thenX-ray structure of R22 (Fig.23-9b), which was determined by Hans Eklund, reveals that the subunits are bundles of eight unusually long helices. Each subunit contains a novel binuclear Fe(III) prosthetic group whose Fe(III) ions are liganded by a variety of groups including an O2- ion (Fig.23-9c).
The Fe(III) complex interacts with Tyr 122 to from an unusual tyrosyl free radical. Tyrosine radicals also take part in the reactions catalysed by cytochrome c oxidase (section 18-2F) and plant Photosystem II (section 19-2C).