Ferredoxin electron transfer site on cytochrome c₃. Structural hypothesis of an intramolecular electron transfer pathway within a tetra‐heme cytochrome

Abstract

To specify electron exchanges involving Desulfovibrio desulfuricans Norway tetra-heme cytochrome c3, the chemical modification of arginine 73 residue, was performed. Biochemical and biophysical studies have shown that the modified cytochrome retains its ability to both interact and act as an electron carrier with its redox partners, ferredoxin and hydrogenase. Moreover, the chemical modification effects on the cytochrome c3 1H NMR spectrum were similar to that induced by the presence of ferredoxin. This suggests that arginine 73 is localized on the cytochrome c3 ferredoxin interacting site. The identification of heme 4, the closest heme to arginine 73, as the ferredoxin interacting heme helps us to hypothesize about the role of the three other hemes in the molecule. A structural hypothesis for an intramolecular electron transfer pathway, involving hemes 4, 3 and 1, is proposed on the basis of the crystal structures of D. vulgaris Miyazaki and D. desulfuricans Norway cytochromes c3. The unique role of some structural features (alpha helix, aromatic residues) intervening between the heme groups, is proposed.

Rubredoxin as a paramagnetic relaxation-inducing probe

“…Nevertheless, this intermediate value for the K d confers a transient character to the complex, which is needed to enable a high-turnover, and a controlled flow of electrons [26,52]. Indeed, a similar K d was obtained for model electron transfer complexes involving cytochrome c 3 from other Desulfovibrio genus [42], and the one of cytochrome c 3 with ferredoxin I from Desulfomicrobium norvegicum [53].…”

“…In the present work, as well as, in other electron transfer complexes involving cytochrome c 3 [42,43,46,53], five of the eleven observed heme methyl resonances shifted upon addition of either Zn-or Fe-rubredoxin, but only two of these resonances experience line broadening due to the paramagnetic effect. These belong to heme IV, which is the one with a prominent positivelycharged patch around it (Fig.…”

NMR characterization and solution structure determination of the oxidized cytochrome c 7 from Desulfuromonas acetoxidans

“…For Cyt c 3 , a pathway for the intramolecular electron transfer from the highest potential heme to the others has been proposed (7,(38)(39)(40)(41): the electron goes from heme IV to heme III. Since the same structural features are all present in the FIG.…”

A proton‐NMR investigation of the fully reduced cytochrome c7 from Desulfuromonas acetoxidans

“…The structure described above represents the first determination for the fully reduced cytochrome c 7 . In Table 2 the results for the fully reduced cytochrome c 7 in solution are compared with those for the fully oxidized cytochrome c 7 in solution [7], and preliminary X‐ray crystallographic data on the oxidized protein [8]. The Fe–Fe distances for hemes I and III are the same in all the cases.…”

“…This loop connects heme III with heme IV. No high-resolution X-ray crystallographic data are available for c 7 cytochromes, although some preliminary data on the iron±iron distances and relative heme orientations are available in the literature [8]. When proteins involved in electron transfer are being considered, a knowledge of the structure of the protein in the two oxidation states is of paramount importance.…”