L6. Biochemistry of Haemoglobin

Question 1

Why is oxygen carried in RBCs (Hb) rather than just diffused into the blood?

Answer 1

• Because oxygen diffusion into the blood is very slow
• Diffusion capacity is not high enough to meet O2 demands of the body
• O2 is highly reactive molecule prone to oxidation

Question 2

What characteristics of Hb make it a good oxygen carrier? [3]

Answer 2

1. Binds to oxygen at high oxygen concentrations (efficient uptake)
2. Gives up oxygen at low oxygen concentrations (efficient delivery)
3. Binds oxygen so it cannot oxidise cellular components

Question 3

What are the two main oxygen carriers of the body?

Answer 3

1. Hb in the blood

2. Mb in the muscles

Question 4

How do oxygen carriers bind to oxygen?

Answer 4

By Haem moieties

Question 5

Describe the coordinating bonds structure around the Fe2 molecules in Hb

Answer 5

4 bonds are the porphyrin ring structure
1 binds to F8 Histidine (proximal histidine)
1 binds to the oxygen (so in deoxyHb this is a free bond)

Question 6

What is important about the binding strength of oxygen to the Fe molecule?

Answer 6

It binds at 120 degree angle which means it is not a very strong bond and so is easily removed when required.

Question 7

What are things is the haem porphyrin ring prosthetic group involved in?

Answer 7

Electron transport (cytochrome C) and Enzymes for redox reactions

Question 8

How is the the structure of the haem affected by O2 binding?

Answer 8

DeoxyHb: the Fe is sitting a little outside the plane of the haem group (domed shape)
OxyHb: when oxygen binds to the Fe, it pulls it closer to the plane of the Haem group (planar) and this in turn affects the whole organisation (and function) of the protein: allosteric

Question 9

How is the colour of the haem group indicative of the redox state of the Fe and the ligand?

Answer 9

HbO2: bright scarlet red
DeoxyHb: Dark red
HbCO: cherry red
MetHb: irreversible oxidisation into Fe3+: aged blood is dark brown

Question 10

What kind of curve describes the binding states of Hb?

Answer 10

At high PO2, binding (O2 saturation) is peaked and at low PO2 the binding is much weaker.
Sigmoidal Graph

Question 11

Describe the myoglobin molecule

Answer 11

A single polypeptide chain with a compact monomeric globin with 8 alpha helicies

Question 12

How does O2 bind to Myoglobin?

Answer 12

There is haem pocket between the F and E alpha helices and the transient breathing or movement of the molecule allows O2 to access this haem group in the hydrophobic pocket

Question 13

Does the myoglobin molecule exhibit cooperativity?

Answer 13

No, the binding of O2 is strong at all levels of O2

Question 14

Describe the structure of the globin of haemoglobin

Answer 14

Tetramer (4 subunits); 2 alpha and 2 beta chains interacting oppositely to each other (alpha to beta)

Question 15

What is meant by cooperativity of the globin?

Answer 15

The affinity for O2 varies with O2 concentration

Question 16

How does cooperativity occur in Hb?

Answer 16

Due to a conformational change in the molecule upon each O2 binding (and Fe being pulled closer and closer to the plane of the haem).
Deoxy: TENSE STATE
- normally the alpha and beta subunits slide past each other and rotate
- binding of O2 breaks electrostatic bonds causing Hydrogen bonds between the subunits to reorganise
-2-3BPG is released
- allowing for more O2 to bind
Oxy: RELAXED STATE

Question 17

How do we describe the changing affinity for binding of O2 of the Hb molecule?

Answer 17

A homotropic allosteric interaction: the binding of the ligand (O2) at one site leads to a positive change to binding properties of another site which binds to the same ligand (homotropic)

Question 18

What is 2-3-BPG and how does it act?

Answer 18

2-3-biphophyoglycerate acts as a heterotropic allosteric effector synthesised in RBCs and at high concentrations in the tissues

• It binds in a cavity between Beta globin subunits and stabilises it in the DEOXY form (no oxygen binding)
• in high O2 environments (the lungs) 2-3-BPG is forced to dissociate and allow O2 binding

Question 19

What is the process of altitude adaptation?

Answer 19

At high altitudes, inspired O2 is lower partial pressure and so the delivery of O2 to the tissues is lower. However about 1-2 days at the altitude, there is an increase in the level of 2-3-BPG production by the RBCs meaning that more O2 is taken out of the Hb than normal (more efficient unloading)

Question 20

How is Hb involved in CO2 transport?

Answer 20

About 15% of the produced CO2 is carried bound to Hb to the lungs as carbamate.
The binding of CO2 is more efficient and stronger to deOxyHb (binds in tissues and lets go in the lungs)

Question 21

How is the majority of CO2 removed and carried from the tissues?

Answer 21

85% is converted into bicarbonate (HCO3) by carbonic anhydrase (from RBCs) and this decreases pH

Question 22

What is the Bohr effect?

Answer 22

The production of CO2 in the tissues leads to acid production and this acid adds charge and stablises the Tense state of the Hb molecules (less O2 binding meaning O2 is taken out more efficiently where it is needed more)

Question 23

What is the difference between adult Hb and foetal Hb?

Answer 23

Foetuses rely on their mothers oxygenating (pulmonary) system to receive oxygen and thus the FeHb is much more capable of binding to O2 with higher affinity: it takes the O2 from the mother.
- 2 alpha chains and 2 gamma chains (gives higher affinity)
- FHb binds to 2-3-BPG with much LESS avidity
(acts more like Mb)

Question 24

Describe the pathogenesis of Sickle Cell Anaemia

Answer 24

The amino acid in HbA for an acidic amino acid residue has a base change mutation that alters it into a hydrophobic amino acid.
The hydrophobicity means it buries into the hydrophobic pockets of deoxyHb
These insoluble pockets join and form insoluble chryrstalline pockets and precipitations if the O2 concentration drops very low