Plant type ferredoxins

Although the redox potential of Rieske-type clusters is approximately 400 mV lower than that of Rieske clusters, it is 300 mV more positive than the redox potential of plant-type ferredoxins (approximately -400 mV). Multiple factors have been considered to be essential for the redox potential of iron sulfur proteins ... 

During the 1960s, research on proteins containing iron—sulfur clusters was closely related to the field of photosynthesis. Whereas the first ferredoxin, a 2[4Fe-4S] protein, was obtained in 1962 from the nonphotosynthetic bacterium Clostridium pasteurianum (1), in the same year, a plant-type [2Fe-2S] ferredoxin was isolated from spinach chloroplasts (2). Despite the fact that members of this latter class of protein have been reported for eubacteria and even archaebacteria (for a review, see Ref. (3)), the name plant-type ferredoxin is often used to denote this family of iron—sulfur proteins. The two decades... 

Fig. 1. Iron-sulfur clusters basic building blocks. In most cases the iron is tetrahe-drally coordinated by sulfur from cysteinyl residues (and labile sulfur). Variability on coordination is allowed (see text). A, Rubredoxin type FeS4 (simplest cluster, no labile sulfur) B, plant-type ferredoxin [2Fe-2S] C, bacterial ferredoxin [3Fe-4S] D, bacterial ferredoxin and HiPIP [4Fe-4S] E, novel cluster [4Fe-2S, 20] ( hybrid cluster ).

Fig. 10. The structure ofZJ. gigas Eildehyde oxidoreductase (AOR) monomer, showing the Mo-MCD site, the two [2Fe-2S] centers, and the tracing of the pol3ipeptide chain. The Fe-S center most exposed is included in a protein domain whose folding is quite similar to the one found in plant-type ferredoxins (199).

Complete sequences of ca. 50 different plant-type ferredoxins(Fd) are known. The invariant sequences nearest to the 2-Fe core are confirmed to be Pro-Tyr-Ser-Cys-Arg-Ala-Gly-Ala-Cys-Ser-Thr-Cys-Ala-Gly and Leu-Thr-Cys-Val. 2Fe-2S complexes of oligopeptides with the Cys-X-Y-Cys sequence have been synthesized by ligand exchange reactions (7,23). We have examined the redox potentials of these model complexes, and the results are shown in Table I. The reversibility improved remarkably and the potential approached the value of the native proteins as the sequence more closely simulated that of the proteins. It is conjectured that hydrogen bonds from the peptide N-H s to thiolate and/or sulfide groups increase the stability of the reduced cluster. It is likely that peptide sequences like those found in the proteins favor the formation of such hydrogen bonds. 

Ferredoxins. Ferredoxins are proteins which contain two or four iron atoms bound to cysteine and inorganic sulfur atoms as shown in Fig. IB. There are two types of ferredoxins plant type ferredoxins (top) which consist of two iron and two labile sulfur atoms coordinated to four cysteine residues, and bacterial type ferredoxins (bottom) consisting of four iron and four labile sulfur atoms coordinated to four cysteine residues. 

Plant type ferredoxins. Tagawa and Arnon (16) described the isolation of a ferredoxin from spinach chloroplast. This ferredoxin is a protein of 12,000 molecular weight, and consists of 97 amino acids (17). Spinach ferredoxin has abosrbance maxima at 325, 420 and 465 nm (18). Ferredoxins of this type have been isolated from other sources of plants and algae, e.g., alfalfa (19), taro (20), Leuceana glauca (21) and Scenedesmus (22). The prot s of thes erredoxins are similar in their properties to ferredoxin from spinach. 

Putidaredoxin. Cushman et al. (36) isolated a low molecular iron-sulfur protein from camphor-grown Pseudomonas putida. This protein, putidaredoxin, is similar to the plant type ferredoxins with two irons attached to two acid-labile sulfur atoms (37). It has a molecular weight of 12,000 and shows absorption maxima at 327, 425 and 455 nm. Putidaredoxin functions as an electron transfer component of a methylene hydroxylase system involved in camphor hydroxylation by P. putida. This enzyme system consists of putidaredoxin, flavoprotein and cytochrome P.cQ (38). The electron transport from flavoprotein to cytochrome P.cq is Smilar to that of the mammalian mixed-function oxidase, but requires NADH as a primary electron donor as shown in Fig. 4. In this bacterial mixed-function oxidase system, reduced putidaredoxin donates an electron to substrate-bound cytochrome P. g, and the reduced cytochrome P. g binds to molecular oxygen. One oxygen atom is then used for substrate oxidation, and the other one is reduced to water (39, 40). 

