Page 12 - Microsoft Word - Lund BioinorgChem 08.doc

12

distales His
N distal His N
N N
H H
O
O
N N
N N

Fe Fe
N N
N N
proximal N N
proximales His
His N
Desoxi-Hb, T-Form N Oxi-Hb, R-Form

Figure 6. Desoxy and oxy forms of haemoglobin/myoglobin. The central ligand system,
protoporphyrin IX, is shown here without the characteristic ring substituents.

Transport, formation and degradation of hydrogencarbonate

Oxygen is finally reduced to water in the mitochondrial respiratory chain (ch. 4). The
reduction equivalents come from organic compounds (such as glucose), which are degraded to
-
-
CO 2. CO 2 is converted enzymatically to hydrogencarbonate according to CO 2 + OH → HCO 3 ,
most of which is extruded out of the erythrocytes (concomitantly, chloride is taken up) and
transported, via the blood plasma, to the pulmonary aveoli, where carbonic acid is formed
+
through the reaction with Hb·H , coupled with binding of O 2 to haemoglobin. Carbonic acid
finally is enzymatically split into CO 2 und H 2O. The enzyme catalysing the formation and
degradation of hydrogencarbonate/carbonic acid is called carbonic anhydrase (CA). CA has a
2+
2+
molecular weight of 29.7 kDa and contains Zn in its active centre. Zn is coordinated to
three histidine residues plus an aqua ligand (in its resting state) or a hydroxido ligand (in its
active state). A histidine close to the active centre participates in the proton shuttle. For the
catalytically conducted mechanism see Fig. 7.

O
O O
H H C
O C O O O C OH H O H
O
Zn Zn Zn H O Zn
2
N N N N
N N N N N N N N
(His) (His) (His) (His)
HCO 3 -
N
N
NH

N

Figure 7. Mechanism of the formation of hydrogencarbonate catalysed by carbonic anhydrase.
The reverse reaction (formation of CO 2 form carbonic acid) is also catalysed by this enzyme.