Tetrahedral iron centers clusters

In the diagram shown in Figure 6.9, a sulfur atom has been inserted at the cluster s center in agreement with current thinking.24 The M center is attached to protein side chains at only two locations, cysteine 275 (at Fel) and histidine 442 (at Mo), and is unusual in having three-coordinate irons rather than the normal four-coordinate tetrahedral iron coordination sphere.29b The molybdenum forms... 

Several models have been proposed for the active center of iron and sulphur in Clostridial ferredoxin in which the cysteine residues in the peptide chain participate in the sulphur bridging. Fig 9 166). Unfortunately X-ray analysis of crystals of these proteins has not been completed. It is difficult to confirm that all the irons are clustered in a single linear array 167, 168). X-ray studies of another non-heme iron protein, the high potential iron protein, hipip, from chromatium, carried out by J. Kraut (personal communication), indicate that the four irons of this molecule may be clustered in a tetrahedral array. 

Ferredoxins and high-potential iron proteins (HiPIPs) involve clusters of two, three, and four tetrahedral Fe centers bridged by sulfido donors. The coordination sphere at Fe is completed by thiolate cysteine ligands. 

All presently known nonredox enzymes with a cubane as catalytic center are hydrolases (see later discussion). In these enzymes Fea is formally equivalent to the tetrahedral Zn(II) site in hydrolytic zinc enzymes. Thus, when the enzyme is ready to act but not yet in turnover (the substrate-free enzyme), its cluster is probably best described as structure ii in Figure 1 with X a hydroxyl group. In shorthand notation this could be written as [FeaOH—3Feb—4S]1+ (Cys)3 or as [Fea3Feb-4S]2+OH(Cys)3 where it has been assumed that, on average, each iron is formally Fe2 5+. 

The structure of crystalline FeO belongs to the NaCl type. When iron(II) oxide is prepared under normal conditions, the composition of the product (wustite) is always Fei 50. In order to retain overall electric neutrality, part of the Fe2+ is oxidized to Fe3+, and the chemical formula becomes Fc Fe. O. Since the radius of Fe3+ is small, the Fe3+ cations tend to occupy the tetrahedral holes to form a short-range ordered Fe40io cluster, which is called the Koch cluster of Fei 0, as shown in Fig. 10.1.3. The Koch clusters are distributed randomly in the crystal structure. To satisfy charge neutrality, the formation of a Koch cluster must be accompanied by the presence of six Fe2+ vacancies, one of which is located at the center of the cluster, and the remaining five are distributed randomly at the centers of the edges of the cubic unit cell. 

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