FeMo protein structure

The Fe-protein, whose molecular structure is shown in Figure 31,40,41 acts as a one-electron donor to the FeMo-protein. This electron-donating ability arises from the propensity of the Fe4S4 cluster to undergo a one-electron oxidation. 

Figure 32 X-Ray structure of the active site of FeMo-protein in the nitrogenase of Azotobacter vinelandii...

This caveat notwithstanding, the Dominant Hypothesls( ) designates [FeMo] as the protein responsible for substrate reduction. The FeMo protein contains an 02fi2 subunit structure due to expression of the nifD and nlfK genes(24,25). Its overall M.W. of about 230,000 reflects the 50-60,000 M.W. of each of its four subunits. The nonprotein composition of 30 Fe, 2 Mo, and 30 s2- betokens the presence of transition metal sulfide clusters, which are presumed to be the active centers of the protein. 

The FeMo-co or M center of the FeMo protein has been identified spectroscoplcally(, 13,30) within the protein and has been extracted from the protein into N-methyl formamlde(31) and other organic solvents(32.33). Its biochemical authenticity can be assayed by its ability to activate FeMo protein from a mutant organism that produces protein that lacks the M center(31). The extracted cofactor resembles the M-center unit spectroscopically and structurally as shown in Table I. It seems reasonable to presume that the differences are due to variation in the ligation of the center between the protein and the organic solvent(34). 

FeMoco, both as a constituent of the FeMo protein and an isolated entity, has been the subject of detailed spectroscopic examination. 57Fe Mossbauer and EPR studies of the cofactor have been interpreted in terms of an S = centre that contains one molybdenum and ca. six irons in a spin-coupled structure. The EPR signal serves as a valuable fingerprint of FeMoco furthermore, release of FeMoco from the FeMo protein produces an EPR spectrum with broader features, but the same profile, thereby indicating that the core of this cluster is little changed by the extraction procedure. Treatment of FeMoco with ca. one equivalent of... 

Kenneth O. Hodgson, Richard H. Hohn, et al. EXAFS (extended X-ray absorption fine structure) study of structure of FeMo protein component of nitrogenase... 

Figure 1 Schematic representation of the Fe-protein (60 kD, 2 dimer) and the MoFe-protein (250 kD, a2fh. tetramer) of the Mo-nitrogenase. Outline of the electron transfer path through the metal clusters. Structure of the Fe4S4 cluster of the Fe-protein. Structure of the P-cluster (in two oxidation states) and of the FeMo-cofactor in the MoFe-Protein. Limiting stoichiometry for the function of Mo-nitrogenase...

Studies of nitrogenase proteins from the bacteria Azoto-bacter vinelandii and Clostridium pasteurianum have provided structural details of the proteins involved. Two metalloproteins make up the nitrogenase system an Fe protein which couples the hydrolysis of ATP to electron transfer, and an FeMo protein which is responsible for binding N2. The dual role of these proteins can be summarized in three steps ... 

Finally, we must consider how protons are delivered to complete the reaction. Without definitive evidence, molecular modeling has identified three likely proton-transfer routes (71). One is the interstitial channel, filled with water molecules, that nms between the a- and )3-subunits from the surface of the MoFe protein to the pool of water molecules around the homocitrate of the FeMo-cofactor. This channel could deliver protons rapidly to boimd substrate and might also provide a pathway for N2 and NH4+ to enter and leave the reduction site (see also the section Substrate-Binding Site). Extensive theoretical studies of the hydrogen-related chemistry of the FeMo-cofactor use this same interstitial channel to deliver protons (75). These studies indicate that the delivery of electrons to the FeMo-cofactor causes its sulfur atoms to become more basic which, in turn, makes them attractive sites for protonation by water molecules in the interstitial pool. Once transferred, these protons become reduced to hydrogen atoms that can then migrate across the FeMo-cofactor structure to other Fe and S atoms and become involved in substrate reduction (76). 

Fe—S Clusters The other surprise in the N2ase structure, apart from the FeMo-co structure, is the nature of the P clusters. To understand this result, we must briefly look at iron-sulfur proteins, which have been known for many years, but the structures of the active sites having become clear only relatively recently. Structures 16.13-16.15 show some the main cluster types that had been recognized. There are also a number of triiron clusters. In each case the R groups represent the cysteine residues by which the metal is bound to the protein chain. In the cases in which there is more than one iron atom, S ions are also present and bridge the metals. The ferredoxin proteins contain Fe4S4 or Fe2S2 cores, and these have been extruded apparently intact... 

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