Electron transfer iron sulfur clusters

The mechanism suggested by Kerscher and Oesterhelt is indicated in Scheme 46 for the enzyme from H. halobium (213). The initial step is identical to that of the 2-oxoacid dehydrogenase complexes and involves binding of pyruvate to thiamin diphosphate and subsequent decarboxylation yielding hydroxyethylthia-min diphosphate. This intermediate undergoes one-electron transfer to the [4Fe-4S] cluster to form the stable free radical. The cluster is then reoxidized by ferredoxin or oxygen to give the enzyme-intermediate complex. Reaction with CoA initiates the second electron transfer to the iron-sulfur cluster, acyl transfer, followed by reoxidation of the enzyme by ferredoxin or O2 to complete the cycle. Two basic questions are yet unanswered (1) What is the mechanism of the enzymic reaction between CoASH and hydroxyethyl-TPP in the absence... 

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

The final step of the reaction involves the transfer of two electrons from iron-sulfur clusters to coenzyme Q. Coenzyme Q is a mobile electron carrier. Its isoprenoid tail makes it highly hydrophobic, and it diffuses freely in the hydrophobic core of the inner mitochondrial membrane. As a result, it shuttles electrons from Complexes I and II to Complex III. The redox cycle of UQ is shown in Figure 21.5, and the overall scheme is shown schematically in Figure 21.6. 

Cluster Fx was also identified via its EPR spectral features in the RCI photosystem from green sulfur bacteria 31, 32) and the cluster binding motif was subsequently found in the gene sequence 34 ) of the (single) subunit of the homodimeric reaction center core (for a review, see 54, 55)). Whereas the same sequence motif is present in the RCI from heliobacteria (50), no EPR evidence for the presence of an iron-sulfur cluster related to Fx has been obtained. There are, however, indications from time-resolved optical spectroscopy for the involvement of an Fx-type center in electron transfer through the heliobacterial RC 56). 

Studies (see, e.g., (101)) indicate that photosynthesis originated after the development of respiratory electron transfer pathways (99, 143). The photosynthetic reaction center, in this scenario, would have been created in order to enhance the efficiency of the already existing electron transport chains, that is, by adding a light-driven cycle around the cytochrome be complex. The Rieske protein as the key subunit in cytochrome be complexes would in this picture have contributed the first iron-sulfur center involved in photosynthetic mechanisms (since on the basis of the present data, it seems likely to us that the first photosynthetic RC resembled RCII, i.e., was devoid of iron—sulfur clusters). 

EPR studies on electron transfer systems where neighboring centers are coupled by spin-spin interactions can yield useful data for analyzing the electron transfer kinetics. In the framework of the Condon approximation, the electron transfer rate constant predicted by electron transfer theories can be expressed as the product of an electronic factor Tab by a nuclear factor that depends explicitly on temperature (258). On the one hand, since iron-sulfur clusters are spatially extended redox centers, the electronic factor strongly depends on how the various sites of the cluster are affected by the variation in the electronic structure between the oxidized and reduced forms. Theoret-... 

Achieving fast electron transfer to enzyme active sites need not be complicated. As mentioned above, many redox enzymes incorporate a relay of electron transfer centers that facilitate fast electron transfer between the protein surface and the buried active site. These may be iron-sulfur clusters, heme porphyrin centers, or mononuclear... 

Tetranuclear iron-sulfur clusters of the type [Fe4S4(SR)4]2, where R = CH2C6H5 and C6H5, were found138 to catalyze the reduction of C02 in DMF solutions. Controlled-potential electrolyses were carried out in a C02-saturated 0.1 M tetrabutylammonium tetrafluoroborate (TBAT)-DMF solution at a mercury pool cathode. In the absence of a catalyst, C02 was substantially reduced only at potentials more negative than -2.4 V versus SCE, while in the presence of a cluster, the reduction took place at around -1.7 V thus, potential shift of ca. 0.7 V was achieved. The products were analyzed by means of gas chromatography and isotachophoresis. Without a catalyst, oxalate was the main product, and addition of small amounts of water to the DMF solution favored formate production, whereas in the presence of the catalyst, formate was produced predominantly even in a dry DMF solution. This result was interpreted in terms of indirect reduction of C02, proceeding by electron transfer from the reduced cluster to C02 in the bulk... 

