The minimum functional unit

The P clusters of nitrogenase. The enzyme nitrogenase consists of two proteins the Fe protein (m.w. 55,000), which contains a single 4Fe-4S center, and the more complex MoFe protein (m.w. 220,000) (48,49). The minimum functional unit of the latter appears to be the half molecule, an asymmetric dimer containing 1 Mo, 14-16 Fe, and 16-18 sulfides. Application of a vast array of spectroscopic methods to the MoFe protein in a variety of oxidation states has led to the conclusion that it contains two types of metal-sulfur cluster in a 2 1 ratio unusual Fe S units termed P clusters, and the protein-bound form of the FeMo-cofactor (50). 

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