Fe II -Cytochrome c Oxidoreductase

Bacterial oxidation of ferrous ion in Acidithiobacillus ferrooxidans occurs by the catalysis of a system consisted of several enzymes and proteins. In the oxidation of ferrous ion by A. ferrooxidans Fel (JCM 7811), electrons are first pulled out of the ion by the catalysis of Fe(II)-cytochrome c oxidoreductase. Then, electrons are transferred to ferricytochrome c-552 (native cytochrome c of the bacterium), fer-rocytochrome c-552 formed is oxidized with oxygen by the catalysis of cytochrome c oxidase (Yamanaka and Fukumori, 1995). However, the mechanism of the oxidation of ferrous ion appears to be a little different between the strains of A. ferrooxidans. Thus, in certain strains of the bacterium the oxidation of ferrous ion is catalyzed by Fe(II)-rusticyanin oxidoreductase (Blake and Shute, 1994). However, as will be pointed out below, the enzyme should be carefully checked. Moreover, in a moderately thermophilic iron-oxidizing bacterium, the oxidation of ferrous ion is reported to be catalyzed by an iron oxidase containing heme A (Takai et al., 1999, 2001). 

Fe(II)-cytochrome c oxidoreductase catalyzes the reduction of ferricytochrome c-552 with ferrous ion at pH 3.5 in vitro, but it does not catalyze the reduction of rusticyanin (a copper protein see below) with ferrous ion. However, it catalyzes the reduction of rusticyanin with ferrous ion in the presence of a catalytic amount of cytochrome c-552 (Yamanaka et al., 1991a). Although the enzyme is extremely labile in the presence of cytochrome c-552 in vitro, it is protected by rusticyanin from the 

The Fe(II)-cytochrome c oxidoreductase catalyzes the reduction of cytochrome c-552 with ferrous ion but does not catalyze the direct reduction of rusticyanin. This agrees with the finding of Hazra et al. (1992) that rusticyanin is reduced via cytochrome c. Blake and Shuts (1994) have claimed that the oxidation of ferrous ion occurs by the catalysis of Fe(II)-rusticyanin oxidoreductase. However, as their enzyme preparation probably contained cytochrome c as a contaminant, its catalysis could be the reduction of rusticyanin by Fe(II)-cytochrome c oxidoreductase mediated by cytochrome c. It has been reported that rusticyanin is not present in Leptospirillum ferrooxidans (Blake et al., 1993). This may mean that rusticyanin is not necessarily required for the oxidation of ferrous ion by the iron-oxidizing bacteria. 

Many kinds of cytochromes c occur in Acidithiobacillus ferrooxidans. One of them is water-soluble cytochrome c-552(s) (14 kDa) (hereafter s means soluble, and m means membrane bound) highly purified by Sato et al. (1989). This cytochrome seems to be the same protein as that partially purified by Vernon et al. (1960) and Ingledew (1982). Besides the cytochrome, several c-type cytochromes are obtained cytochrome c-552(m) (22.3 kDa), cytochrome c-550(m) (51 kDa) (Tamegai et al., 1994) Valkova-Valchanova and Chan, 1994) and cytochrome c-552(m) (30 kDa) (Elbehti and Lemesle-Meunier, 1996), cytochrome c4 (21.2 kDa) (Cavazza et al., 1996), and brown soluble cytochrome c-553 (12789 Da) (Cavazza and Bruschi, 1995). Furthermore, occurrence of several cytochromes c besides those mentioned above has been indicated (Yarzabal et al., 2002a). It has also been reported that cytochrome c (46 kDa) occurs in the outer membrane of the bacterium and participates in removing electrons from insoluble iron compounds such as pyrite (Yarzabal et al., 2002b). 

A cytochrome having the a peak at 579 nm is obtained from Leptospirillum ferrooxidans DSM 2706 (Hart et al., 1991). The cytochrome has 1 atom of zinc in addition to heme iron. Its molecular mass is 17.9 kDa and its Emj5 is +0.68 V (Blake et al., 1993). From L. ferrooxidans P3A, a cytochrome with the molecular mass of 12 kDa is obtained, while the 17.9 kDa cytochrome is not obtained. In any case, nothing is known about the function of the L. ferrooxidans cytochromes. Moreover, Metallosphaera sedula and Acidianus brierleyi have a membrane-bound yellow cytochrome which is reduced by ferrous ion (Blake and McGinness, 1993 Blake et al., 1993). However, the description here about cytochromes c will be limited to the proteins that have been purified from A. ferrooxidans and well characterized. 

