Fe3S4 clusters

On the basis of the available structural data, evidence has accumulated for a common ancestral 8Fd with two [Fe4S4l clusters for both monocluster (3Fe and 4Fe) and dicluster (7Fe and 8Fe) Fds. The evolutionary aspects of Fds have been discussed in detail in several reviews (52, 60) and the arguments will not be repeated here. It is worth noting, however, that the higher potential [Fe3S4] ° clusters are believed to have evolved from their low potential [Fe4S4] counterparts in order to facilitate higher potential electron transport processes. 

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... 

Metric Data for Cuboidal [Fe3S4] Clusters as Determined by Crystallographic Studies ... 

Average Fe-Fe and Fe-S Distances for [Fe3S4] Clusters AS Determined by EXAFS... 

Fig. 2. Metrical data for [Fe3S4] clusters taken from the high-resolution crystsd structures of D. gigas Fdll (1.7 A resolution) (19, 61, 166), A. vinelandii Fdl (1.35 A resolution) (165), Emd the model complex (Et4N)3[Fe3S4(LSs)] (50).

Fig. 3. Ground state spin (S) and valence delocalization schemes for the known oxidation states of [Fe3S4] clusters. Discrete [Fe3S4] clusters have not been observed in siny protein, but they have been identified as fragments in heterometallic cubane clusters. Reduction of the [Fe3S4]+ cluster by three electrons, to yield a putative aU-ferrous cluster, occurs with the concomitant addition of three protons. Key S , grey Fe +, black Fe +, white Fe, white with central black dot.

Fig. 4. X-band EPR spectra of [Fe3S4]+ clusters in wild type and mutant forms of P. furiosus Fd. All spectra were recorded at 4.2 K microwave power, 1 mW microwave frequency, 9.60 GHz modulation amplitude, 0.63 mT. All samples were in 100 mM Tris-HCl buffer, pH 7.8.

Fig. 6. Comparison of VTMCD spectra for biological [Fe3S4] clusters. (A) D. gigas Fdll (20) (B) P. furiosus 3Fe Fd (42) (C) A. vinelandii Fdl (70) (D) T. thermophilus 7Fe Fd (70) (E) E. coli nitrate reductase (24) (F) E. coli fumarate reductase (53) (G) spinach glutEimate synthase (25) (H) beef heart aconitase (27). Spectra were recorded at temperatures between 1.5 and 70 K with an apphed magnetic field of 4.5 T (sdl trEmsitions increase in intensity with decreasing temperature). BEmds originating from minor heme contaminEmts Eire indicated by an asterisk.

Fig. 9. Zero-field splitting of the S = 2 ground state of [Fe3S4] clusters. Constructed for D = -2.5 and E/D = 0.23, where D and E are the axial and rhombic zero-field splitting parameters, respectively.

Fig. 12. UV-visible absorption and VTMCD spectra of the [Fe3S4] cluster in wild-type P. furiosus Fd. The absorption spectrum was recorded at room temperature and the MOD spectra were recorded at 1.67, 4.22, 9.0, 15.8, and 51.0 K (all transitions increasing in intensity with decreasing temperature) with an applied magnetic field of 4.5 T. The band at 314 nm in the absorption spectrum (marked with an asterisk) is due to excess dithionite.

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