Details of the M Center

More recent crystallographic data at 2.0-A resolution were collected on two crystals, one believed to have the P-cluster and M center in oxidized states 

Concurrently with the X-ray crystallographic studies, extended X-ray absorption fine structure (EXAFS) studies confirmed many of the bond distances proposed for nitrogenase s FeMoco cluster. The EXAFS data of reference 25 indicate short Fe-Fe distances of 2.61, 2.58, and 2.54 A for M+, M (resting state), and M forms, respectively. The authors believe that the short M center bond lengths indicate Fe-Fe bonds in this cluster. In another study using dithionite-reduced MoFe-protein Fe-S, Fe-Fe, Fe-Mo distances of 2.32, 2.64, and 2.73 A, respectively, were found in the 1 to 3 A region and Fe-Fe, Fe-S and Fe-Fe distances of 3.8, 4.3, and 4.7 A, respectively, were found in the 3 to 5 A region.30 

The authors of reference 28 find a strikingly similar FeMoco environment in Kpl as compared to that of Avl discussed above, although the Kpl interatomic distances were generally slightly longer. 

Hypotheses for dinitrogen binding and reduction at nitrogenase s M center include  

A possible increased coordination number for Mo by adding hydrides and/or r 2-dihydrogen ligands for transfer to dinitrogen 

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