Haemoglobin variants:
1)
Methaemoglobin (MetHb)
- The
iron molecule in the heme can be in the ferrous (+2) or the ferric (+3)
oxidation state. The corresponding forms of haemoglobin are called ferrohaemoglobin and ferrihaemoglobin , respectively. Only
ferrohaemoglobin can bind oxygen, Ferrihaemoglobin is also called methemoglobin..
- MetHb
cannot carry any oxygen.
- Normally
present in less than 1% in blood . . . it is maintained at this low level
by the NADH-MetHb reductase system present in normal rbc.
- Causes:
- genetic
defects in Hb synthesis
- genetic
defects in NADH-Methemoglobin reductase enzyme system
- drugs/toxins:
nitrates, nitrites, nitrous gases, GTN, sulphonamides
2)
Caboxyhaemoglobin (CO-Hb)
CO + Hb ————>
CO-Hb (CarboxyHb)
- Each
heme in Hb can bind one carbon monoxide molecule, but O2 & CO cannot
bind simultaneously to the same heme.
- The
binding affinity for CO is approx 200x as great as that for oxygen.
- Therefore,
even at minuscule partial pressures, CO is able to displace oxygen.
- Develops
into hypoxemic hypoxia.
3) Foetal
haemoglobin (Hb F)
- Normal
Hb of fetus, replaced by adult Hb A in the first days after birth.
- Has
an increased capacity to carry oxygen thereby optimizing the transfer of
oxygen from the maternal (Hb A) to the fetal circulation (Hb F).
|
|
|
Type |
Subunit
structure |
Description |
|
Hb A |
a2b2 |
principle Hb in adults |
|
Hb A2 |
a2d2
(delta) |
2% in adults |
|
Hb F |
a2g2
(gamma) |
|
4) Carboaminohemoglobin
(HbCO2)
20-30% of transported CO2 is carried
within rbc as carboaminoglobin
CO2 + Hb 
HbCO2 carboaminohemoglobin
K. C. Potger
Copyright © 2001