Iron-molybdenum-sulfur clusters crystal structure

The next two entries to Table 3 are cited for completeness. Nitrogenase is treated in Chapter 7 and CO dehydrogenase in Chapter 9. Nitrogenase contains a very complex iron-sulfur cluster that includes another metal, molybdenum or vanadium. The crystal structure of the Mo variant has been determined. There is a third variant, alternative nitrogenase [92], whose cluster apparently does not contain any heterometal. That cluster would thus be a perfect candidate for our definition of a redox-catalytic iron-sulfur cluster. Unfortunately, this third nitrogenase has thus far been characterized to a much lesser extent than the other two forms. For all nitrogenases holds that the binding of N2 to the cluster has not been established [53] therefore, formally these enzymes have not yet been positively identified as redox iron-sulfur catalysts. 

The Mo enzyme consists of two components (1) the Fe protein (molecular weight 57,000 daltons), which contains iron and sulfur (4 atoms of each per protein) and (2) the MoFe protein (220,000 daltons, Uz subunits), which contains both metals (1 atom Mo, 32 atoms Fe). Each also contains S ions (ca. one per iron), which act as bridging ligands for the metals. The protein contains special Fe—S clusters called P clusters that have EPR resonances like those of no other Fe—S cluster. A soluble protein-fiee molybdenum and iron-containing cluster can be separated from the enzyme. This iron-molybdenum cofactor, or FeMo-co, was known to have approximately 1 Mo, 7-8 Fe, 4-6 S ", and one molecule of homocitrate ion. As for the P cluster, there was no agreement on the structure of FeMo-co for many years. In purified form FeMo-co does crystallize, and it can restore N2 reducing activity to samples of mutant N2ase that are inactive because they lack FeMo-co. On the other hand, no crystal structure of FeMo-co proved possible, and no synthetic model complex was found that could activate the mutant enzyme. 

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