A new player in the biochemistry of Anammox bacteria: a multidomain HAO-like protein

Anaerobic ammonium-oxidizing (anammox) bacteria are essential players in the global nitrogen cycle, responsible for converting ammonium and nitrite directly to nitrogen gas. Anammox bacteria have unique features such as a specialized cellular compartment - the anammoxosome. Candidatus (Ca.) Brocadia pituitae genome, as other anammox bacteria, encodes for a diversity of hydroxylamine oxidoreductase (HAO) paralogs, often pointed out as the enzymes responsible for most of the reactions of the anammox cycle. One of this Ca. B. pituitae HAO paralogs is an 840-amino acids protein, named here as BpMHAO, that stands out for its unprecedented domain organization, which includes a multicopper oxidase-like (MCo-like) domain followed by the HAO-like one. Sequence and structural analyses classified this MCo-like domain as homologous to the small laccase family. Spectroscopic characterization revealed a distinct UV-visible spectrum, tentatively assigned to the T3 center, whereas the EPR spectra confirmed the presence of T1, T2 and T3 copper centers. Enzymatic studies demonstrated limited laccase and oxygen-dependent ferroxidase activities. On the other hand, enzymatic assays performed in cell extracts from Escherichia coli and Shewanella oneidensis, harbouring the recombinant HAO-like domain, exhibited a robust hydroxylamine reductase activity using methyl viologen as the electron donor. Our results showed that the BpMHAO potentially plays a role in the anammox process/reactions by converting hydroxylamine into hydrazine. This feature can be relevant to anammox bacteria either by i) mitigating unwanted hydroxylamine, obtained by incorrect formation of this compound, by converting it into hydrazine and enabling its use in the anammox reaction or ii) using hydroxylamine from the outside medium as a substitute for ammonium, delivering hydrazine directly to the last step of the cycle, short-circuiting its first steps.