FeMo-coenzyme

Each P-cluster is actually a joined pair of cubane-type clusters, one Fe4S4 and one Fe4S3 with two bridging cysteine -SH groups and one iron atom bonded to three sulfide sulfur atoms (Fig. 24-3).17/23 The FeMo-coenzyme can be released from the MoFe-protein by acid denaturation followed by extraction with dimethylformamide.24 While homocitrate was identified as a component of the isolated coenzyme, the three-dimensional structure of FeMo-co was deduced from X-ray crystallography of the intact molybdenum-iron protein.14/17/18... 

Only one electron is transferred to the MoFe-protein in each catalytic cycle of the Fe-protein. Thus, the cycle must be repeated eight times to accomplish the reduction of N2 + 2 H+. Where in the MoFe-protein does a transferred electron go EPR spectroscopic and other experiments with incomplete and catalyti-cally inactive molybdenum coenzyme40 have provided a clear answer. The electron is transferred first to one of the two P-clusters, both of which are close to the Fe4S4 cluster of the Fe-protein. The transfer causes an observable change both in the spectroscopic properties and in the three-dimensional structure of the P-cluster.23/40a Since protons are needed at the active site for the reduction reactions (the FeMo-coenzyme), it is probable that hydrolysis of ATP in the Fe-protein is accompanied by transport of protons across the interface with the MoFe-protein. Tire electron transfer from the P-cluster on to the FeMo-co center would be assisted by a protic force resulting from ATP cleavage. 

Nitrogenase, which catalyzes the reduction of N2 to two molecules of NH3, has a different molybdenum -iron cofactor (FeMo-co). It can be obtained by acid denaturation of the very oxygen-labile iron-molybdenum protein of nitrogenase followed by extraction with d i methyl formamide.655,656 The coenzyme is a complex Fe-S-Mo cluster also containing homocitrate with a composition MoFe7S9-homocitrate (see Fig. 24-3). Nitrogenase and this coenzyme are considered further in Chapter 24. 

A) Stereoscopic view of the FeMo-co coenzyme with interacting side chains from the MoFe-protein of A. vinlandii. After Kim et al.27 (B) FeMo-co with atom labels. From Kim et al.18 (C) The structure of the oxidized form of the P-cluster. From Peters et alP Recent studies by Einsle et al,22a indicate that the cluster probably also contains a nitrogen atom that is held within the cluster by coordination to six of the iron atoms. 

Many other synthetic complexes have been studied including cubic MoFe3S4 clusters.46 However, no exact chemical model for the FeMo-co coenzyme has been developed, and the rates of reaction for all of the model reactions are much slower than those of nitrogenases. 

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