Anaerobic ammonium oxidation (anammox) and denitrification are two different microbial reactions that form nitrogen gas. The initial step in the anammox reaction-reduction of nitrite to nitric oxide-is thought to be catalyzed by homologs of dissimilatory nitrite reductase, which is known to be involved in denitrification. Here, we reveal the crystal structure of the copper-containing nitrite reductase (CuNIR) of strain KSU-1, an anammox bacterium. CuNIR had a unique homohexameric structure with three disulfide bridges between homotrimers, although the trimer was similar to that of known CuNIRs. Kinetic and mutagenesis analyses suggested that the hexameric structure is important for the electron transfer reaction. (C) 2020 Elsevier Inc. All rights reserved.