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the +III state, in the reduced form, the ferric iron coordinated to four sulphur functions turns to
the ferrous state.

NADH Ferredoxin +
Ubichinon
2.5+ 2.25+ 2H Ubiquinone Rieske-Protein
Fe Fe 4 Fe 3+ Fe 2.5+
NAD S S Fe S MeO O Me OH 2
Fe S S S N
H O O S Fe S MeO O R Fe Fe
NH 2 NH 2 S Fe OH S S N
N N S R = H
R R Me 6-10
+
Cytochrome-c oxidase
H O O 2 , 4H Cytochrom-c-Oxidase Cytochromes-b/c
Cytochrom-b/c
2
2+
Cu 2Fe 3+ /Fe 3+ Cu 2+ Fe 3+ Fe 2+
L
Cu 1.5+ 2Fe 2+ /Fe 2+ Cu +
N Fe N
N N
N
Figure 10. Reaction cascade in the mitochondrial respiratory chain (shortened).

Step 4: The reduction equivalents are now transferred to cytochrome-b (Cyt-b) and further to
cytochrome-c (Cyt-c). In these haeme-type electron transporters (for details see below), iron
switches between the ferric and ferrous state.

II
Step 5: The reduced (Fe ) form of Cyt-c is re-oxidised by cytochrome-c oxidase, an enzyme
that contains 5 redox active centres: 2 haeme type Fe II/III (Cyt-a and Cyt-a 3), a dinuclear cupper
A
B
II
centre{Cu 2/Cu 1.5 2} = Cu and a mononuclear Cu I/II = Cu centre. In addition, there are two
2+
2+
structural metal centres (Zn und Mg ).

Step 6: Cytochrome-c oxidase (Cyt-c-Ox) can take up 4 electrons from 4 Cyt-c. These
electrons are employed for the reduction of O 2 to H 2O. 8 protons are handled in this process: 4
of the protons are needed to form water; 4 additional protons are translocated across the
membrane (from the intra- to the extra-mitochondrial space); i.e. Cyt-c-Ox also works as a
proton pump. Activation and reduction of the oxygen (via peroxide) occurs between the
B
adjacent Cyt-a 3 and Cu centres For the organisation of Cyt-c-Ox see Fig. 13.

2+
3+
4Cyt-c(Fe ) + [Cyt-c-Ox] ox → 4Cyt-c(Fe ) + [Cyt-c-Ox] red
+ +
[Cyt-c-Ox] red + O 2 + 8H in → [Cyt-c-Ox] ox + 2H 2O + 4H ex

The iron-sulphur proteins

The more important (in the sense that they are more generally used) members of this
family are collated in Fig. 11. (1) Rubredoxins contain one iron centre tetrahedrally
coordinated to four cysteinates. (2) [2Fe,2S] ferredoxines, with two iron centres, constitute two
2-
edge-bridged FeS 4 tetrahedra. The bridging sulphur functions are inorganic sulphide S , the
remaining ligands are cysteinate. (3) [4Fe,4S] ferredoxins have a cubane structure. The four
trebly bridging functions are again sulphide, also termed labile sulphur because they can be