Haemoglobin
From Ganfyd
Haemoglobin is a pigmented protein integral to gaseous exchange and oxygen delivery to the tissues. When oxygenated, haemoglobin is coloured red, and is responsible for imparting colour to the red blood corpuscles that contain the haemoglobin.
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Biochemistry
It has two alpha chains and two beta chains surrounding an iron-containing haem group. Each haemoglobin molecule can carry up to four oxygen molecules.
The affinity of oxygen to haemoglobin is not linear. Instead the uptake and release of oxygen is governed by a sigmoid oxygen dissociation curve, which itself can be influenced by several local physiological variables. This curve is explained by the allosteric properties of haemoglobin: binding of the first oxygen molecule to deoxyhaemoglobin subtly alters the structure of the protein which increases its affinity for oxygen, facilitating binding of the further oxygen molecules. As the oxygenated haemoglobin reaches an area with low oxygen concentration, the process is reversed.
Subtypes
Several varieties of the chains exist, determined by genes which probably first became duplicated, and then differentiated through mutation and have been conserved through natural selection.
The foetal variant of haemoglobin (HbF) has an oxygen dissociation curve more optimal for the deep compartment the foetus occupies than would be the adult forms - A1 and A2.
There are a large number of minor variants in adult Hb, most of which work perfectly well, and some clinically important ones which only work moderately well, of which the major one is HbS - the variety which causes erythrocytes to sickle in low oxygen tension, thus sickle cell diseas.
HbS is presumed to be conserved in certain populations as it gives a degree of resistance against malaria to those heterozygous for it.
Other diseases
Thalassaemia are disorders of haemoglobin caused by dispropotionate production of one or other of the chains.
Measurement
By a spectrophotometric method.
- Full blood count
- Haemoglobin becomes glycosylated (HbA1c), more so in diabetes mellitus, and usefully depending upon the degree of control over time.
