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Redox properties, iron center

The MoFe proteins exhibit complex redox properties. Each tetra-meric a2/32 molecule of MoFe protein contains two P clusters and two FeMoco centers and, as normally isolated in the presence of sodium dithionite, the FeMoco centers are EPR-active, exhibiting an S = spin state with g values near 4.3 and 3.7 and 2.01 (Fig. 6). The P clusters are EPR silent and there is a wealth of evidence (39) using a variety of techniques that indicates that the iron atoms in these clusters are all reduced to the Fe state. [Pg.170]

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]. [Pg.33]

Fig. 10. Hypothetical reaction cycle for D. gigas hydrogenase, based on the EPR and redox properties of the nickel (Table II). Only the nickel center and one [4Fe-4S] cluster are shown. Step 1 enzyme, in the activated conformation and Ni(II) oxidation state, causes heterolytic cleavage of H2 to produce a Ni(II) hydride and a proton which might be associated with a ligand to the nickel or another base in the vicinity of the metal site. Step 2 intramolecular electron transfer to the iron-sulfur cluster produces a protonated Ni(I) site (giving the Ni-C signal). An alternative formulation of this species would be Ni(III) - H2. Step 3 reoxidation of the iron-sulfur cluster and release of a proton. Step 4 reoxidation of Ni and release of the other proton. Fig. 10. Hypothetical reaction cycle for D. gigas hydrogenase, based on the EPR and redox properties of the nickel (Table II). Only the nickel center and one [4Fe-4S] cluster are shown. Step 1 enzyme, in the activated conformation and Ni(II) oxidation state, causes heterolytic cleavage of H2 to produce a Ni(II) hydride and a proton which might be associated with a ligand to the nickel or another base in the vicinity of the metal site. Step 2 intramolecular electron transfer to the iron-sulfur cluster produces a protonated Ni(I) site (giving the Ni-C signal). An alternative formulation of this species would be Ni(III) - H2. Step 3 reoxidation of the iron-sulfur cluster and release of a proton. Step 4 reoxidation of Ni and release of the other proton.
The extent of CPO immobilized on the sol-gel was determined by the difference between the activity of the initial enzyme solution and that measured in cumulative washes. Based on the cumulative activity lost in six washes, a second preparation of the CPO-bound sol-gel contained 10, 24, and 55 mg of CPO/g of sol-gel for the 50-, 150-, and 200-A CPO sol-gels, respectively. In prior experiments, the total activity was measured and an estimated 80% of the bound CPO was active. The sol-gel immobilization is expected to limit the unfolding of the protein bound inside pores of the sol-gel. Thus, immobilization is expected to affect solvent stability and thermostability. Immobilization would probably not impact peroxide stability, since the mechanism of peroxide inactivation is associated with changes in the redox properties and oxidation state of the heme iron and the active center, which cannot be protected by immobilization. Experimental studies of immobilized CPO were therefore limited to temperature and solvent stability. [Pg.280]

Another important contribution of pulse radiolysis is in the evaluation of redox processes in native SODs and development of SOD mimics. SOD is an endogenous antioxidant enzyme which catalyzes the conversion of Oj radicals to H2O2. Different types of SODs are present in cells such as Mn-SOD in mitochondria and Cu, Zn-SOD in the cytosol and in extracellular surfaces. Reactions of O " radicals with the active site of native SODs from bacterial and animal sources have been examined. In one recent study involving superoxide reductase (SOR) from Desulfoarculus baarsii, the precise step responsible for the catalytic action was examined. Its active site contains an unusual mononuclear ferrous center. Since protonation processes are essential for the catalytic action, the pH dependence of the redox properties of the active site, both in the absence and in the presence of O radicals, was studied using pulse radiolysis. The results confirmed that the reaction of SOR with O2" radicals involves two reaction intermediates, an iron(III)-peroxo species and an iron(III)-hydroperoxo species. The protonation takes place in the second step, and therefore responsible for its catalytic activity. [Pg.586]

