Iron—sulfur proteins crystallography

Stout, C. D. Iron-Sulfur Protein Crystallography in Iron-Sulfur Proteins, vol. 4 (ed. Spiro, T. G.) New York, WUey 1982... 

Among the electron transport proteins, there are four known classes that have been isolated and studied by X-ray crystallography. These include cytochromes, iron-sulfur proteins, copper blue proteins, and... 

Chloroplast ferredoxin containing the [(2Fe-2S)-(S-Cys)4] cluster is one common type of iron-sulfur protein. Another [2Fe-2S]-type protein is the Rieske iron-sulfur protein, present in the Cyt >6/complex as well as the Cyt Ac, complex. The pair of iron atoms in the cluster ofthe Rieske iron-sulfur protein are bound to two cysteine and two histidine residues, in addition to two sulfur atoms. The three-dimensional structures of ferredoxins and that of the Rieske iron-sulfur protein have been determined by X-ray crystallography (see Chapters 34 and 35, respectively, for the structure ofthe chloroplast ferredoxin and the Rieske iron-sulfur protein). The sulfide ions in iron-sulfur proteins urt acid-labile this provides a simple means for detecting the iron-sulfur proteins, as the sulfide is released as H2S upon acidification. The oxidized and reduced states of iron-sulfur clusters differ by just one unit of formal charge, corresponding to and Fe. Iron-sulfurproteins are commonly characterized by optical absorption, circular-dichro-... 

The existence of this HiPIP-type structure was the starting point of an interesting development in the study of the iron-sulfur active center. X-ray crystallography showed that there is little difference between the 4Fe-4S cluster in a ferredoxin (E0 = —400 mV) and in HiPIP (E0 = +350 mV). This anomaly was elucidated by the so-called C state hypothesis of Carter et al. (7) (Figure 3), in which the existence of a super-reduced HiPIP and a super-oxidized ferredoxin was postulated. The super-reduced HiPIP was shown to exist by Cammack (8) utilizing 80% DMSO (dimethylsulfoxide) to distort the protein environment of HiPIP. In this case a super-reduced HiPIP with EPR signal similar to a... 

To reassess the electron-transfer sequence on the acceptor side of photosystem I, Shinkarev et al used the above calculated average lifetimes for the back-reactions involving FeS-X and FeS-A and applied the known electron-transfer rate-vx.-distance relationship" and by taking into consideration of the asymmetrical position of iron-sulfur clusters FeS-A and FeS-B relative to FeS-X as determined by X-ray crystallography. The work of Moser on electron transfer in proteins deduced that the rate of electron transfer between the electron carriers decreases exponentially with distance. According to the relationship established by Moser etal, a change in distance of 1.7 A would result in a one-order change in the electron-transfer rate in proteins. 

Oarter has reviewed the comparative crystallography of oxidized and reduced C. vinosum HiPIP (1), and the dimensional changes of the iron-sulfur cube following oxidation or reduction have also been extensively tabulated and discussed for both model complexes and protein-bound clusters (118). In spite of the low sequence homology in HiPIPs, there is a remarkable similarity in tertiary structure, especially around the cluster (114). No significant secondary structure is observed in the HiPIPs, with only two short a-helical segments, three strands of antiparallel /3-pleated sheet, and one small helix near the N terminus (Fig. 1). The 4Fe-4S cluster is buried in the protein interior and is inaccessible to solvent (Fig. 2). This feature has been pro-... 

The impetus for the development of iron-sulfur cluster chemistry over the last three decades derives largely from the occurrence of iron-sulfur clusters in an extensive variety of proteins and enzymes.Five cluster types (l)-(5) have been demonstrated by protein crystallography and are shown in Figure 1. Clusters (l)-(3) represent the fundamental set found in proteins (ferredoxins) from a variety of prokaryotic and eucaryotic sources. Rhomb-like cluster (1) is especially prevalent in green plants. Cuboidal cluster (2) and cubane-type cluster (3) are found in bacterial sources... 

Each P-cluster is actually a joined pair of cubane-type clusters, one Fe4S4 and one Fe4S3 with two bridging cysteine -SH groups and one iron atom bonded to three sulfide sulfur atoms (Fig. 24-3).17/23 The FeMo-coenzyme can be released from the MoFe-protein by acid denaturation followed by extraction with dimethylformamide.24 While homocitrate was identified as a component of the isolated coenzyme, the three-dimensional structure of FeMo-co was deduced from X-ray crystallography of the intact molybdenum-iron protein.14/17/18... 

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