Conformations ferredoxin

A second example is that of an Ala-to-Cys mutation, which causes the fonnation of a rare SH S hydrogen bond between the cysteine and a redox site sulfur and a 50 mV decrease in redox potential (and vice versa) in the bacterial ferredoxins [73]. Here, the side chain contribution of the cysteine is significant however, a backbone shift can also contribute depending on whether the nearby residues allow it to happen. Site-specific mutants have confirmed the redox potential shift [76,77] and the side chain conformation of cysteine but not the backbone shift in the case with crystal structures of both the native and mutant species [78] the latter can be attributed to the specific sequence of the ferre-doxin studied [73]. 

For the cytochrome c-plastocyanin complex, the kinetic effects of cross-linking are much more drastic while the rate of the intracomplex transfer is equal to 1000 s in the noncovalent complex where the iron-to-copper distance is expected to be about 18 A, it is estimated to be lower than 0.2 s in the corresponding covalent complex [155]. This result is all the more remarkable in that the spectroscopic and thermodynamic properties of the two redox centers appear weakly affected by the cross-linking process, and suggests that an essential segment of the electron transfer path has been lost in the covalent complex. Another system in which such conformational effects could be studied is the physiological complex between tetraheme cytochrome and ferredoxin I from Desulfovibrio desulfuricans Norway the spectral and redox properties of the hemes and of the iron-sulfur cluster are found essentially identical in the covalent and noncovalent complexes and an intracomplex transfer, whose rate has not yet been measured, takes place in the covalent species [156]. 

Antitumor drugs cisplatin as, history, 37 175-179 platinum compounds future studies, 37 206-208 resistance to, 37 192-193 second-generation, 37 178 Antiviral agents, 36 37-38 AOR, see Aldehyde oxidoreductase Aphanothece sacrum, ferredoxins, amino acid sequence, 38 225-227 Apo-calcylin, 46 455 Apo-caldodulin, 46 449-450 Apoenzyme, 22 424 Apoferritin biosynthesis, 36 457 cystalline iron core, 36 423 Fe(III)distribution, 36 458-459 Fe(II) sequestration, 36 463-464 ferroxidase centers, 36 457-458 iron core reconstruction in shell, 36 457 mineralization, 36 25 Mdssbauer spectra, 36 459-460 optical absorbance spectra, 36 418-419 subunit conformation and quaternary structure, 36 470-471... 

These complexes are known with planar and non-planar M3X3 cores, and with tetrahedral or square-planar metal coordination. The X3M3 cycle in chair conformation and with tetrahedral metal coordination occurs in (p-SMe)3B3X6 (X = Cl, Br)38 and is proposed in the form of (p-Scys)3Cd3(Scys)6 for one of the two clusters with cysteinate coordination in metallothionein proteins.39 The twist-boat conformation occurs in S3Sn3Me640 and probably in the related compounds S3M3R6 (M = Ge, Sn, Pb R = Me, Bu, Ph).41 The same structural unit with distorted twist-boat stereochemistry occurs in the (p-S)3Fe3(Scys)5L cluster recently discovered in several ferredoxins.15 42... 

The protein sequence data in Table 2 show that the cysteine residues in all the proteins occur in identical positions (18, 39, 44, 47, 77) in the sequence. Thus, the ligand field produced by the cysteinyl-sulfur atoms is not likely to be different among these proteins unless there is a difference in protein conformation which causes a displacement in one or more of the cysteinyl sulfur atoms. Note that a displacement of any cysteinyl sulfur atom in the model in Fig. 15 results in rhombic distortion at the iron to which it is ligated. Since, according to the spin-coupled model, this rhombic distortion will manifest itself in the difference between gx and gx for a particular protein, the EPR data in Table 1 provide a measure of the rhombic distortion around the ferrous iron in the reduced proteins. In particular, the g-values of adrenodoxin are axially symmetric while the g-values of spinach ferredoxin show a rhombic distortion. Thus, the observation of Kimura et al. (168) that adrenodoxin and spinach ferredoxin have different protein conformations is consistent with the prediction of the above model. 

The bacterial ferredoxins, in general, are similar to each other in amino acid content, but differ in detail with each ferredoxin having a characteristic composition. Each contains about fifty total amino acid residues with an abundance of acidic and a paucity of basic amino acids. The abundance of acidic residues accounts for the affinity of ferredoxin for DEAE-cellulose and its low isoelectric point (Lovenberg, Buchanan, and Rabinowitz (65)). Each of the bacterial ferredoxins lacks histidine, methionine, tryptophan, and at least one additional amino acid, which is characteristic of a particular ferredoxin. The possible significance of these differences in either the conformation or function of these proteins has not been established, although minor differences in enzymic activity do exist (Lovenberg, Buchanan, and Rabinowitz (65)). 

The synthetic [2Fe-2S] model complex of the 20-peptide complex exhibits two LMCT absorption maxima at 423 and 461 nm in DMF, maxima which are near to those of the native plant-type ferredoxin (423 and 466 nm) (69). Two redox couples for — 3/—2 were observed at — 0.64 V versus SCE and at —0.96 V versus SCE in DMF. One of them is very close to the value (—0.64 V versus SCE) of native ferredoxin. The 20-peptide complex containing invariant sequences Cys-A-B-C-D-Cys-X-Y-Cys and Leu-Thr-Cys-Val possesses all essential factors for a model of the active site except for the peptide conformation. The positive-shifted redox potential of the 20-peptide complex in DMF is undoubtedly due to the interactions between the Fe2S22+ core and adjacent amino-acid residues, giving rise to NH--S hydrogen bonding. 

Figure 24.2. Nitrogen Fixation. Electrons flow from ferredoxin to the reductase (iron protein, or Fe protein) to nitrogenase (molybdenum-iron protein, or MoFe protein) to reduce nitrogen to ammonia. ATP hydrolysis within the reductase drives conformational changes necessary for the efficient transfer of electrons.

Morales, R., Chron, M., Hudry-Clegeon, G., Pelillot, Y., Norager, S., Medina, M., Frey, M. (1999). Refined X-ray structures of the oxidized, at 1.3 A, and reduced, at 1.17 A, [2Fe-2S] ferredoxin from the cyanobacterium Anabaene PCC7119 show redox-linked conformation changes. Biochemistry, 38,16764-15773. 

It is not our purpose to survey here the intriguing aspects of the molecular evolution of these proteins but rather to stress, at this stage, the underlying unity of their chemical design. This justifies discussion of the conformational properties of the bacterial and plant ferredoxins under a common approach in spite of their distinctive molecular weights and iron-sulfur contents. The evolutionary relationship should not, however, be overemphasized indeed, the compositional differences provide valuable information regarding the structural factors that determine the conformational state of iron proteins in general. 

A number of reviews have been published recently concerning biochemical and functional characteristics of the ferredoxins (276 a, b, 265, 271,280—282). In what follows we will center our attention on the conformational aspects only. 

If iron is such an important structural determinant of ferredoxin, it might be expected that apoferredoxin and the iron-sulfide protein could exhibit different conformations. Such differences are not obvious from the ORD spectra in the far ultraviolet peptide absorption region (Fig. 22) native clostridial ferredoxin has a secondary structure which upon... 

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