Electron Transfer System Coupled to Oxidation of Ferrous Ion

In the oxidation of ferrous ion by A. ferrooxidans Fel (JCM 7811), first, Fe(II)-cytochrome c oxidoreductase catalyzes the reduction of ferricytochrome c-552(s) with ferrous ion, while it catalyzes the reduction of rusticyanin with ferrous ion only in the presence of a catalytic amount of cytochrome c-552(s) (14 kDa) (Fukumori et al., 1988b Yamanaka and Fukumori, 1995). Although Fe(II)-cytochrome c 

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). 

The redox proteins participating in the oxidation of ferrous ion other than cytochrome c oxidase are thought to be present in the periplasm, and they are fairly strongly bound to the outside of the plasma membrane. Thus, the spheroplasts of the bacterium retain 80% of the ferrous-oxidizing activity which the intact cells 

Yarzabal et al. (2002b) hypothesized that as A. ferrooxidans oxidizes pyrite, which is insoluble, the initial electron acceptor for electron from ferrous ion in its respiratory pathway should be located in the outer membrane, and that cytochrome c (46 kDa) occurring in the outer membrane is a candidate for the initial electron acceptor. They proposed a pathway of electron transfer from ferrous ion to oxygen as follows  

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 

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