Rieske Fe-S center

Efforts toward developing synthetic models for the Rieske Fe-S centers focussed initially on preparing Fe2S2 cores with non-thlolate ligands, and have centered on nitrogenous ligands since the realization of their probable occurrence in the Rieske protein. In addition to the [Fe2S2(0Ar) ] " ions (Ar = aryl)... 

Figure 9. A schematic view of the probable structure of the Rieske Fe-S center (based on the data in ref. 47.)...

Complex III crystallizes in two distinct conformations (not shown). In one, the Rieske Fe-S center is close to its electron acceptor, the heme of cytochrome c, but relatively distant from cytochrome b and the QH2-binding site at which the Rieske Fe-S center receives electrons. In the other, the Fe-S center has moved away from cytochrome c, and toward cytochrome b. The Rieske protein is thought to oscillate between these two conformations as it is reduced, then oxidized. 

QH2 donates one electron (via the Rieske Fe-S center) to cytochrome c, and one electron (via cytochrome b) to a molecule of Q near the n side, reducing it in two steps to QH2. This reduction also uses two protons per Q, which are taken up from the matrix. 

Britt, R. D., Sauer, K., Klein, M. P., Knaff, D. B., Kriauciunas, A., Yu, C. A., Yu, L., and Malkin, R., 1991, Electron spin echo envelope modulation spectroscopy supports the suggested coordination of two histidine ligands to the Rieske Fe-S centers of the cytochrome b6f complex of spinach and the cytochrome bcl complexes of Rhodospirillum rubrum, Rhodobacter sphaeroides, and bovine heart mitochondria. Biochemistry 30 1892nl901. 

Complex III 280 kDa 11 28 type hemes (b and bg) bound to same mitochondrially coded peptide 1 C heme (cytochrome c,) 1 Fe-S center Rieske factor Spans membrane, cytochrome b, and b in membrane, cytochrome c, and Fe-S center on outer face 0.25-0.53 Pumps protons out of matrix during electron transport/2e"... 

The action of some inhibitors is indicated in Figure 17.4. It is sometimes difficult to pinpoint exactly where an inhibitor may act, however, because our knowledge of the composition and function of the four complexes is far from complete. Complex I inhibitors, such as rotenone, piericidin A, and the barbiturates, are believed to inhibit the transfer of elctrons from the Fe-S centers to UQ. In complex III, antimycin appears to inhibit the reduction of UQ by cytochrome b. Myxothiazol and 2,3-dimercaptopropanol (BAL) inhibit the transfer of electrons from UQH2 to Rieske s protein, because they destroy the Fe-S centers. The action of cyanide and azide on complex IV is also unclear, but it is believed that these substances combine with the Fe3+ moiety of the a3 heme prosthetic group. 

Figure 4 Ranges of midpoint potentials (mV vs. NHE) for biological Fe-S centers. [2Fe-2S]R + +, Rieske-type Fe-S center...

Fig. 18. Thermodynamic profile of iron-sulfur centers in pigeon heart mitochondria SDH, succinate dehydrogenase Rieske s Fe/S, iron-sulfur protein of complex III. From Ohnishi (ISl).

Spectroscopic and crystallographic studies have identified four Fe S clusters in the membrane-bound photosynthetic electron transport chain of plant and cyanobacterial chloro-plasts. One is the Rieske-type [2Fe-2S] + + center in the cyt b(,f complex, which catalyzes electron transfer from plasto-quinol to plastocyanin with concomitant proton translocation, and is functionally analogous to the cyt bc complex, with cyt / in place of cyt The remainder are low-potential [4Fe 4S] + + centers in Photosystem I which constitute the terminal part of the electron transfer chain that is initiated by the primary donor chlorophyll. One is a very low-potential [4Fe S] + + center, Fx (Em =-705 mV), that bridges two similar subunits (PsaA and PsaB) and is coordinated by two cysteines from each subunit in a C-Xg-C arrangement. This cluster transfers electrons to the 2Fe-Fd acceptor via an electron transfer chain composed of Fa, a [4Fe S] + + cluster with Em = -530 mV, and Fb, a [4Fe S] + + clusters with Em = -580 mV. Fa and Fb are in a low-molecular weight subunit (PsaC, 9 kDa) that shows strong sequence and structural homology with bacterial 8Fe-Fds. The center-to-center distance between Fx and Fa and between Fa and Fb are 14.9 A and 12.3 A, respectively, well... 

Fe—2 S] centers cover a wide range of negative redox potentials (— 450 to — 220 mV) excluding the Rieske centers66). 

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