Ferredoxin stability

In the Rieske proteins from bci or b f complexes, loops (34-/35 and (36-/37 both contain an additional cysteine residue (Cys 144 and Cys 160 in the ISF and Cys 112 and Cys 127 in RFS) these cysteines form a disulfide bridge connecting the two loops (Fig. 3b). These cysteines are not present in the sequences of Rieske-type proteins, that is, in neither NDO nor Rieske-type ferredoxins. In Rieske proteins, the disulfide bridge appears to be important for the stabilization of the fold around the cluster as the two loops are not shielded by other parts of the protein in NDO, the Rieske cluster is stabilized without a disulfide bridge since it is completely buried by surrounding a and (3 subunits. 

Uhlmann, H., Iametti, S., Vecchio, G., Bonomi, R, and Bernhardt, R. 1997. Prol08 is important for folding and stabilization of adrenal ferredoxin, but does not influence the functional properties of the protein. European Journal of Biochemistry 248 897-902. 

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. 

Perutz MF, Raidt H. Stereochemical basis of heat stability in bacterial ferredoxins and in haemoglobin A2. Nature 1975 255 256-259. 

The molar masses of the 2-oxoacid ferredoxin oxidoreductases are 200,000-300,000 g/mol and they are composed of four subunits of the kind a2p2. It has been shown that halobacteria have only these systems of 2-oxoacid ferredoxin oxidoreductases. The two enzymes of H. halobium (pyruvate and oxoglutarate) were isolated and characterized by Kerscher and Oesterhelt (1981a). These systems proved to be thiamin diphosphate-containing iron-sulfur proteins. The relative stability of the halobacterial enzymes enabled detailed analysis of the various steps of the catalytic cycles (Kerscher and Oesterhelt, 1981b), demonstrating two distinct steps of one-electron transfer reactions. 

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... 

Table 3 summarizes some of the properties of PS I reaction center and specific functions of its individual subunits. The purified preparation contains about 100 Chi a molecules per P-700 [9,10]. However, this number can be decreased to about 40 while the order of the Chi a molecules increases [70]. Washing out more of the Chi molecules caused a decrease in the dichroic ratio, indicating that those 40 Chls are the highly oriented primary light-harvesting antenna of the reaction center. It was also shown that the S-carotene is in very close proximity to P-700 and it is highly oriented with respect to the latter [70]. P-700 as well as the primary electron acceptor (Aj) may be composed of specialized Chi a molecules [80]. There are at least three more electron acceptors which are part of the reaction center and their function is to slow down the rate of the reaction and thereby stabilize the redox potential difference [72]. It was not until Malkin and Bearden [81] discovered the bound ferredoxins that this part of PS I started to be understood. Today it appears that at least four different clusters are involved in the electron-accepting site of the... 

As 2 electrons move from each water molecule to NADP1 (blue arrows), about two H+ are pumped from the stroma into the thylakoid lumen. Two additional H+ are generated within the lumen by the oxygen-evolving complex. The flow of protons through the proton pore in CF0 drives the synthesis of ATP in CF (MSP = manganese stabilizing protein ph = pheophytin Fd = ferredoxin FNR = ferredoxin-NADP oxidoreductase)... 

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