Fe2S2 centers

Figure 5.4. Resonance Raman spectra of [Fe2S2] centers in A. vinelandii. A, NifU as isolated. B, D37A NifU-1 as isolated. C, NifU-1 repnrified after NifS-mediated cluster assembly. D, IscU containing two [Fc2S2] clnsters per dimer pnrified fraction after IscS-mediated clnster assembly. All spectra were recorded nsing 457-nm excitation at 17 K and with 6cm resolntion. Vibrational modes resnlting from lattice modes of ice have been snbtracted.

The active site structures of the three classes of molybdenum-containing enzymes are compared in Fig. 16-32. In the DMSO reductase family there are two identical molybdopterin dinucleotide coenzymes complexed with one molybdenum. However, only one of these appears to be functionally linked to the Fe2S2 center. 

Figure 23-17 The zigzag scheme (Z scheme) for a two-quantum per electron photoreduction system of chloroplasts. Abbreviations are P680 and P700, reaction center chlorophylls Ph, pheophytin acceptor of electrons from PSII QA, Qg, quinones bound to reaction center proteins PQ, plastoquinone (mobile pool) Cyt, cytochromes PC, plastocyanin A0 and Aj, early electron acceptors for PSI, possibly chlorophyll and quinone, respectively Fx, Fe2S2 center bound to reaction center proteins FA, FB, Fe4S4 centers Fd, soluble ferredoxin and DCMU, dichlorophenyldimethylurea. Note that the positions of P682, P700, Ph, Qa/ Qb/ Ay and A, on the E° scale are uncertain. The E° values for P682 and P700 should be for the (chlorophyll / chlorophyll cation radical) pair in the reaction center environment. These may be lower than are shown.

The soluble electron carriers released from the reaction centers into the cytoplasm of bacteria or into the stroma of chloroplasts are reduced single-electron carriers. Bacterial ferredoxin with two Fe4S4 clusters is formed by bacteria if enough iron is present. In its absence flavodoxin (Chapter 15), which may carry either one or two electrons, is used. In chloroplasts the carrier is the soluble chloroplast ferredoxin (Fig. 16-16,C), which contains one Fe2S2 center. Reduced ferredoxin transfers electrons to NADP+ (Eq. 15-28) via ferredoxin NADP oxidoreductase, a flavoprotein of known three-dimensional structure.367 369... 

FIGURE 3. The overall protein fold of the D. gigas aldehyde oxidoreductase. The two Fe2S2 centers and the molybdenum center are indicated in wireframe/spacefilling representation. The two Fe2S2 domains are at bottom left and bottom center, respectively (structure rendered from the data of Rom o et al., 1995). 

Figure 9 The structure of PdR and Pdx, the electron-transfer proteins of the P450cam system. The PdR molecule is viewed from the si side of the flavin from which Pdx is proposed to approach and accept electrons from PdR. The Fe2S2 center of Pdx is close to the protein surface, and the C-terminal Trp-106 residue is important for electron transfer...

Xanthine is converted to uric acid at the molybdenum center of the enzyme, and the electrons are removed from the enzyme by oxidation of the flavin center. From early reductive titrations of xanthine oxidase with sodium dithionite, it was proposed that reducing equivalents were equilibrated among the four redox-active centers (Mo-co, two separate Fe2S2 centers, flavin) at a rate that was rapid relative to the overall catalytic rate of substrate turnover (243). Under such conditions, the flux of reducing equivalents through the enzyme should be influenced by the relative reduction potentials of the redox centers involved (244). Any effects of pH and temperature on the reduction potentials of individual redox components would affect the apparent rates of intramolecular transfer of the enzyme. 

The Fe2S2 center shows nicely how spectroscopy can be used to deduce the structure of an active site. Indeed, in this case the now well-established active-site structure was deduced by a combination of chemical, spectroscopic, and magnetic methods, and the site was successfully modeled long before the first protein crystallographic study was reported. 

The last three steps in the reduction of nitrate to ammonia are carried out by an enzyme called nitrite reductase. It contains one Fe2S2 center and one molecule of siroheme, a partially reduced iron porphyrin. The electron donor for each step is ferredoxin. 

Transcriptional regulation of antioxidant proteins. Certain proteins with easily accessible Fe-S clusters, e.g., aconitase, are readily inachvated by oxidants such as perox5mitrite. At least two proteins of this type function as transcription factors in E. coli. These are known as SoxR and OxyR. The SoxR protein is sensihve to superoxide anion, which carries out a one-electron oxidahon on its Fe2S2 centers. ... 

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