Detection and classification of hyperfine-shifted 1H, 2H, and 15N resonances of the Rieske ferredoxin component of toluene 4-monooxygenase.

Abstract

T4MOC is a 12.3 kDa soluble Rieske ferredoxin that is obligately required for electron transfer between the oxidoreductase and diiron hydroxylase components of toluene 4-monooxygenase from Pseudomonas mendocina KR1. Our preliminary 1H NMR studies of oxidized and reduced T4MOC [Markley, J. L., Xia, B., Chae, Y. K., Cheng, H., Westler, W. M., Pikus, J. D., and Fox, B. G. (1996) in Protein Structure Function Relationships (Zaidi, Z., and Smith, D., Eds.) pp 135-146, Plenum Press, London] revealed the presence of hyperfine-shifted 1H resonances whose short relaxation times made it impractical to use nuclear Overhauser effect (NOE) measurements for assignment purposes. We report here the use of selective isotopic labeling to analyze the hyperfine-shifted 1H, 2H, and 15N signals from T4MOC. Selective deuteration led to identification of signals from the four Hbeta atoms of cluster ligands C45 and C64 in the oxidized and reduced forms of T4MOC. In the reduced state, the Curie temperature dependence of the Hbeta protons corresponded to that predicted from the simple vector spin-coupling model for nuclei associated with the localized ferric site. The signal at 25.5 ppm in the 1H spectrum of reduced T4MOC was assigned on the basis of selective 2H labeling to the His Hepsilon1 atom of one of the cluster ligands (H47 or H67). This assignment was corroborated by a one bond 1H-13C correlation (at 25.39 ppm 1H and 136.11 ppm 13C) observed in spectra of [U-13C]T4MOC with a 1H-13C coupling constant of approximately 192 Hz. The carbon chemical shift and one bond coupling constant are those expected for 1Hepsilon1-13Cepsilon1 in the imidazolium ring of histidine and are inconsistent with values expected for cysteine 1Halpha-13Calpha. The His Hepsilon1 proton exhibited weak Curie temperature dependence from 283 to 303 K, contrary to the anti-Curie temperature dependence predicted from the spin coupling model for nuclei associated with the localized ferrous site. A 1H peak at -12.3 ppm was observed in spectra of reduced T4MOC; this signal was found to correspond to a hydrogen (probably in an H-bond to the cluster) that exchanged with solvent with a half-time of about 2 days in the oxidized state but with a much longer (undetectable) half-time in the reduced state. These results with T4MOC call into question certain 1H assignments recently reported on the basis of NOE measurements for the comparable Rieske ferredoxin component of an evolutionarily related alkene monooxygenase from Xanthobacter sp. Py2 [Holz, R. C., Small, F. J., and Ensign, S. A, (1997) Biochemistry 36, 14690-14696]. Selective 15N labeling was used to identify hyperfine-shifted 15N NMR signals from the backbone nitrogens of all four cluster ligands (C45, H47, C64, and H67), from the Nepsilon2 atoms of the two histidine ligands (H47 and H67), and from nonligand Gln and Ala residues (Q48 and A66) present in the cluster-binding motif of T4MOC in the oxidized and reduced states. The results indicate that the Ndelta1 of each of the two ligand histidines of T4MOC are ligated to an iron atom and reveal a pattern of H-bonding to the Rieske [2Fe-2S] center involving four (H47, Q48, A66, and H67 of T4MOC) of the five backbone amide H-bonds expected on the basis of comparison with the crystal structures of other related Rieske proteins; the fifth backbone amide (I50 of T4MOC) failed to exhibit a hyperfine shift. This anomaly may arise from the lack of an associated disulfide in T4MOC, a fundamental structural difference between the three types of Rieske proteins that may be related to functional diversity in this protein family.