Chemical Modelling of the Active Site of Molybdenum—Iron Protein. Synergism of MoFe₃S₄ Cubane-Like Unit from Physical and Chemical Evidence

Abstract

Investigations on molybdenum-iron -sulfur cluster compounds have been a hot subject since the isolation of FeMoco from MoFepr of nitrogenases (Shah 1977). Both spectroscopic (Conradson 1985, 1987; Hedman 1988) and chemical analysis (Burgess 1984) results have implicated a mixed molybdenum-iron cluster nature with sulfur and oxygen ligand atoms of composition MoFe_6–9S_4–9 for the active site of FeMoco. Many structural models (Lu 1975, 1980; Cai 1982; Cramer 1978; Teo 1979; Christou 1982; Orme-Johnson 1983) having mostly MoFe₃S_3–4 as a basic structural unit have been proposed, of which the first one that was suggested by Lu in 1973 was named Fuzhou model. It has a string-bag MoFe₃S₃ composition with a defected cubane-like skeleton, where the open site is for substrate coordination. Mo-EXAFS (Conradson 1987) and other studies on MoFepr or FeMoco have revealed a distorted octahedral Fuzhou model environment for molybdenum atom with 2 ~ 3 oxygen (and/or nitrogen) atoms at 2.10 ~ 2.12 Å, 3 ~ 4 sulfur atoms at 2.37 Å, and 3 ~ 4 iron atoms at 2.68 ~ 2.70 Å. Chemically, the MoFe₃S₄ cubane-like unit composed of these atoms is one of the most stable arrangements known. Compounds containing this basic structural fragment have many of the characteristics and properties of FeMoco. Although the exact molecular structure of FeMoco remains unknown, cluster compounds with MoFe₃S₄ cubane-like unit chelated by thiolato ligands have been regarded as the closest chemical models and widely studied for their chemical and physical properties, and their relevance to the activity of nitrogen fixation is proposed.