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Cytochrome c-552(s) (14kDa) of Acidithiobacillus ferrooxidans acts not only as the electron acceptor for Fe(II)-cytochrome c oxidoreductase but also as the electron donor for cytochrome c oxidase. Cytochromes c-552(m) (22.3 kDa) and c-550(m) (51 kDa) also act as the electron donor for cytochrome c oxidase although it has not yet been clarified whether they act as the electron acceptors for Fe(II)-cytochrome c oxidoreductase. The reactivity with cytochrome c oxidase of cytochrome c-550(m) is larger than that of cytochrome c-552(s), while the reactivity of cytochrome c-552(m) is much less than that of cytochrome c-552(s) (Kai et al., 1992 Yamanaka and Fukumori, 1995). However, a big difference is observed in the effect of sulfate on the reactions with the oxidase between these cytochromes c the reaction with the oxidase of cytochrome c-552(s) is much stimulated by sulfate ion, while those of cytochromes c-552(m) and c-550(m) are inhibited by the ion. Considering that Acidithiobacillus ferrooxidans requires sulfate for its growth (Lazaroff, 1963), the stimulation of the reaction with the oxidase of cytochrome c by sulfate ion seems to suggest that cytochrome c-552(s) is a real electron donor for cytochrome c oxidase, although several kinds of cytochromes c occur in the bacterium (cf. Fig. 5.2). 

Although the reduced forms of cytochromes c-552(m) (22.3 kDa) (or cytochrome c4) and c-550(m) (51 kDa) are also oxidized with molecular oxygen by the catalysis of cytochrome c oxidase, it has not yet been verified whether these cytochromes are reduced with ferrous ion by the catalysis of Fe(II)-cytochrome c oxidoreductase. On the basis of the studies of DNA that encodes the redox proteins of A. ferrooxidans, it is suggested that cytochrome c4 is the direct electron donor for cytochrome c oxidase in vivo (Appia-Ayme et al., 1999). However, as already mentioned, the reactivity with cytochrome c4 of cytochrome c oxidase is much lower than that of cytochrome c-552(s) (14 kDa) and is depressed with sulfate, while the enzymatic reaction of cytochrome c-552(s) (14 kDa) is stimulated by the salt. So cytochrome c-552(s) (14 kDa) seems to function as the real electron donor for the oxidase, if cytochrome c oxidase catalyzes the reduction of molecular oxygen at the outside of the plasma membrane as already mentioned (see also Fig. 5.2). 

However, Fe(II)-cytochrome c oxidoreductase may occur in the outer membrane, although it seems difficult to detect. Thus, it has been reported that an iron-lattice... 

The electron carriers in the respiratory assembly of the inner mitochondrial membrane are quinones, flavins, iron-sulfur complexes, heme groups of cytochromes, and copper ions. Electrons from NADH are transferred to the FMN prosthetic group of NADH-Q oxidoreductase (Complex I), the first of four complexes. This oxidoreductase also contains Fe-S centers. The electrons emerge in QH2, the reduced form of ubiquinone (Q). The citric acid cycle enzyme succinate dehydrogenase is a component of the succinate-Q reductase complex (Complex II), which donates electrons from FADH2 to Q to form QH2.This highly mobile hydrophobic carrier transfers its electrons to Q-cytochrome c oxidoreductase (Complex III), a complex that contains cytochromes h and c j and an Fe-S center. This complex reduces cytochrome c, a water-soluble peripheral membrane protein. Cytochrome c, like Q, is a mobile carrier of electrons, which it then transfers to cytochrome c oxidase (Complex IV). This complex contains cytochromes a and a 3 and three copper ions. A heme iron ion and a copper ion in this oxidase transfer electrons to O2, the ultimate acceptor, to form H2O. 

This topic has been reviewed by Ingledew (55). The major components of the respiratory chain for T. ferrooxidans are a cytochrome oxidase of the Ci type, cytochromes c, and the blue copper protein rusticyanin. Initial electron transfer from Fe(II) to a cellular component takes place at the outer surface of the plasma membrane in the periplasmic space. The rate of electron transfer from Fe(II) to rusticyanin is too slow for rusticyanin to serve as the initial electron acceptor. Several proposals have been made for the primary site of iron oxidation. Ingledew (56) has suggested that the Fe(II) is oxidized by Fe(III) boimd to the cell wall the electron then moves rapidly through the polynuclear Fe(III) complex to rusticyanin or an alternative electron acceptor. Other proposals for the initial electron acceptor include a three-iron-sulfur cluster present in a membrane-bound Fe(II) oxidoreductase (39, 88), a 63,000 molecular weight Fe(II)-oxidizing enzyme isolated from T. ferrooxidans (40), and an acid-stable cytochrome c present in crude extracts of T. ferrooxidans (14). 

Yamazaki Takehiro, Oyanagi H, Fujiwara T, Fukumori Y (1995) Nitrite reductase from the magnetotactic bacterium Magnetospirillum magnetotacticum. A novel cytochrome cdt with Fe(II) nitrite oxidoreductase activity. Eur 1 Biochem 233 665-671 Yamazaki Takeshi, Fukumori Y, Yamanaka T (1985) Cytochrome at of Nitrosomonas europaea resembles aa3-type cytochrome c oxidase in many respects. Biochim Biophys Acta 810 174-183... 

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