Cuboidal iron-sulfur

The [3Fe-4S] core is now considered an unique basic iron-sulfur core whose structure was determined in D. gigas Fdll 56, 84) (as well as in aconitase 85-87) and A. vinelandii Fd (57, 59, 80)). The cluster in these proteins have Fe-Fe and Fe-S distances around 2.8 and 2.2 A and the core described as a cuboidal geometry with one comer missing (Fe S stoichiometry of 3 4). 

Trinuclear Cuboidal and Heterometallic Cubane-Type Iron-Sulfur Clusters New Structural and Reacticity Themes in Chemistry and Biology R. H. Holm... 

The next class of iron-sulfur proteins contains Fe3S4 clusters. The commonest example of such proteins is that of 3Fe ferredoxins, which are found in bacteria. As shown in Figure 23, in these proteins the Fe3S4 cluster belongs to the class previously defined as having an incomplete cuboidal geometry (Chapter 8, Section l.l).46... 

The discovery of the different dinuclear or cuboidal-type biological iron-sulfur clusters is associated with their natural occurrence in two oxidation states. They can all function as one-electron transferring agents. This redox function has been well established in many studies over a period of almost five decades [1-5], However, electron transfer is generally not considered to be a catalytic activity. It is typically a stoichiometric transfer between two complex redox proteins. Mechanistically, it is probably best described as outer sphere or not involving the breaking and making of covalent bonds other than those related to hydrons. 

FIGURE 4.5 Structures of the four common iron—sulfur centres (C—Cys). (a) Rubredoxin (b) rhombic two iron—two sulfide [Fc2—S2] cluster (c) cuboidal three-iron—four sulfide [Fes—S4] cluster and (d) cubane four iron—four sulfide [Fc4-S4] cluster. 

Iron—sulfur proteins contain four basic core structures which have been characterized crystallographically both in model compounds and in iron—sulfur proteins (Rao and Holm, 2004). These are (Figure 13.16), respectively, (a) rubedoxins found only in bacteria, in which the [Fe—S] cluster consists of a single Fe atom hganded to four Cys residues — the iron atom can be in the +2 or +3 valence (b) rhombic two iron—two sulfide [Fe2 S2] clusters — typical stable cluster oxidation states are +1 and +2 (the charges of the coordinating cys-teinate residues are not considered) (c) cuboidal three-iron—four sulfide [Fe3—S4] clusters — stable oxidation... 

B7.13 Trinuclear cuboidal and heterometallic cubane-type iron-sulfur clusters new structural and reactivity themes in chemistry and biology... 

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... 

Cuboidal iron-vanadium-sulfur clusters have been synthesized in an effort to understand the role of vanadium in this enzyme, and to delineate the differences between clusters with Mo and V... 

Aconitase was initially crystallized in the [3Fe-4S]" " form, and these crystals could be converted to the [4Fe S] " " form by addition of ferrous ammonium sulfate. Alternatively, anaerobic crystallization methods were used to crystallize the [4Fe-4S] " " form directly." The iron-sulfur cluster of aconitase sits within a solvent-filled cleft in the protein structure and is coordinated by Cys 358, Cys 421, and Cys 424. The [3Fe S]+ cluster shows the typically cuboidal geometry, with the open iron site directed toward the active site cleft. Conversion to the active [4Fe-4S] " " form results in almost no structural perturbation, with the extra iron atom simply being inserted into the vacant site in the cluster. The fourth iron is tetrahedrally coordinated, but with a solvent hydroxide rather than a cysteine as the fourth ligand. (The electron density clearly reveals a bound solvent, and previous ENDOR studies had demonstrated that the bound species was associated with only a single proton.)... 

The percursor for the synthesis of hetero metal cuboidal type cores in proteins has been the [3Fe-4S] cluster. This structure has unusual electronic and magnetic properties which were explored in detailed in the cluster contained In ferredoxin II (Fdll) isolated from Desulfovibrio (D.) gigas, a sulfate reducer. This protein is an electron transfer protein isolated as a tetramer of identical subunits of know amino add sequence (6 kDa) and consists of 57 amino-adds, induding six cysteinyl residues (44). Each subunit contains a trinudear iron-sulfur duster of the [3Fe-4S] type. 

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