Structure of T4moC, the Rieske-type ferredoxin component of toluene 4-monooxygenase

The structure of the Rieske-type ferredoxin (T4moC) from toluene 4-monooxygenase was determined by X-ray crystallography in the [2Fe–2S]²⁺ state at a resolution of 1.48 Å using single-wavelength anomalous dispersion phasing with the [2Fe–2S] center. The structure consists of ten β-strands arranged into the three antiparallel β-sheet topology observed in all Rieske proteins. Trp69 of T4moC is adjacent to the [2Fe–­2S] centre, which displaces a loop containing the conserved Pro81 by ∼8 Å away from the [2Fe–2S] cluster compared with the Pro loop in the closest structural and functional homolog, the Rieske-type ferredoxin BphF from biphenyl dioxygenase. In addition, T4moC contains five hydrogen bonds to the [2Fe–2S] cluster compared with three hydrogen bonds in BphF. Moreover, the electrostatic surface of T4moC is distinct from that of BphF. These structural differences are identified as possible contributors to the evolutionary specialization of soluble Rieske-type ferredoxins between the diiron monooxygenases and cis-dihydrodiol-forming dioxygenases.