An unexpected reactivity of the P₄₆₀ cofactor in hydroxylamine oxidoreductase

Hydroxylamine oxidoreductases (HAOs) contain a unique haem cofactor called P₄₆₀ that consists of a profoundly ruffled c-type haem with two covalent bonds between the haem porphyrin and a conserved tyrosine. This cofactor is exceptional in that it abstracts electrons from a ligand bound to the haem iron, whereas other haems involved in redox chemistry usually inject electrons into their ligands. The effects of the tyrosine cross-links and of the haem ruffling on the chemistry of this cofactor have been investigated theoretically but are not yet clear. A new crystal structure of an HAO from Candidatus Kuenenia stuttgartiensis, a model organism for anaerobic ammonium oxidation, now shows that its P₄₆₀ cofactor has yet another unexpected reactivity: when ethylene glycol was used as a cryoprotectant, the 1.8 Å resolution electron-density maps showed additional density which could be interpreted as an ethylene glycol molecule covalently bound to the C₁₆ atom of the haem ring, opposite the covalent links to the conserved tyrosine. Possible causes for this unexpected reactivity are discussed.