Electron paramagnetic resonance iron-sulfur clusters

Valuable spectroscopic studies on the dithiolene chelated to Mo in various enzymes have been enhanced by the knowledge of the structure from X-ray diffraction. Plagued by interference of prosthetic groups—heme, flavin, iron-sulfur clusters—the majority of information has been gleaned from the DMSO reductase system. The spectroscopic tools of X-ray absorption spectroscopy (XAS), electronic ultraviolet/visible (UV/vis) spectroscopy, resonance Raman (RR), MCD, and various electron paramagnetic resonance techniques [EPR, electron spin echo envelope modulation (ESEEM), and electron nuclear double resonance (ENDOR)] have been particularly effective probes of the metal site. Of these, only MCD and RR have detected features attributable to the dithiolene unit. Selected results from a variety of studies are presented below, chosen because their focus is the Mo-dithiolene unit and organized according to method rather than to enzyme or type of active site. 

Abbreviations FAD, flavin adenine dinucleotide Fe-S, iron-sulfur proteins that can he identified in separate clusters by electron paramagnetic resonance analysis (the s-1, s-2 subscripts identify these iron-sulfur proteins as part of the succinate dehydrogenase complex) His, the histidine linkage between FAD and the large (70,000 daltons) protein moiety of the enzyme FMN, flavin mononucleotide N-la, N-2 subscripts identify these iron-sulfur proteins as part of the NADH-dehydro-genase complex UQ, ubiquinone Cyt bf and Cyt b, cytochrome b-566 and b-563, respectively. 

The application of electron paramagnetic resonance (EPR) spectroscopy has shown that electron transfer from FMN to UQ involves reduction of eight or more iron sulfur clusters, of which that with the highest redox potential, center N2, is responsible for UQ reduction [35], EPR spectroscopy has also revealed the presence of two ubisemiquinone species, suggesting the presence of multiple quinone binding sites. There may also be other redox centers involved in electron transport through the complex [26, 36-38]. With the exception of fenaminosulf, none of the inhibitors listed in Table 13.1.1, or elsewhere in this volume, have been distinguished in their site of action based on effects on the reduction or reoxidation of detectable redox centers - all that have been studied in detail seem to act to prevent electron transfer somewhere between center N2 and UQ [14]. 

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