One of the earliest recognized Fe S proteins was that associated with mitochondrial electron transport (Rieske et al., 1964). Even in the first partial in vivo characterization it was apparent that the protein had spectral properties that set it apart from the bacterial and plant-type ferredoxins which had just been discovered. Namely, the EPR spectrum had a gave near 1.91 and the high-held g value was shifted upheld. Furthermore, the protein had an Eq of approximately -t-250 mV, 600 mV more positive than the ferredoxins. Due to the instability of the protein, a more detailed analysis was not possible until the 1980s, when an analogous protein was isolated from bacterial sources (Fee etal., 1984). The ensuing... 

Fe Sj core. X-ray structure, [2Fe-2S] plant type ferredoxin, 33 52-53 Fc4S4 + core aconitase, 33 61 [Fe4S4] + couple, potential shifts, 38 10 Fe(S3)Cp, 35 14-15 Fe—S dimers, 38 441-452 density differences and spin densities, 38 443-445... 

Meanwhile, Blumberg and Peisach (145) showed that the interaction between a low-spin ferrous atom and an adjacent free radical can give rise to a g= 1.94 EPR signal. Brintzinger, Palmer, and Sands (146) proposed the first two-iron model for the active center of a plant-type ferredoxin. Their model, which consisted of two spin-coupled, low-spin ferric atoms in the oxidized protein and one low-spin ferric and one low-spin ferrous atom in the reduced protein, explained much of the chemical data on the proteins. Later, they (Brintzinger, Palmer, and Sands, (147)) presented EPR data for a compound, bis-hexamethylbenzene, Fe(I), which demonstrated all the properties fo the g= 1.94 signal observed in the ferredoxins. 

Table 3. Previously-proposed models for the active center of the plant-type ferredoxins...

Several Mossbauer spectroscopic papers have dealt with members of the plant-type ferredoxins. In these papers, the Mossbauer spectra for a particular protein were interpreted to yield information such as the oxidation state and spin state of the iron atoms in the protein, and in some cases this information was extended to validate a proposed model for the active site. However, problems with denatured protein material or incorrect interpretation of the Mossbauer data have prevented any of these models from being accepted as valid. 

Fig. 7 shows the Mossbauer spectra obtained by Dunham et al. (153) of the oxidized state of all the plant-type ferredoxins. The isomer shift and quadrupole splittings for these spectra are listed below ... 

These spectra, taken at variable temperatures and a small polarizing applied magnetic field, show a temperature-dependent transition for spinach ferredoxin. As the temperature is lowered, the effects of an internal magnetic field on the Mossbauer spectra become more distinct until they result at around 30 °K, in a spectrum which is characteristic of the low temperature data of the plant-type ferredoxins (Fig. 11). We attribute this transition in the spectra to spin-lattice relaxation effects. This conclusion is preferred over a spin-spin mechanism as the transition was identical for both the lyophilized and 10 mM aqueous solution samples. Thus, the variable temperature data for reduced spinach ferredoxin indicate that the electron-spin relaxation time is around 10-7 seconds at 50 °K. The temperature at which this transition in the Mossbauer spectra is half-complete is estimated to be the following spinach ferredoxin, 50 K parsley ferredoxin, 60 °K adrenodoxin, putidaredoxin, Clostridium. and Axotobacter iron-sulfur proteins, 100 °K. 

Fig. 11. Mossbauer spectra at low temperature and small applied magnetic field for reduced plant-type ferredoxins. Abbreviations AZI = Azotobacter Fe-S Protein I, 4.2 °K, 1.15 kG AZIl — Azotobacter Fe-S Protein II, 4.2 °K, 300 G Put. = Putid-aredoxin, 4.6 °K, 580 G Clos. = Clostridial Paramagnetic Protein, 4.7 K, 3.4 kG Ad. = Pig Adrenodoxin, lyophilized, 5.3 C>K, 580 G PPNR = Spinach Ferredoxin, lyophilized, 4.3 °K, 580 G Parsley = Parsley Ferredoxin, 5.1 °K, 580 G. Applied magnetic field is parallel to gamma-ray direction. Velocities are relative to platinum source matrix...

In the preceding section we presented the experimental evidence in support of the spin-coupled model proposed by Gibson et al. (148) and Thornley et at. (150) for the plant-type ferredoxins. However, the spin-coupled model does not provide a spatial or configurational model for the active center. Therefore we proceed to a more detailed analysis with the goal of asserting a proper chemical and structural model of the active center. The following properties of the active site of these proteins are well-substantiated experimentally. 

The active center of the oxidized plant-type ferredoxins contains two iron atoms with almost identical electronic environments at the nuclei. These irons are high-spin ferric (S = 5/2), spin-coupled to give a resultant diamagnetism for the complex. 

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