Solution structure of T4moC, the Rieske ferredoxin component of the toluene 4-monooxygenase complex

Toluene 4-monooxygenase, a four-protein complex from Pseudomonas mendocina KR1, catalyzes the NADH- and O₂-dependent hydroxylation of toluene to form p-cresol. The solution structure of the 112-amino-acid Rieske ferredoxin component, T4moC, was determined from 2D and 3D ¹H, ¹³C, and ¹⁵N NMR data. The structural model was refined through simulated annealing by molecular dynamics in torsion angle space with input from 1650 experimental restraints, including 1264 inter-proton distance restraints obtained from NOEs, 247 non-redundant intra-residue NOEs, 26 hydrogen bond restraints, and 113 dihedral angle (φ, ψ) restraints. The 20 calculated conformers that best satisfied the input restraints were submitted to refinement in explicit solvent to improve the stereochemical quality. With exclusion of ill-defined N- and C-terminal segments (Ser2; His111-Ser112) and residues near to the [2Fe-2S] cluster, the atomic root mean square deviation for the 20 conformers with respect to the mean coordinates was 1.09 Å for the backbone and 1.60 Å for all non-hydrogen atoms. The T4moC structure consists of 10 β-strands arranged in the three anti-parallel β-sheet topology observed in all Rieske [2Fe-2S] domain proteins. The S ^γ of Cys45 and Cys64 and the N^δ1 of His47 and His67 provide the ligands to the [2Fe-2S] cluster of T4moC. ¹H- ¹⁵N HSQC measurements show that both His47-N^ε2 and His67-N^ε2 are protonated at the pH of the NMR experiments. Comparisons are made between the present NMR structure, previous paramagnetic NMR studies of T4moC, and the X-ray structures of other members of the Rieske protein family.