Fe-S clusters

As its name implies, this complex transfers a pair of electrons from NADH to coenzyme Q a small, hydrophobic, yellow compound. Another common name for this enzyme complex is NADH dehydrogenase. The complex (with an estimated mass of 850 kD) involves more than 30 polypeptide chains, one molecule of flavin mononucleotide (FMN), and as many as seven Fe-S clusters, together containing a total of 20 to 26 iron atoms (Table 21.2). By virtue of its dependence on FMN, NADH-UQ reductase is a jlavoprotein. 

Why has nature chosen this rather convoluted path for electrons in Complex 111 First of all. Complex 111 takes up two protons on the matrix side of the inner membrane and releases four protons on the cytoplasmic side for each pair of electrons that passes through the Q cycle. The apparent imbalance of two protons in ior four protons out is offset by proton translocations in Complex rV, the cytochrome oxidase complex. The other significant feature of this mechanism is that it offers a convenient way for a two-electron carrier, UQHg, to interact with the bj and bfj hemes, the Rieske protein Fe-S cluster, and cytochrome C, all of which are one-electron carriers. 

Synthetic analog clusters have played a pivotal role in development of Fe-S cluster biochemistry. Indeed, the synthesis and characterization of clusters with [Fe2( t2-S)2], [Fe4( (,3-S)4], and linear [Fe3( t2-S)4l cores by Holm and co-workers (47, 48) were crucial in establishing the properties of these clusters and identifying these types of centers in biological systems. However, the synthesis of a cluster with the physi-... 

The spatial arrangement of the Fe-S clusters in D. gigas NiFe-hydrogenase (see Fig. 1) suggests an active role for the [Fe3S4] ° cluster in mediating electron transfer from the NiFe active site to the... 

Fe-S clusters can be found in a set of commentaries in the Journal of Biological Inorganic Chemistry 204, 210-213). 

The elucidation of the crystal structures of two high-spin EPR proteins has shown that the proposals for novel Fe-S clusters are not without substance. Two, rather than one novel Fe-S cluster, were shown to be present in nitrogenase, the key enzyme in the biotic fixation of molecular nitrogen 4, 5). Thus the FeMoco-cofactor comprises two metal clusters of composition [4Fe-3S] and [lMo-3Fe-3S] bridged by three inorganic sulfur atoms, and this is some 14 A distant from the P-cluster, which is essentially two [4Fe-4S] cubane moieties sharing a corner. The elucidation of the crystal structure of the Fepr protein (6) provides the second example of a high-spin EPR protein that contains yet another unprecedented Fe-S cluster. 

This review presents an overview of the discovery of the Fepr protein, the spectroscopy that led to the suggestion that it contained a [6Fe-6S] cluster, and the subsequent crystal structure analysis that disproved this hypothesis, yet uncovered what is at a present a unique Fe-S cluster in biology. 

A second unusual EPR spectrum was observed in the oxidized (as-isolated) protein (Fig. 3). This spectrum, which was assigned to an S = z system, was not reminiscent of any Fe-S cluster. Indeed, with g-values of 1.968, 1.953, and 1.903, it looked more like a molybdenum or tungsten spectrum. However, chemical analysis ruled out the possibility that this EPR spectrum arose from Mo or W, and the spectrum was assigned to an Fe-S center instead. The spin concentration, however, was sub stoichiometric and sample-dependent. Furthermore, when the as-isolated protein was oxidized with ferricyanide, it became EPR silent. This, together with the iron determination and the fingerprint of the reduced protein, led Hagen and colleagues to the... 

MCD results more or less confirmed the conclusions drawn from previous EPR data (27). The shapes of the MCD spectra of the putative prismane protein in the 3+, 4+, and 5+ states had not been observed for any Fe-S protein. This was not surprising, since every single type of Fe-S cluster is considered to exhibit a unique MCD spectrum. Magnetization data confirmed the S = ground state of the 5-1- state, as well as the S = 4 ground state of the 4+ state. Unexpectedly, in addition to the S = 4 contribution, a considerable diamagnetic contribution was observed for the 4-1- state. The nature of the diamagnetic contribution was not understood a physical spin mixture was considered to be a possible explanation. 

Resonance Raman studies on the putative prismane protein would provide other important information. In the frequency region of 200-430 cm the putative prismane protein showed bands that at first sight seemed to be typical for Fe-S clusters, but at a closer look appeared to be broader than those observed in basic Fe-S proteins. Also, the resonance frequencies were slightly different from known Fe-S clusters, and it was contended that A prismane-type [6Fe-6S] core is clearly an excellent candidate in light of the available analytical and biophysical data [28]. 

Fig. 9. An overall view of the Fepr molecule from D. vulgaris showing the three domains. Domain 1 is predominantly a-helical and contains an unusual configuration of two three-helix bundles approximately perpendicular to one another (see Fig. 11). Domains 2 and 3 have central /3-sheets surrounded by helices. The two Fe-S clusters are at the center of the figure the hybrid cluster is on the left, and located near the interfaces of the three domains.

Fig. 15. The two Fe-S clusters are some 12-13 A apart and within possible electron transfer range. A tyrosine residue, Y493, is situated roughly halfway between the two clusters, but whether it plays a role in any electron transfer is unclear. Two adjacent tryptophan residues are also located close to cluster 2 again, their possible roles in any enzymatic reaction remain to be defined.

C. Cluster Stmcturrd Interconversions and S3mthesis of Heterometal Clusters Fuscoredoxin (Novel Fe-S Cluster)... 

The larger AEq values observed for the ferrous sites in reduced [2Fe-2S] clusters and the hyperfine parameters obtained for the Fe-S clusters in the D. gigas AOR are very similar to those of the [2Fe-2S] centers in plant ferredoxins. 

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