Evidence for N coordination to Fe in the [2Fe-2S] clusters of thermus Rieske protein and phthalate dioxygenase from pseudomonas

Rieske-type iron/sulfur protein and several NADH-dependent oxygenases contain Fe/S clusters with similar spectral and magnetic properties. Purified Rieske iron/sulfur protein from thermus thermophilus contains two apparently identical [2Fe-2S] clusters in a polypeptide having only four cysteine residues, and it has been proposed that each Fe/S cluster is coordinated to two cysteine S-atoms and to an unknown number of other non-sulfur atoms. We have examined the Rieske protein from Thermus and the phthalate dioxygenase from Pseudomonas cepacia with electron nuclear double resonance (ENDOR) and pulsed EPR methods and report here evidence for the direct coordination of nitrogenous ligands to the Fe/S clusters in these proteins. The electron nuclear double resonance signals arising from ¹⁴N have been interpreted in terms of a strongly coupled ligand with A(N) = ~26-28 MHz and a weakly coupled ligand with A(N) = ~9 MHz. The pulsed EPR spectrum shows a rich pattern of lines in the Fourier transformed data having peaks in the range of 0.8 to 6.7 MHz. The lower frequency resonances are tentatively associated with coupling of the unpaired spin to the remote N-atoms of coordinated imidazole rings.