Ferredoxin proteins

Other electron transferases include the rubredoxin and ferredoxin iron-sulfur proteins, so named because they contain iron-sulfur clusters of various sizes. Rubredoxins are found in anaerobic bacteria and contain iron ligated to four cysteine sulfurs. Ferredoxins are found in plant chloroplasts and mammalian tissue and contain spin-coupled [2Fe-2S] clusters. Further discussion of rubredoxin and ferredoxin proteins can be found in Chapters 6 and 7 of reference 15, and cytochromes will be extensively discussed in Chapter 7 of this text. 

The biological functions of chloroplast ferredoxins are to mediate electron transport in the photosynthetic reaction. These ferredoxins receive electrons from light-excited chlorophyll, and reduce NADP in the presence of ferredoxin-NADPH reductase (23). Another function of chloroplast ferredoxins is the formation oT" ATP in oxygen-evolving noncyclic photophosphorylation (24). With respect to the photoreduction of NADP, it is known that microbial ferredoxins from C. pasteurianum (16) are capable of replacing the spinach ferredoxin, indicating the functional similarities of ferredoxins from completely different sources. The functions of chloroplast ferredoxins in photosynthesis and the properties of these ferredoxin proteins have been reviewed in detail by Orme-Johnson (2), Buchanan and Arnon (3), Bishop (25), and Yocum et al. ( ). 

Protein fraction Ferredoxin fraction NAD reduction (nmol min mg protein" ) Total protein (mg) Ferredoxin protein (mg)... 

Just as the absorption of a photon of light converts P680 to P680, which is a better reductant, so too does the absorption of light convert the chlorophyll of PSI to a species that gives up an electron more easily. P700 donates an electron to a series of mobile quinones and then to a ferredoxin protein. The ferredoxin reduces NADP to NADPH. This provides reducing power for conversion of CO2 to carbohydrate. 

A wide variety of synthetic binuclear iron complexes (39) bridged by sulfide, disulfide and/or thiolate groups are known. They have proven to be good models for the two-iron ferredoxin proteins, as demonstrated by comparisons of structure and properties. Early attempts to isolate two-iron complexes by direct reaction of a monothiol with FeCl3, NaSH and NaOMe afforded only four-iron products suggesting that a particularly high stability is associated with the tetrameric... 

Figure 2 shows the stmcture of the [Fc2S2] core. All clusters of this type contain an Fe2S2 rhomb which is planar or with small nonplanar deviations from D2h symmetry. Protein clusters tend toward longer Fe-Fe distances, while most model complexes fall within a small range of Fe-Fe = 2.69 2.71 A. The trend in Fe-S distances is not as clear-cut overall, however, the synthetic analogs accurately reflect the important structural features of the active sites of native oxidized ferredoxin proteins, as shown in Table 3. 

The [Fc4S4] cluster, (5), appears in ferredoxins, in high-potential ferredoxin proteins (HP or HiPiP), and in a variety of enzymes. As shown in Table 8, the cluster itself is of mixed-valency (Fe and Fe°) for all forms except the most reduced, [FC4S4] . 

While the protein chain C surroimding the iron-sulphur clusters (Fig. 2.28) is similar in structure to ferredoxin, it needs a real ferredoxin protein docked on the stroma side to transfer the electron from FeS-B away from PS I. The subsystems C, D and E on the stroma side of PS I (Fig. 2.27) allow docking not only of the fairly small ferredoxin system (an example of which is shown in Fig. 2.30 - there are variations in structure between species such as plants and cyanobacteria), but also of the larger flavodoxin protein (Fromme et al, 2003), an example of which is shown in Fig. 2.31. 

Fig. 3 Molecular model [Fe4S4(SCH3)4] of the iron-sulfur active center of ferredoxine proteins. The carbon, iron, sulfur, and hydrogen atoms are represented by spheres of different shades of gray, from dark to bright, respectively...

Fe—S Clusters The other surprise in the N2ase structure, apart from the FeMo-co structure, is the nature of the P clusters. To understand this result, we must briefly look at iron-sulfur proteins, which have been known for many years, but the structures of the active sites having become clear only relatively recently. Structures 16.13-16.15 show some the main cluster types that had been recognized. There are also a number of triiron clusters. In each case the R groups represent the cysteine residues by which the metal is bound to the protein chain. In the cases in which there is more than one iron atom, S ions are also present and bridge the metals. The ferredoxin proteins contain Fe4S4 or Fe2S2 cores, and these have been extruded apparently intact... 

The compound 6 is a model of the Fe2Se unit found in ferredoxin proteins. Electrochemical and homogeneous chemical redox reactions establish the existence of the series Fe(III)Fe(III), Fe(III)Fe(II), and Fe(II)Fe(II), and epr and Mossbauer spectra show the middle members to be of the class II mixed-valence type. " As yet, however, the IT band has not been detected. 

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