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2+
3+
+
3+
3+
3+
2Fe + O 2 → Fe (µ-O 2)Fe ; Fe (µ-O 2)Fe + 2H 2O + 2 [H] → 2FeO(OH) + 4H

Figure 4. The iron storage protein ferritin. Left: Apoferritin (the inside of the hollow sphere is
lined with carboxylates); centre: subunit structure and channels of C 2, C 3 (for iron exchange
with the surroundings) and C 4 symmetry; right: one of the subunits.

Ferritins – like many other proteins – exhibit high symmetry. High symmetry (also found with
higher organised forms of life such as viruses, bacteria and even proteins in plants and animals)
makes less reactive – as a consequence of minimised overall polarity – and thus has “a
protective function”. For some basic considerations on symmetry, also of relevance in the
context of the electronic configuration of metal ions in coordination compounds (and thus for
oxygen binding by haeme; next chapter), see the inset on page 10.

3. Oxygen transport

Dry air contains 20.96 vol.-% of O 2; 1 L of water at 20 °C can dissolve 31 ml of O 2; with
increasing temperature, the solubility decreases (23 ml at 40 °C). In the pulmonary alveoli, O 2
is taken up by haemoglobin (Hb, M = 65 kDa; Fig. 5); at saturation, 1 L of blood can dissolve
ca. 200 ml of oxygen. Simultaneously, hydrogencarbonate is converted to carbonic acid, which
in turn is catalytically degraded into CO 2 und H 2O (by the zinc enzyme carbonic anhydrase):

+
-
Hb·H + O 2 + HCO 3 ' Hb·O 2 + H 2CO 3
Desoxi-Hb Oxi-Hb

H 2CO 3 ' H 2O + CO 2

Figure 5. Left: Schematic view of haemoglobin (a tetramer, mainly α 2β 2 in adults; there is one
haeme group per subunit). Myoglobin is a monomer. Right: Affinity of haemoglobin and
myoglobin to oxygen. The O 2 partial pressure at saturation (100%) is ca. 100 Torr (ca. 0.13 bar
= 13 kPa). The graphs apply to the normal blood pH of 7.35 and temperature of 37 °C.
Decreasing the pH and increasing the temperature decreases the affinity for O 2.