Anammox Biochemistry: a Tale of Heme c Proteins

Anaerobic ammonium-oxidizing (anammox) bacteria are one of the latest scientific discoveries in the biogeochemical nitrogen cycle. These microorganisms are able to oxidize ammonium (NH4+) with nitrite (NO2−) as the oxidant instead of oxygen and form dinitrogen (N2) as the end product. Recent research has shed a light on the biochemistry underlying anammox metabolism with two key intermediates, nitric oxide (NO) and hydrazine (N2H4). Substrates and intermediates are converted exploiting the catalytic and electron-transfer potentials of c-type heme proteins known from numerous biochemical reactions and that have acquired new functionality in anammox biochemistry. On a global scale, anammox bacteria significantly contribute to the removal of fixed nitrogen from the environment and the process finds rapidly increasing interest in wastewater treatment. About 30–70% of all nitrogen that is released into the atmosphere is produced by microorganisms that have been considered impossible for a long-time, the anaerobic ammonium-oxidizing (anammox) bacteria. Anammox bacteria oxidize ammonium in the absence of oxygen with NO2− as the terminal electron acceptor. Substrate conversion proceeds through two highly toxic intermediates, NO and N2H4. Anammox metabolism relies on multiheme proteins that structurally resemble ones known from other organisms, but that have new functions in the anammox bacteria. Anammox metabolism resides in a special and unique cell organelle, the anammoxosome. Here, energy released in the anammox reaction is used to generate proton-motive force that drives ATP synthesis. This respiratory process is supported by novel membrane-bound protein complexes.