Reduction potentials iron-sulfur proteins

Note A HiPIP stands for high potential iron sulfur protein, a trivial name to indicate that the reduction potential of this type of small ET proteins is relatively high +0. < +0.5 volts. Formally,... 

Iron-sulfur proteins can be observed by EPR spectroscopy, either in their oxidized or in their reduced state. As a method of observing iron-sulfur clusters, EPR is discriminating but not particularly sensitive lack of a detectable EPR signal cannot be taken as evidence of absence. However, a positive EPR signal is good evidence for the intactness of an iron-sulfur cluster in a protein. Moreover, EPR can be used to follow reduction of the clusters and, by use of mediated electrochemical titrations, to estimate redox potentials. 

Iron-sulfur proteins belong to the class of electron-transport proteins [29]. They contain an iron sulfur cluster, e.g. [4Fe-4S], which shuttles between different oxidation states. The structure of the cluster is quite consistent among a series of these proteins, but their redox potentials vary widely. Synthetic models of iron-sulfur proteins have been designed [30] to investigate the factors that determine the reduction potential of the core and to mimic other biologically... 

FIGURE 19-5 Iron-sulfur centers. The Fe-S centers of iron-sulfur proteins may be as simple as (a), with a single Fe ion surrounded by the S atoms of four Cys residues. Other centers include both inorganic and Cys S atoms, as in (b) 2Fe-2S or (c) 4Fe-4S centers, (d) The ferredoxin of the cyanobacterium Anabaena 7120 has one 2Fe-2S center (PDB ID 1 FRD) Fe is red, inorganic S2 is yellow, and the S of Cys is orange. (Note that in these designations only the inorganic S atoms are counted. For example, in the 2Fe-2S center (b), each Fe ion is actually surrounded by four S atoms.) The exact standard reduction potential of the iron in these centers depends on the type of center and its interaction with the associated protein. 

If an enzyme binds a flavin radical much more tightly than the fully oxidized or reduced forms, reduction of the flavoprotein will take place in two one-electron steps. In such proteins the values of E° for the two steps may be widely separated. The best known examples are the small, low-potential electron-carrying proteins known as flavodoxins.266 269a These proteins, which carry electrons between pairs of other redox proteins, have a variety of functions in anaerobic and photosynthetic bacteria, cyanobacteria, and green algae. Their functions are similar to those of the ferredoxins, iron-sulfur proteins that are considered in Chapter 16. 

These are involved in a wide range of electron-transfer processes and in certain oxidation-reduction enzymes, whose function is central to such important processes as the nitrogen cycle, photosynthesis, electron transfer in mitochondria and carbon dioxide fixation. The iron-sulfur proteins display a wide range of redox potentials, from +350 mV in photosynthetic bacteria to —600 mV in chloroplasts. 

Some detailed comparisons of the protein environments around the HiPIP and Fd clusters have been made.769,770 It is noteworthy that the HiPIP cluster is more deeply buried (about 4.5 A) than is the case for the clusters in the other iron-sulfur proteins. All iron-sulfur proteins for which structural data are available, with the exception of the three-iron protein from Azotobacter vinelandii, have hydrogen bonding between the cysteine sulfur in the iron-sulfur cluster and the backbone peptide link. It appears that there is an approximate correlation between the number of NH S hydrogen bonds in the environment of a cluster and its redox potential. In HiPIP, these hydrogen bonds become more linear and shorten on reduction of the cluster. It is possible, therefore, that the oxidation states of the cluster may be controlled by the geometries of the hydrogen bonds.770... 

All iron-sulfur proteins, whether of the plant-type or the bacterial-type have three characteristics in common all contain the acid-labile sulfide in equimolar ratio to iron all show reduction potentials in the range from —240 to —420 mV (E0,pli = 7.0) and when these proteins are chemically-reduced (typically with dithionite), they display an uncommon EPR signal, known as the g = 1.94 signal. The oxidized forms of the proteins are not paramagnetic (159). 

N. A. Strombaugh, J. E. Sundquist, R. H. Burris, and W. H. Orme-Johnson, Oxidation-Reduction Properties of Several Low Potential Iron-Sulfur Proteins of Methylviologen. Biochem., 15, 2533 2641 (1976). 

HiPIP Formerly used abbreviation for high-potential iron-sulfur protein, now classed as a ferredoxin. An ELECTRON-TRANSFER PROTEIN from photosynthetic and other bacteria, containing a [4FE-4S] CLUSTER which undergoes oxidation-reduction between the [4Fe-4S]2+ and [4Fe-4S]3+ states. 

In DPP, after application of the pulse, the potential returns to a continually increasing value, which eventually is sufficient to cause electrolysis during the nonpulse part of the experiment. Therefore, DPP does not have the advantage of restricted electrolysis times seen for normal-pulse polarography (NPP). Besides its application to trace analytical work, DPP can be advantageous because of the better resolution inherent in a peak-shaped output. The reaction of iron-sulfur protein site analogues [Fe S (SR)4] , with electrophiles is studied by DPP, where closely spaced reduction waves of the reactant and product are adequately resolved". The reduction of cobaltocene in the presence of phenol studied using DPP, allows quantitative measurement of the amount of cyclopentadienylcobalt cyclopentadiene produced in the electrolysis at the dme by" ... 

B Ke, RE Hansen and H Belnert (1973) Oxidation-reduction potentials of bound iron-sulfur proteins of photosystem I, Proc Nat Acad Scl, USA 70 2941-2945... 

MOW Evans SG Reeves and R Cammack (1974) Determination of the oxidation-reduction potential of the bound iron-sulfur proteins of the primary electron acceptor complex of photosystem I in spinach chloroplasts. FEBS Lett 49 111-114... 

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