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coordinated to three histidines, the other one to two His, leaving one of its coordination sites
vacant for the access of oxygen. As in the case of haemocyanins, oxygen is coordinated in the
sense of an oxidative addition, i.e. the ferrous centres become ferric centres, and the oxygen is
converted to peroxide. Concomitantly, the bridging hydroxide converts to a µ-oxido group by
-
protonating the peroxide to hydroperoxide, HO 2 .

(His)N O O O O N(His) (His)N O O O O N(His)
(His)N Fe 2+ O Fe 2+ + O 2 (His)N Fe 3+ O Fe 3+ N(His)
(His)N H N(His) (His)N O
H O

Figure 9. Desoxy form (left) and oxy form (right) of haemerythrin.

4. The mitochondrial respiratory chain

The overall reaction can be represented in the following way:
-
+
O 2 + {CH 2O} → HCO 3 + H + energy (commonly stored in the form of ATP)
+
+
or: O 2 + 2 (NADH + H ) → 2H 2O + 2 NAD

The free enthalpy of reaction (∆G) of this reaction amounts to -217 kJ/mol, the redox potential
to 1.14 V. The reduction equivalents are delivered by, e.g., products formed in the course of the
degradation of glucose, such as lactate:
H
- + -
H CC CO 2 + NAD H CC CO 2 + NADH + H +
3
3
OH O
L a c t a t P y r u v a t
Lactate Pyruvate

The reduction of O 2 to H 2O takes place step by step in order to prevent damage to cellular
constituents by the burst of energy liberated in a single step. The reaction cascade is termed
respiratory chain, which takes place in the mitochondria, and serves the generation of energy.
For the overall process, cf. Fig. 10. For some general remarks on oxidation and reduction, see
inset on p. 16.

Step 1: Primary acceptor for two reduction equivalents delivered by NADH is an iron-sulphur
protein belonging to the [4Fe,4S] ferredoxin family (for a systematic treatment of ferredoxins
see below). Such an iron-sulphur cluster can accept and deliver just one electron per cluster.
The charge is delocalised over the complete cluster system; the mean oxidation state of iron is
2.5 in the oxidised and 2.25 in the reduced form.

Step 2: Electron acceptor for the ferredoxin is a quinone (so-called ubiquinone, containing a
polyisoprene side-chain in position 2), a two-electron acceptor which becomes reduced to the
hydroquinone.

Step 3: Another iron-sulphur protein, the Rieske protein (or Rieske centre) then takes over.
Rieske proteins are two-centre iron proteins with one of the irons carrying two His (the other
-
2-
one is coordinated to 2 Cys and two bridging S ). In the oxidised form, both iron ions are in