2fe 2s ferredoxins sequenced which implies that it

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2Fe-2S ferredoxins sequenced, which implies that it plays a crucial structural or functional role. The structure of the 2Fe-2S core in Figure 7,2 reveals a tetrahedron of S ligands surrounding each Fe atom. The two tetrahedra share an edge defined by the two bridging sulfide ions, and the core structure is designated Fe2(JLTSh. Fe-S distances and angles cannot be measured accurately in the structure at the present 2.5-A resolution; 70a so we will later discuss these details in terms of model compounds. 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 presence of acid-labile, inorganic sulfide is a key feature of both the Fe2S2 and the Fe4S4 centers. The 1: 1 stoichiometry between iron and acid-labile sulfide was eventually established analytically for Fe2S2 centers. 9 - 11 Care must be taken to ensure that both the protein and its active-site complement are ho- mogeneous. Although protein homogeneity is usually established by electropho- retic methods, these methods may not distinguish between pure proteins and those with absent or incomplete active centers. Fortunately, absorption at 420 run
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380 Figure 7.8 The x-ray crystal structure of the FezSz ferredoxin from Spirulina platensis. 70a is due solely to the Fe2S2 cluster, whereas the 275-nm absorption is dominated by the protein. Therefore a good criterion for active-site saturation and homo- geneity is the ratio of the absorbances at 420 and 275 nm, A 420 nm/Am nm, which is ~ 0.48 for pure spinach ferredoxin. 81 Once homogeneous protein is obtained, the Fe2S2 composition of the "plant" ferredoxins can be correctly deduced an- alytically. The Fe2S2 center displays two redox states that differ by a single electron. The potential range for the couple is - 250 to - 420 mV, revealing the highly reducing nature of the ferredoxin. The correct structure of the Fe2S2 center was first proposed in 1966 based on EPR studies. 82 The reduced state of the cluster shows a rhombic EPR signal with g values of 1.88, 1.94, and 2.04 (Figure 7.6B) characteristic of an S = t center. The oxidized state is EPR-silent. The weakness of the sulfur ligand field causes the iron atoms to be high-spin. But how can two sulfur-ligated iron atoms, each with a tendency to be high-spin, produce a state with a single unpaired electron? The individual Fe atoms in the Fe2S2 cluster resemble those in rubredoxin quite closely. The two redox states of the Fe2S2 protein correspond to an Fe 3 + - Fe3+ and an Fe 3+ -Fe 2+ pair, respectively, as shown in Figure 7.9.
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