COX-1 inhibition up to 100 yM in human blood (152,153). (Cyclic voltammetry and iron chelation measurements confirmed that this (methoxyalkyl)thiazole series is free from redox and iron-complexing properties. The series differs from redox and theiV-hydroxyurea series in that it demonstrates enantioselective inhibition of 5-LO both in vitro and in vivo (154). This was the first evidence of 5-LO inhibitors forming enantiospecific interactions with the enzyme. Thus, unlike the a-methyl-ene center in the Abbott N-hydroxyurea series, the stereoselectivity of these ligands indicates a close contact of the stereogenic center and active site of the protein. [Pg.215]

Structure of the Iron Center Formation of the Iron Center and Tyrosyl Radical Spectroscopy of the Diferric Iron Center Spectroscopy of the Tyrosyl Radical Redox Properties of the Iron Center Mixed-Valent Form of the Iron Center Diferrous Form of the Iron Center Inhibitors to Iron-Containing Ribonucleotide Reductase Methane Monooxygenase A. Spectroscopy of the MMOH Cluster X-Ray Structure of MMOH... [Pg.359]

The metals are generally found either bound directly to proteins or in cofactors such as porphyrins or cobalamins, or in clusters that are in turn bound by the protein the ligands tire usually O, N, S, or C. Proteins with which transition metals and zinc are most commonly associated catalyze the intramolecular or intermolecular rearrangement of electrons. Although the redox properties of the metals are important in many of the reactions, in others the metal appears to contribute to the structure of the active state, e.g., zinc in the Cu-Zn dismutases and some of the iron in the photosynthetic reaction center. Sometimes equivalent reactions are catalyzed by proteins with different metal centers the metal binding sites and proteins have evolved separately for each type of metal center. [Pg.2]

High-potential iron proteins (HiPIPs) comprise a subset of the 4Fe-4S cluster family of metalloproteins that are characterized by a positive reduction potential, F , in the range of +50 to +450 mV. This class is differentiated from the 4Fe-4S centers in low-potential ferredoxins, which show a negative E, tjrpically varying between -100 and -650 mV. The origin of these distinct redox properties has been rationalized in terms of the three-state hypothesis of Carter (1), summarized in Scheme 1, and can be attributed to the stability of the common [Fe4S4(SR)4] state. [Pg.313]

Figure 17-6 Proposed model for redox-dependent activity of the Fe-Zn and Fe-Fe forms of calcineurin. Native calcineurin isolated from bovine brain contains a dinuclear Fe +-Zn + cluster which is catalytically active. The presence of Zn + in the M2 site precludes further oxidation of the cluster, a property which may prevent oxidative damage to this form of the enzyme. Reduction of the Fe ion by one electron produces the inactive Fe +-Zn + cluster [14]. Calcineurin can also be assembled to contain a dinuclear iron center. Three oxidation states are accessible in this form, of which only the mixed valence oxidation state is catalytically active [35]. The dependence of the activity on the redox state of the Fe-Fe center of calcineurin parallels observations made of calcineurin activity in crude brain extract [48]. Figure 17-6 Proposed model for redox-dependent activity of the Fe-Zn and Fe-Fe forms of calcineurin. Native calcineurin isolated from bovine brain contains a dinuclear Fe +-Zn + cluster which is catalytically active. The presence of Zn + in the M2 site precludes further oxidation of the cluster, a property which may prevent oxidative damage to this form of the enzyme. Reduction of the Fe ion by one electron produces the inactive Fe +-Zn + cluster [14]. Calcineurin can also be assembled to contain a dinuclear iron center. Three oxidation states are accessible in this form, of which only the mixed valence oxidation state is catalytically active [35]. The dependence of the activity on the redox state of the Fe-Fe center of calcineurin parallels observations made of calcineurin activity in crude brain extract [48].

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See also in sourсe #XX -- [ Pg.206 , Pg.209 ]




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