Ferredoxin, iron-sulfur anions

Activation of an M-CO bond for nucleophilic substitution in anion radical metallo-complexes appears to be quite a general effect (Kaim 1987 Mao et al. 1989,1992 Shut et al. 1995 Klein et al. 1996). Such activation seems to be the basis of metal-cluster catalytic activity. The iron-sulfur cluster (Bu4N)2Fe4S4(SPh)4 deserves to be mentioned here. The cluster is considered as a ferredoxin model (Inoue Nagata 1986) it catalyzes an electron transfer from //-butyl lithium or phenyl lithium to 5-phenyl thiobenzoate or phenylbenzoate (Inoue Nagata 1986). 

Figure 4.5 (a) the positions of the three amide NH atoms in the anion-binding nest motif found in a wide range of functional anion binding proteins, (b) the five-amide compound nest in the P-loop of p21 ras - a nucleotide triphosphate binding protein and (c) a six-amide compound nest surrounding the iron sulfur core in ferredoxin. (Reproduced from Section Key Reference with permission from Elsevier). 

The second main class of enzymes capable of activated C=C bioreductions is that of enoate reductases (EC 1.3.1.31), which are also able to act on a, 3-unsaturated carboxylate anions, substrates typically not accepted by ene reductases. Up to now, only a few enoate reductases have been identified, all from anaerobic bacteria, mainly of the Clostridium species (Table 3.1c) [35,36]. They belong to the family of ferredoxins, containing an iron-sulfur cluster (Fe4-S4) that makes them extremely sensitive to molecular oxygen, and therefore much more difficult to isolate and employ in preparative chemical transformations. 

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