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Abstract
The 2-halobenzoate 1,2-dioxygenase from Burkholderia (Pseudomonas) cepacia 2CBS (Fetzner, S., Müller, R., and Lingens, F. (1992) J. Bacteriol. 174, 279-290) contains both a ferredoxin-type and a Rieske-type 2Fe2S center. These two significantly different 2Fe2S clusters were characterized with respect to their EPR spectra, electrochemical properties (Rieske-type cluster with gz = 2.025, gy = 1.91, gx = 1.79, gav = 1.91, Em = -125 +/- 10 mV; ferredoxin-type center with gz = 2.05, gy = 1.96, gx = 1.89, gav = 1.97, Em = -200 +/- 10 mV) and pH dependence thereof. X band electron spin echo envelope modulation and electron nuclear double resonance spectroscopy was applied to study the interaction of the Rieske-type center of the 2-halobenzoate 1,2-dioxygenase with 14N and 1H nuclei in the vicinity of the 2Fe2S cluster. The results are compared to those obtained on the Rieske protein of the cytochrome b6f complex (Em = +320 mV) and the water-soluble ferredoxin (Em = -430 mV) of spinach chloroplasts, as typical representatives of the gav = 1.91 and gav = 1.96 class of 2Fe2S centers. Properties common to all Rieske-type clusters and those restricted to the respective centers in bacterial oxygenases are discussed.
Highlights
For a long time, the membership in either of the two classes was considered to be correlated with significantly different redox midpoint potentials, i.e. with an Em1 Ͻ Ϫ400 mV for the ferredoxins and an Em Ͼ ϩ260 mV for the Rieske centers of the cytochrome bc complexes, the enzymes in which such a 2Fe2S center had been discovered (Rieske et al, 1964)
The model of the cluster suggesting a coordination via two cysteines and two histidines is supported by 15N Q band ENDOR experiments on the terminal oxygenase of the phthalate dioxygenase from Pseudomonas cepacia (Gurbiel et al, 1989)
These clusters show all characteristic features of the Rieske centers of cytochrome bc complexes except that their redox midpoint potential is about 150 mV lower ranging between ϩ100 and ϩ165 mV. (b) In bacterial oxygenases, 2Fe2S centers with gav ϭ 1.91 have been reported having midpoint potentials as low as Ϫ155 mV (Geary et al 1984; Rosche et al 1995b). (c) Ferredoxin-type gav ϭ 1.96 clusters were found titrating as high as Ϫ174 mV in bacterial dioxygenases (Correll et al, 1992) or even at Ϫ7 mV for the center S1 in the mitochondrial succinate dehydrogenase (Cammack and Palmer, 1977; Shergill and Cammack, 1994b)
Summary
The membership in either of the two classes was considered to be correlated with significantly different redox midpoint potentials, i.e. with an Em1 Ͻ Ϫ400 mV for the ferredoxins and an Em Ͼ ϩ260 mV for the Rieske centers of the cytochrome bc complexes, the enzymes in which such a 2Fe2S center had been discovered (Rieske et al, 1964). (a) 2Fe2S clusters contained in cytochrome bc complexes involved in oxidation of menaquinol have been characterized (Liebl et al, 1992; Riedel et al, 1993) These clusters show all characteristic features of the Rieske centers of cytochrome bc complexes except that their redox midpoint potential is about 150 mV lower ranging between ϩ100 and ϩ165 mV. The EPR spectroscopic, electrochemical, 14N ESEEM, and 1H ENDOR characteristics of the gav ϭ 1.91 and gav ϭ 1.96 2Fe2S centers in 2-halobenzoate 1,2-dioxygenase from Burkholderia (Pseudomonas) cepacia 2CBS (Fetzner et al, 1992) are described and compared to other examples of these two classes of 2Fe2S centers, such as the Rieske center of the cytochrome b6 f complex and the watersoluble ferredoxin from spinach
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