4 - Haemoglobin

Question 1

Haemoglobin

Answer 1

Human haemoglobin is a tetrameric protein of Mr 64,400.
The four subunits each contain a haem group with its Fe2.
Tetrameric Hb can combine reversibly with 4 O2 molecules

Question 2

Alpha and Beta Chains

Answer 2

Normal adult Hb (HbA) has 2 identical α-chains and 2 identical β-chains.
The subunits are packed in a tetrahedral array.

Question 3

Similar to myoglobin

Answer 3

The α- and β-chains of Hb closely resemble Mb, but Mb carries charged AAs at positions which are hydrophobic in Hb and which are important in bonding the Hb subunits together

Question 4

Key Aspects of Deoxyhaemoglobin Structure

Answer 4

• Tetramer (α2β2)
• α-chains - 141 AAs
• β-chains - 146 AAs
• Hb in urea dissolves into two αβ dimers
• The dimers, called α1β1 and α2β2, are the two stable units.

Question 5

Two alpha and two beta chains

Answer 5

Little contact between the 2 α-chains

No contact between the 2 β-chains.

Question 6

Alpha – Beta Dimers

Answer 6

There are strong contacts between the chains in an α1β1 dimer.
There are strong contacts between the chains in an α2β2 dimer

Question 7

Hb complexity > Mb complexity

Answer 7

• Hb transports H+ and CO2 as well as O2.
• Hb-O2 binding is regulated by three molecules: H+ , CO2 and 2,3 – Bisphosphoglycerate (2,3-BPG)
These regulatory molecules affect O2 binding despite being bound to sites distant from the haem.
Such interactions between spatially distinct sites are called allosteric interactions

Question 8

Haemoglobin-oxygen graph

Answer 8

The sigmoidal shape means that the binding of O2 to Hb is (positively) cooperative, the binding of O2 at one haem enhances the binding of O2 to the other haems
Cooperative binding makes Hb a more efficient transporter so that it can deliver twice as much O2 as it would if the sites were independent.
If the curve shifts to the right (by allosteric regulators, p50 is larger, Hb can more readily give up its O2
p50 =26.6 mm Hg in RBCs, Mb with p50 = 2.75 mm Hg binds O2 an order of magnitude more tightly than Hb

Question 9

Concentration gradient of oxygen

Answer 9

In humans myoglobin facilitates the diffusion of oxygen.

Question 10

Pure Hb

Answer 10

Pure Hb, in the absence of significant quantities of the regulators H+ , CO2 and 2,3-BPG, binds oxygen too tightly for physiological needs.
For pure Hb, p50 = 2–5 mm Hg (more like Mb).
The regulators each weaken oxygen affinity to enable Hb to unload oxygen more easily.

Question 11

Effect of H+ : Bohr Effect

Answer 11

In exercising muscles lactic acid is formed and there is acidification of venous blood from the dissolution of CO2 and its subsequent ionisation, so [H+] rises.
At these acidic conditions the oxygen dissociation curve shifts to the right and more O2 is released.
Increased acidity enables the delivery of more oxygen without a fall of oxygen tension.

Question 12

Effect of 2,3 – Bisphosphoglycerate

Answer 12

2,3-BPG is produced in RBCs from glycolysis in concentrations of 4-5 mM.
It is generated by activity which uses oxygen and so the presence of it is a signal that oxygen is being used and so more is needed.

Question 13

2,3-BPG binding

Answer 13

It binds only to deoxyhaemoglobin with one molecule of BPG between the two β-subunits of Hb.
BPG displaces the oxygen dissociation curve to the right, weakening oxygen affinity.
BPG is physiologically important because in its absence Hb would unload little oxygen in passing through the tissue capillaries, because the pO2 there is about 26 mm Hg