Chapter 6). Other iron-sulfur proteins, so named because they contain iron sulfur clusters of various sizes, include the rubredoxins and ferredoxins. Rubredoxins are found in anaerobic bacteria and contain iron ligated to four cysteine sulfurs. Ferredoxins are found in plant chloroplasts and mammalian tissue and contain spin-coupled [2Fe-2S] clusters. Cytochromes comprise several large classes of electron transfer metalloproteins widespread in nature. At least four cytochromes are involved in the mitrochondrial electron transfer chain, which reduces oxygen to water according to equation 1.29. Further discussion of these proteins can be found in Chapters 6 and 7 of reference 13. 

In the fall of 2000 an international workshop on the chemistry and biology of iron-sulfur clusters was held at Virginia Polytechnic Institute and State University, Blacksburg.19 Many functions for iron-sulfur clusters besides electron transfer and... 

Cytochromes, catalases, and peroxidases all contain iron-heme centers. Nitrite and sulfite reductases, involved in N-O and S-O reductive cleavage reactions to NH3 and HS-, contain iron-heme centers coupled to [Fe ] iron-sulfur clusters. Photosynthetic reaction center complexes contain porphyrins that are implicated in the photoinitiated electron transfers carried out by the complexes. 

NADH-coenzyme Q (CoQ) oxidoreductase, transfers electrons stepwise from NADH, through a flavoprotein (containing FMN as cofactor) to a series of iron-sulfur clusters (which will be discussed in Chapter 13) and ultimately to CoQ, a lipid-soluble quinone, which transfers its electrons to Complex III. A If, for the couple NADH/CoQ is 0.36 V, corresponding to a AG° of —69.5 kJ/mol and in the process of electron transfer, protons are exported into the intermembrane space (between the mitochondrial inner and outer membranes). 

For the cytochrome c-plastocyanin complex, the kinetic effects of cross-linking are much more drastic while the rate of the intracomplex transfer is equal to 1000 s in the noncovalent complex where the iron-to-copper distance is expected to be about 18 A, it is estimated to be lower than 0.2 s in the corresponding covalent complex [155]. This result is all the more remarkable in that the spectroscopic and thermodynamic properties of the two redox centers appear weakly affected by the cross-linking process, and suggests that an essential segment of the electron transfer path has been lost in the covalent complex. Another system in which such conformational effects could be studied is the physiological complex between tetraheme cytochrome and ferredoxin I from Desulfovibrio desulfuricans Norway the spectral and redox properties of the hemes and of the iron-sulfur cluster are found essentially identical in the covalent and noncovalent complexes and an intracomplex transfer, whose rate has not yet been measured, takes place in the covalent species [156]. 

Figure 8.3 Outline reaction cycle of NiFe hydrogenase.The minimal hydrogenase is depicted, consisting of the [NiFe] centre in the large subunit, and the proximal [4Fe-4S] cluster (C) in the small subunit.The reaction is written in the direction of the oxidation of H2. Electrons are transferred out through the other iron-sulfur clusters to an acceptor protein (not shown).The equivalent states of the NiFe centre B, SR, R and C are indicated. Reduced centres are shaded. Electron transfers are accompanied by transfers of hydrons (not shown).

One large class of non-heme iron-containing biomolecules involves proteins and enzymes containing iron-sulfur clusters. Iron-sulfur clusters are described in Sections 1.7 (Bioorganometallic Chemistry) and 1.8 (Electron Transfer) as well as in Section 3.6 (Mossbauer Spectroscopy). See especially Table 3.2 and the descriptive examples discussed in Section 3.6.4. Iron-sulfur proteins include rubredoxins, ferrodoxins, and the enzymes aconitase and nitrogenase. The nitrogenase enzyme was the subject of Chapter 6 in the hrst edition of this text—see especially Section 6.3 for a discussion of iron-sulfur clusters. In this... 

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