Simple and Complex Iron-Sulfur Proteins

SIMPLE AND COMPLEX IRON-SULFUR PROTEINS IN SULFATE REDUCING BACTERIA... 

Simple and Complex Iron-Sulfur Proteins in Sulfate Reducing Bacteria... 

Simple and Complex Iron-Sulfur Proteins in Sulfate Reducing Bacteria Isabel Moura, Alice S. Pereira, Pedro Tavares, and Jose J. G, Moura... 

This chapter will focus on simple and complex iron-sulfur-con-taining proteins isolated from sulfate reducing bacteria (SRB), in order to review the following topics types and distribution of proteins metal clusters involved and their association with other centers and... 

In particular, the study of SRB ferredoxins enables us to survey the different properties of simple iron-sulfur proteins, including electron transfer, flexibility in coordination chemistry, and ability to undergo cluster interconversions. Most of the observations can be extrapolated to more complex situations. 

The iron-sulfur proteins include small proteins that function as remarkably simple electron carriers and large multisubunit complexes with multiple activities. Although many of these enzymes function in electron transfer in bioener-getic or biosynthetic pathways, it has become clear that iron-sulfur proteins catalyze a broad array of reactions not always involving electron transfer. 

A. Peptide Model and Simple Thiolate Complexes of High-Potential Iron-Sulfur Proteins... 

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 fully characterization of both simple or more complex (but well defined in terms of active center composition) iron-sulfur proteins leads to a well of information. The compilation of typical spectroscopic features of the known basic iron-sulfur structures enables a preliminary characterization of centers in a new simple situation or even in some more complex ones. EPR spectroscopy of the iron-sulfur cores in the appropriated oxidation states have characteristics that can be used to readily distinguish certain type of centers58). This technique has also been used to analyse components in complex systems. However the use of EPR as the sole technique can be misleading, when applied to new situations as we have seen for the case of the [3 Fe—xS] core. In this particular case only the conjunction of EPR and Mossbauer can lead to a proper characterization27 33 34 55). 

Whereas many cognate apoenzymes can be reconstituted with iron/sulfur clusters by simple and essentially alchemistic procedures using Fe + and sulfide ions under anaerobic conditions, a highly complex enzymatic machinery is used in vivo for the synthesis of iron/sulfur clusters and their transfer to the target enzymes. Sulfide ions required for cluster synthesis are obtained from cysteine (1) via a persulfide of a protein-bound cysteine residue (2) pyridoxal phosphate is required for the formation of the persulfide intermediate (Fig. 1) (12). 

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