogen cycle. These bacteria perform ANaerobic AMMonium OXidation, combining ammonium with nitrite into molecular dinitrogen (N2) and water, yielding energy for the cell. The anammox process is of global importance, being responsible for up to 50% to the total yearly nitrogen removal from the oceans. Biochemical studies, mainly by the group of Mike Jetten in Nijmegen, have identified the enzymes involved, and showed that the process relies on the extremely unusual, highly toxic and reactive intermediate hydrazine. To elucidate how bacteria perform such extraordinary chemistry, we have determined the structures of the key enzymes in this process. Central to harvesting the energy from hydrazine is the hydrazine dehydrogenase complex, which converts hydrazine into dinitrogen gas, liberating four extremely low-potential electrons (-750 mV). Our 2.9 Å resolution crystal structure reveals that this 1.7 MDa complex contains an extended system of 192 heme groups spanning the entire complex, which is only accessible via narrow holes in the side of the complex. Moreover, we identified an unexpected assembly factor for this complex. The findings have consequences for the way in which the electrons harvested from hydrazine oxidation are transferred to other components of the anammox metabolism. Anammox bacteria obtain additional reducing equivalents through the oxidation of nitrite to nitrate, which is catalyzed by a nitrite oxidoreductase (NXR) related to the NXR from nitrifying bacteria. By combining crystallography and biochemistry with cryo-electron tomography and helical reconstruction cryo-electron microscopy performed by the group of Kristian Parey, we show that anammox NXR forms long tubules, held together by a novel tubule-inducing subunit we call NXR-T. As with the hydrazine dehydrogenase structure, our multiscale structure of the anammox NXR tubules suggest how electrons are passed on to redox partners.