Iron-sulfur clusters oxidation states

The first signs of this spin richness are seen in the magnetism of the trinuclear iron-sulfur cluster [3Fe-4S], a distorted cube of alternating Fe and S comers from which one Fe has been removed. In the fully oxidized state, [3Fe-4S]1+ all three iron... 

Table 3.4 lists values for A Eq and for some important oxidation and spin states found in bioinorganic molecules. Data are taken from reference 24 and from Table 1 of reference 25 for hemoglobin, myoglobin, and the picket-fence porphyrin model compound, FeTpivPP(l-Melm).25 The myoglobin and hemoglobin model compounds are discussed in Section 4.8.2. Reference 26 provides the Table 3.4 data on iron sulfur clusters found in many bioinorganic species.26 The unusual iron-sulfur and iron-molybdenum-sulfur clusters found in the enzyme nitrogenase are discussed more fully below and in Chapter 6. 

The fourth state with [Fe4S4]° shown in Table 6.1 was recently described as the most reduced form possible for the Fe-protein s [Fe4S4] cluster.16 Usually, only two oxidation states for a given metal-sulfur cluster are stable. Therefore a stable [Fe4S4]° state in Fe-protein s iron-sulfur cluster (as appears likely from experimental evidence presented in reference 16) would be unique because the cluster would then have three stable oxidation states, [Fe4S4]2+/1+/0. It appears also that the all-ferrous state is only stable in the protein-bound cluster and not for model... 

Dithionite-reduced P-clusters are diamagnetic and consistent with an all-ferrous state having 5=0. Oxidation of the reduced P-cluster results in iron-sulfur clusters with the following spin identities ... 

Figure 5.6 Activities and states of the [NiFe] centre and iron-sulfur clusters in the [NiFe] hydrogenase of D. gigas. Higher oxidation states are at the top, lower at the bottom.

Figure 8.3 Outline reaction cycle of NiFe hydrogenase.The minimal hydrogenase is depicted, consisting of the [NiFe] centre in the large subunit, and the proximal [4Fe-4S] cluster (C) in the small subunit.The reaction is written in the direction of the oxidation of H2. Electrons are transferred out through the other iron-sulfur clusters to an acceptor protein (not shown).The equivalent states of the NiFe centre B, SR, R and C are indicated. Reduced centres are shaded. Electron transfers are accompanied by transfers of hydrons (not shown).

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. 

The complexity of the low temperature MCD spectra of the oxidized and reduced trinuclear cluster shows the multiplicity of the predominantly S — Fe charge transfer transitions that contribute to the absorption envelope. While MCD spectroscopy provides a method of resolving the electronic transitions, assignment cannot be attempted without detailed knowledge of the electronic structure. However, the complexity of the low temperature MCD spectra is useful in that it furnishes a discriminating method for determining the type and redox state of protein bound iron-sulfur clusters. Each well characterized type of iron-sulfur cluster, i.e. [2Fe-2S], [3Fe-4S], and [4Fe-4S], has been shown to have a characteristic low temperature MCD spectrum in each paramagnetic redox state (1)... 

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... 

Nitrite reductase and sulfite reductase are enzymes found in choroplasts and in prokaryotes that reduce nitrite to ammonia and sulfite to sulfide (Scott et al., 1978). Sulfite reductase also catalyzes reduction of nitrite at a lower rate. Both enzymes contain a siroheme prosthetic group linked to an iron-sulfur cluster. In siroheme, the porphyrinoid moiety is present in the more reduced chlorin form. Because NO lies between nitrite and ammonia in oxidation state, it is a potential intermediate. 

Many of the Ni hydrogenases contain an iron-sulfur cluster presumed to be of the [3Fe-xS] type, which is paramagnetic with S = 1/2 in the oxidized state and S = 2 in the reduced state (68, 69). Thi function of these clusters is unknown. In some hydrogenases, typifie< by C. vinosum hydrogenase and the membrane-bound hydrogenase o Alcaligenes eutrophus (70), the EPR spectra of the iron-sulfur cluster and the oxidized nickel center show complex lineshapes (Fig. 8). In C vinosum the nickel is also EPR detectable and its spectrum also shows ... 

In D. gigas hydrogenase, splittings are not observed in the Ni-A and Ni-B signals from oxidized nickel centers (Fig. 4a), but are seen in the reduced Ni-C species at low temperatures (Fig. 5b) (41, 72). The splitting of Ni-C correlates with the reduced state of a [4Fe-4S] cluster (72). The spin-spin interactions observed in EPR are consistent with a distance between the nickel and iron-sulfur cluster of less than 1.2 nm (73). 

It is difficult to distinguish these possibilities on the basis of the EPR spectra since the spectra of low-spin d1 Ni(III) and d9 Ni(I) are similar. A disadvantage of Scheme (1) is that it requires the nickel to take up oxidation states from Ni(III) to Ni(O) over a potential range of about 240 mV, whereas in inorganic complexes they span several volts (80). In Scheme (2) (41), the nickel is coupled to an iron-sulfur cluster. On... 

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.

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... 

A more specific operational definition excludes the unique clusters of the nitrogenase enzymes (which are treated in Chapter 7) from our considerations biological iron-sulfur clusters in this context only deal with Fe and do not contain any other metal. Under physiological conditions they can occur in two, and not more than two, oxidation states. All clusters are either dinuclear or one of the... 

The discovery of the different dinuclear or cuboidal-type biological iron-sulfur clusters is associated with their natural occurrence in two oxidation states. They can all function as one-electron transferring agents. This redox function has been well established in many studies over a period of almost five decades [1-5], However, electron transfer is generally not considered to be a catalytic activity. It is typically a stoichiometric transfer between two complex redox proteins. Mechanistically, it is probably best described as outer sphere or not involving the breaking and making of covalent bonds other than those related to hydrons. 

Aconitase was the first protein to be identified as containing a catalytic iron-sulfur cluster [24-26]. It was also readily established that the redox properties of the [4Fe-4S](2+ 1+) cluster do not play a role of significance in biological functioning the 1 + oxidation state has some 30% of the activity of the 2+ state [25], Since then several other enzymes have been identified or proposed to be nonredox iron-sulfur catalysts. They are listed in Table 2. It appears that all are involved in stereospecific hydration reactions. However, these proteins are considerably less well characterized than aconitase. In particular, no crystal structural information is available yet. Therefore, later we summarize structural and mechanistic information on aconitase, noting that many of the basic principles are expected to be relevant to the other enzymes of Table 2. 

Thus, the hybrid cluster is a putative iron-sulfur redox catalyst. It is, however, a very uncommon cluster (perhaps only comparable to the nitrogenase active site) in two aspects (1) it is a hybrid cluster i.e., it contains intrinsic building blocks that are distinctly strange to iron-sulfur clusters and (2) it can exist in more than two (in fact, four [63]) oxidation states. 

Figure 3 Speculative model for the hydrogenase enzyme cycle such as that from D. gigas. The highest oxidation states of the enzyme are at the top, and each step down corresponds to a one-electron reduction. Some hydrons that are transferred to sites in the protein are not shown. Redox states of the iron-sulfur clusters are omitted.

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