ACTIVE-SITE STRUCTURE OF RIESKE-TYPE PROTEINS - ELECTRON-NUCLEAR DOUBLE-RESONANCE STUDIES OF ISOTOPICALLY LABELED PHTHALATE DIOXYGENASE FROM PSEUDOMONAS-CEPACIA AND RIESKE PROTEIN FROM RHODOBACTER-CAYSULATUS AND MOLECULAR MODELING STUDIES OF A RIESKE CENTER

Continuous wave electron nuclear double resonance (CW ENDOR) spectra o f [delta-N-15,epsilon-N-14]-histidine-labeled phthalate dioxygenase (P DO) from Pseudomonas cepacia were recorded and found to be virtually i dentical to those previously recorded from [delta,E-N-15(2)]histidine- labeled protein [Gurbiel, R. J., Batie, C. J., Sivaraja, M., True, A. E., Fee, J. A., Hoffman, B. M., & Ballou, D. P. (1989) Biochemistry, 2 8, 4861-4871]. Thus, the two histidine residues, previously shown to l igate one of the irons in the cluster [cf: Gurbiel ct al. 1989)], both coordinate the metal at the N(delta) position of their imidazole ring s. Pulsed ENDOR studies showed that the ''remote'', noncoordinating ni trogen of the histidine imidazole ring could be observed from the Ries ke protein in a sample of Rhodobacter capsulatus cytochrome bc(1) comp lex uniformly labeled with N-15 but not in a sample of PDO labeled wit h [delta-N-15,epsilon-N-14]histidine, but this atom was easily observe d with a sample of Rh. capsulatus cytochrome bc(1) complex that had be en uniformly labeled with N-15; this confirmed the conclusion from the CW ENDOR studies that ligation is exclusively via N(delta) for both l igands in the PDO center. Modifications in the algorithms previously u sed to simulate N-14 ENDOR spectra permitted us to compute spectra wit hout any constraints on the relative orientation of hyperfine and quad rupole tensors. This new algorithm was used to analyze current and pre viously published spectra, and slightly different values for the N-Fe- N angle and imidazole ring rotation angles are presented [cf: Gurbiel et al. (1989) Gurbiel, R. J., Ohnishi, T., Robertson, D. E., Daldal, F ., and Hoffman, B. M. (1991) Biochemistry 30, 11579-11584]. This analy sis has permitted us to refine the proposed structure of the [2Fe-2S] Rieske-type cluster and rationalize some of the properties of these no vel centers. Although the spectra of cytochrome bc(1) complex from Rh, capsulatus are of somewhat lower resolution than those obtained with samples of PDO, our analysis nevertheless permits the conclusion that the geometry of the cluster is essentially the same for all Rieske and Rieske-type proteins. Structural constraints inferred from the spectr oscopic results permitted us to apply the principles of distance geome try to arrive at possible three-dimensional models of the active site structure of Rieske protein from Rh. capsulatus. Results from this tes t case indicate that similar procedures should be generally useful in metalloprotein systems. We also recorded the pulsed and CW ENDOR spect ra of Fe-57-labeled PDO, and the resulting data were used to derive th e full hyperfine tensors for both Fe(III) and Fe(II) ions, including t heir orientations relative to the g tenser. The A tensor of the ferric ion is nominally isotropic, while the A tensor of the ferrous ion is axial, having A(parallel to) > A(perpendicular to); both tensors are c oincident with the observed g tensor, with A(parallel to) of the ferro us ion lying along the maximum g-value, g(1). These results were exami ned using refinements of existing theories of spin-coupling in [2Fe-2S ](+) clusters, and it is concluded that current theories are not adequ ate to fully describe the experimental results.