A 60-heme reductase complex from an anammox bacterium shows an extended electron transfer pathway

The hydroxylamine oxidoreductase/hydrazine dehydrogenase (HAO/HDH) protein family constitutes an important group of octaheme cytochromes c (OCCs). The majority of these proteins form homotrimers, with their subunits being covalently attached to each other via a rare cross-link between the catalytic heme moiety and a conserved tyrosine residue in an adjacent subunit. This covalent cross-link has been proposed to modulate the active-site heme towards oxidative catalysis by distorting the heme plane. In this study, the crystal structure of a stable complex of an HAO homologue (KsHAOr) with its diheme cytochrome c redox partner (KsDH) from the anammox bacterium Kuenenia stuttgartiensis was determined. KsHAOr lacks the tyrosine cross-link and is therefore tuned to reductive catalysis. The molecular model of the KsHAOr–KsDH complex at 2.6 Å resolution shows a heterododecameric (α6β6) assembly, which was also shown to be the oligomeric state in solution by analytical ultracentrifugation and multi-angle static light scattering. The 60-heme-containing protein complex reveals a unique extended electron transfer pathway and provides deeper insights into catalysis and electron transfer in reductive OCCs.