The BU72-μ Opioid Receptor Crystal Structure Is a Covalent Adduct

In the crystal structure of BU72 bound to the μ opioid receptor, the opioid clashes with an adjacent residue, and unexplained electron density connects the two. It has been reported that this density can be filled by a magnesium ion. However, this proposal requires unrealistically short bonds and an incomplete coordination shell. Moreover, the crystals were prepared without magnesium salts, but with components that can generate reactive oxygen species: HEPES buffer, nickel ions, and an N-terminus that forms redox-active nickel complexes. Here I show that an oxygen atom fills the unexplained density, giving a known type of covalent adduct with reasonable geometry and no clashes. Strain is evident, but is consistent with tension from the tethered N-terminus.