Iron-sulfur proteins solution structure

Contact shifts give information on the electronic structure of the iron atoms, particularly on the valence distribution and on the magnetic coupling within polymetallic systems. The magnetic coupling scheme, which is considered later, fully accounts for the variety of observed hyperfine shifts and the temperature dependence. Thus, through the analysis of the hyperfine shifts, NMR provides detailed information on the metal site(s) of iron-sulfur proteins, and, thanks to the progress in NMR spectroscopy, also the solution structure 23, 24 ). 

The solution structure determination of iron-sulfur proteins is feasible in most cases. To achieve this result, it is necessary to have enough structural constraints to make it possible to obtain a family of conformers with similar folding from many randomly generated structures (95). 

Table III reports structural statistics relative to the solution structures of iron-sulfur proteins available from the Protein Data Bank (118). The lowest percentage of residue assignment occurs for oxidized Synechococcus elongatus Fd (119). The highest percentage of proton assignment is instead obtained for oxidized E. halophila HiPIP, with a value as high as 95% (120). A close figure was also obtained for the reduced protein (94%). In the latter case, such high values are obtained also thanks to the availability of labeled...

NMR Assignments, Structural Constraints, and Structural Parameters for the Available NMR Solution Structures of Iron-Sulfur Proteins"- ... 

The remarkable range of redox potentials in the iron sulfur proteins, already noted, illustrates the principle that nature, having discovered a ligand system, attempts to extract from it the maximum utility. Certainly the outstanding problem awaiting solution in these proteins is an explanation for the relationship between structure and redox potential. Thus... 

EPR and Mossbauer study of two mutants of the mononuclear iron sulfur protein rubredoxin72 compared the results of chemical and cryoradiolytic reduction and concluded that the latter reduction method produces exclusively one reduced species (serine coordinated iron known from the X-ray structure of the ferric protein) because it leaves the coordination of the ferric precursor unaltered. The identity of another spectral form observed in the chemically reduced C42S mutant in solution could not be unambiguously identified. 

R 121 J.M. Nocek, K. Huang and B.M. Hoffman, An Approach to NMR Treatment of Structural Perturbation in Paramagnetic Proteins too Big for Solution Structure Determination , p. 227 R 122 I. Bertini, F. Capozzi and C. Luchinat, Electronic Isomerism in Oxidized High-Potential Iron-Sulfur Proteins Revisited , p. 272 R 123 C.O. Fernandez and A.J. Vila, Paramagnetic NMR of Electron Transfer Copper Proteins , p. 287 Vol. 859, 2003 Oriental Foods and Herbs... 

It is known that FNR forms a 1 1 complex with Fd in solution, as well as when FNR is bound to the membrane. However, dissociation of the flavoprotein from the membrane results in a substantial decrease in the affinity of the enzyme for Fd. No crystallographic data are yet available for the FNR-Fd complex but from known crystal-structure information on FNR, it was suggested by P. Andrew Karplus that Fd could bind on one face of the flavin when its iron-sulfur cluster approaches the Cl a. and C8a atoms of the dimethylbenzyl portion of the flavin. The graphic representation in Fig. 20 shows a plausible orientation for a favorable interaction of the two proteins. 

The complete structure of clostridial rubredoxin has been solved by Jensen and his associates using x-ray analysis (49), and the structure of the iron-sulfur center of the high potential iron protein, but not of the polypeptide, has been determined by x-ray analysis by Kraut and associates (50). Both of these are relatively atypical proteins in this class, either having no inorganic sulfide or having a very high redox potential. So far, we still have no solution from x-ray work for the 2-iron-2-sulfur or the 8-iron-8-sulfur type protein. 

Ramelot TA, Cort JR, Goldsmith-Fischman S, Kornhaber GJ, Xiao R, Shastry R, Acton TB, Honig B, Montelione GT, Kennedy MA Solution NMR structure of the iron-sulfur cluster assembly protein U (IscU) with zinc bound at the active site. J Mol Biol 2004, 344(2) 567—583. 

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