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Abstract

Anaerobic ammonium oxidation (anammox) is a major process in the biogeochemical nitrogen cycle in which nitrite
and ammonium are converted to dinitrogen gas andwater through the highly reactive intermediate hydrazine. So far,
it is unknown how anammox organisms convert the toxic hydrazine into nitrogen and harvest the extremely low
potential electrons (−750 mV) released in this process. We report the crystal structure and cryo electron microscopy
structures of the responsible enzyme, hydrazine dehydrogenase, which is a 1.7MDa multiprotein complex containing
an extended electron transfer network of 192 heme groups spanning the entire complex. This unique molecular arrangement
suggests a way inwhich the protein stores and releases the electrons obtained from hydrazine conversion,
the final step in the globally important anammox process.