Cluster conversions

Fig. 21. Cluster conversions involving [Fe3S4]+ clusters. The cluster conversion in (c) has been proposed on the basis of EPR, VTMCD, and resonance Raman studies, but has yet to be confirmed by Mossbauer or more direct structural techniques.

The Rieske protein in mitochondrial bci complexes is assembled when the protein is incorporated into the complex. The Rieske protein is encoded in the nucleus and synthesized in the cytosol with a mitochondrial targeting presequence, which is required to direct the apoprotein to the mitochondrial matrix. The C-terminus is then targeted back to the outside of the inner mitochondrial membrane where the Rieske cluster is assembled. In addition, the presequence is removed and the protein is processed to its mature size after the protein is inserted into the bci complex. In mammals, the presequence is cleaved in a single step by the core proteins 1 and 2, which are related to the general mitochondrial matrix processing protease (MPP) a and (3 subunits the bovine heart presequence is retained as a 8.0 kDa subunit of the complex (42, 107). In Saccharomyces cerevis-iae, processing occurs in two steps Initially, the yeast MPP removes 22 amino acid residues to convert the precursor to the intermediate form, and then the mitochondrial intermediate protease (MIP) removes 8 residues after the intermediate form is in the bci complex (47). Cleavage by MIP is independent of the assembly of the Rieske cluster Conversion of the intermediate to the mature form was observed in a yeast mutant that did not assemble any Rieske cluster (35). However, in most mutants where the assembly of the Rieske cluster is prevented, the amount of Rieske protein is drastically reduced, most likely because of instability (35, 44). 

DNA, 43 132-133 metal chlorides graphite and, 1 254-259 metal oxides graphite and, 1 260-262 metal sulfldes graphite and, 1 260-262 Interconversions, see Cluster conversions... 

Another interesting cluster conversion is the joining of two Fe2S2 clusters in a protein to form a single Fe4S4 cluster at the interface between a dimeric protein. Such a cluster is present in the nitrogenase iron protein (Fig. 24-2) and probably also in biotin synthase.294 The clusters in such proteins can also be split to release the monomers. 

Rousset M, Montet Y, Guigliarelli B, Forget N, Asso M, Bertrand P, Fontecilla-Camps JC, Hatchikian EC (1998) [3Fe-4S] to [4Fe-4S] cluster conversion in Desulfovibrio fructosovorans [NiFe] hydrogenase by site-directed mutagenesis. Proc. Natl. Acad. Sci. USA 95 11625-11630... 

Aconitase is currently the only well-characterized example of a cubane-type [3Fe-4S]+ to linear [3Fe-4S]+ cluster conversion. This remarkable transformation occurs at pH > 9.5 or under conditions of partial denaturation, despite requiring a major protein rearrangement. Only two of the cysteine ligands to the cubane-type [3Fe-4S]+ cluster... 

From One Type to Another Type of Fe-S Cluster. Iron-sulfur clusters, such as [2Fe-2S], [3Fe-4S], and [4Fe-4S], are found in proteins see Iron-Sulfur Proteins). One of the most interesting aspects of the iron-sulfur proteins is their ready conversion from one type to another through chemical oxidation and reduction, pH changes, or site-directed mutagenesis.More importantly, several studies have indicated that some of the cluster conversions are physiologically relevant in that they play important roles in regulation of enzyme activity. 

Summarized in Table II arc examples of cluster conversion reactions. Those and others resulting in the formation of [3Fe-4S] from [4Fe-4S] were effected by aerial oxidation or by treating the proteins with a chemical oxidant, usually ferricyanide. In several instances of the conversion [3Fe-4S]------[4Fe-4S], dithionite, but no iron source, was added. For-... 

In defining a subsite in terms of-its position in the protein structure, this type of process is subsite-specific if the same atom is added and removed, and the vacant subsite of the [3Fe-4S] cluster does not migrate during cluster conversion. An equivalent statement, under the same requirement for specificity, is that the [4Fe-4SJ subsite voided in the formation of [3Fe-4S] is reoccupied upon reconstitution of the initial cluster. Observations relevant to this matter have been made mainly with two proteins, aconitase from beef heart and Dg Fd II, in the notable experiments of Beinert, Miinck, and their co-workers. 

Figure 9. Schematic representation of cluster interconversion reactions of aconitase and Dg Fd II. Mdssbauer spectroscopic parameters refer to [4Fe-4S] clusters (124, 132) ox = oxidant. With aconitase, subsite a is occupied by Fe upon cluster conversion, whereas with Dg Fd II one or more of the subsites b are populated in this process.

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