Nitrogenase iron centers

More complex assemblies of iron and sulfur, sometimes extended to other metals like nickel, molybdenum, vanadium, or other iron centers are found in some enzymes, that catalyze the transformation of small molecules [1, 14]. Among these centers, we will focus next on the P cluster and the FeMo cofactor of nitrogenase and on the H cluster of the iron-only hydrogenase. 

Lanzilotta, W.N and Seefeldt, L.C. (1997) Changes in the midpoint potential of the nitrogenase metal centers as a result of iron protein-molybdenum-iron protein complex formation, Biochemistry 36, 12976-12983. 

S-adenosylmethionine (SAM or AdoMet) superfamily, aconitase, and others), enzymes that contain Fe-S heteroatomic clusters (nitrogenase iron-molybdenum cofactor (FeMoco), carbon monoxide dehydrogenase (CODH), and acetyl CoA synthase (ACS)), and enzymes that contain unique ligation sets around specialized iron centers ([NiFe] and [FeFe] hydrogenases) (Figure 1). ... 

These model complexes do not include an interstitial atom nor do they capture the geometry of the central iron core of the cofactor which recent calculations point to as vital for nitrogenase activity [73-76]. One possibility is that the central atom is a nitride if this is so, then the coordination geometry is unprecedented in small-molecule model chemistry of iron nitrides. Holm and coworkers have recently begun to examine the coordination chemistry of iron nitrides to see if a nitride which bridges six iron centers can be obtained [96]. Literature precedent shows ample evidence for linear Fe-N-Fe bridges for octa-hedrally coordinated iron [97-100] and, more recently, a bent Fe-N-Fe bridge for pseudotetrahedral iron centers from our laboratory at Caltech vide infra) [89,... 

Part 1 of the nitrogenase protein contains another interconnected group of Fe-S atoms, this one with eight iron atoms and seven sulfur atoms. This [8Fe-7S] group collects electrons and transmits them to the binding center. Part 2 of nitrogenase contains a third Fe-S group, this one made up of four iron atoms and four sulfur atoms. This part of the enzyme also binds two molecules of ATP. 

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