Flashcards in L6. Biochemistry of Haemoglobin Deck (24):
Why is oxygen carried in RBCs (Hb) rather than just diffused into the blood?
- 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
What characteristics of Hb make it a good oxygen carrier? 
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
What are the two main oxygen carriers of the body?
1. Hb in the blood
2. Mb in the muscles
How do oxygen carriers bind to oxygen?
By Haem moieties
Describe the coordinating bonds structure around the Fe2 molecules in Hb
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)
What is important about the binding strength of oxygen to the Fe molecule?
It binds at 120 degree angle which means it is not a very strong bond and so is easily removed when required.
What are things is the haem porphyrin ring prosthetic group involved in?
Electron transport (cytochrome C) and Enzymes for redox reactions
How is the the structure of the haem affected by O2 binding?
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
How is the colour of the haem group indicative of the redox state of the Fe and the ligand?
HbO2: bright scarlet red
DeoxyHb: Dark red
HbCO: cherry red
MetHb: irreversible oxidisation into Fe3+: aged blood is dark brown
What kind of curve describes the binding states of Hb?
At high PO2, binding (O2 saturation) is peaked and at low PO2 the binding is much weaker.
Describe the myoglobin molecule
A single polypeptide chain with a compact monomeric globin with 8 alpha helicies
How does O2 bind to Myoglobin?
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
Does the myoglobin molecule exhibit cooperativity?
No, the binding of O2 is strong at all levels of O2
Describe the structure of the globin of haemoglobin
Tetramer (4 subunits); 2 alpha and 2 beta chains interacting oppositely to each other (alpha to beta)
What is meant by cooperativity of the globin?
The affinity for O2 varies with O2 concentration
How does cooperativity occur in Hb?
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
How do we describe the changing affinity for binding of O2 of the Hb molecule?
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)
What is 2-3-BPG and how does it act?
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
What is the process of altitude adaptation?
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)
How is Hb involved in CO2 transport?
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)
How is the majority of CO2 removed and carried from the tissues?
85% is converted into bicarbonate (HCO3) by carbonic anhydrase (from RBCs) and this decreases pH
What is the Bohr effect?
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)
What is the difference between adult Hb and foetal Hb?
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)