30 - Haemoglobin Flashcards
Proportion of RBC dry weight is haemoglobin?
97%
Proportion of body CO2 carried by haemoglobin
~15%
How is variable sigmoid binding of haemoglobin achieved?
Subunit co-operativity, allosteric effectors
How do foetal and adult haemoglobins differ?
Foetal has alpha and gamma subunits, adult has alpha and beta subunits
Proportion of O2 needs that would be met by diffusion
~1%
Why does O2 need a carrier in the blood?
Deliver more O2 to tissues.
O2 extremely reactive, so need to stop it reacting with body tissues without control
Part of haemoglobin that binds O2
The haem prosthetic group, NOT the protein
Iron ion in haem
Iron II
Number of coordinating positions on haemoglobin Fe2+
Six
What in haemoglobin co-ordinates with Fe2+?
1)
2)
3)
1) Four co-ordination positions with N in porphyrin ring
2) Fifth bond to histidine F8 (8th histidine of helix F)
3) 6th bond unoccupied in deoxygenated Hb. Oxygen bonds here in oxygenated Hb
Angle at which O2 binds Fe2+ in Hb
120 degrees
Example of other things that Fe2+ haem prosthetic group is involved in
E- transport, EG: Cytochrome C in electron transport chain
Colour of oxygenated Hb
Scarlet red
Colour of deoxygenated Hb
Dark red, appears blue through skin
Colour of HbCO (carbon monoxide)
Cherry red
Colour of MetHb
Old Hb with Fe3+ (oxidised)
Dark brown
Tissue that has myoglobin in it
Muscle
Difference in saturation curves against partial pressure of oxygen between haemoglobin and myoglobin
Myoglobin has a hyperbolic curve.
Haemoglobin has a sigmoidal curve.
Myoglobin has the higher affinity for O2
Saturation of Hb in lungs
90%
Saturation of Hb in venous blood
64% saturated
Difference in Hb and Mb structure
Hb is tetrameric, Mb is monomeric.
Share 50% of structure.
Tertiary structure of Mb
Eight alpha helices
Where does haem bind in myoglobin?
Between helices F and E
P50
Partial pressure of oxygen that gives Hb or Mb 50% saturation with O2
Alpha and beta subunit homology in Hb
50%
Things that alter Hb affinity for O2
O2 binding, CO2 concentration, pH, 2,3-bisphosphoglycerate concentration
Why don’t two alpha subunits, or two beta subunits of Hb join together?
Have higher affinity for other subunit than for subunits of the same type
How does O2 binding affect Hb structure?
O2 binding pulls Fe2+ into same plane as the porphyrin ring. Histidine F8 is pulled, which changes conformation of entire protein.
This leads to a shift from deoxy (tense) state to osygenated (relaxed) state.
Change between tense and relaxed state of Hb 1) 2) 3) 4) 5) 6)
1) Fe2+ pulled into middle of porphyrin ring.
2) Histidine on F8 helix moved.
3) Alpha and beta subunits pull past each other, rotate.
4) Electrostatic bonds are broken
5) H-bonds between alpha and beta subunits reorganise
6) If BPG is attached to deoxy Hb, it is released
Function that O2 serves i nHb conformational change
Homotropic allosteric modulator
P50 of Hb
26 Torr
Heterotrophic allosteric effector of Hb
2,3-bisphosphoglycerate
2,3-BPG function
Product of glycolysis in RBCs particularly.
Has negative charge, enters positively-charged groove between beta subunits of Hb, stabilises deoxygenated Hb.
Increases O2 unloading from Hb
How is CO2 transported in Hb?
On amino terminal groups of deoxy Hb as carbamate
How is CO2 delivered to the lungs from Hb?
Oxygenated Hb binds CO2 less readily than deoxygenated Hb. When Hb binds O2, releases CO2 (EG: in lungs)
How is most CO2 transported in the blood?
CO2 converted to HCO3- by carbonic anhydrase, which is soluble in plasma
Bohr effect
H+ binding to Hb reduces affinity for O2.
When CO2 converted to HCO3- by carbonic anhydrase, increases number of protons binding Hb, therefore O2 is dumped in low-pH tissue.
Hb also transports ~40% of protons back to lungs and kidneys
HbF properties
Gamma subunit has higher O2 affinity, so can pull O2 out of mother’s blood.
Binds 2,3-BPG less avidly than HbA
When does beta globin production exceed gamma globin production?
Between 1 and 6 weeks postnatal
Sickle cell anaemia cause
Amino acid in position 6 of beta globulin changed from glutamate to valine.
Valine is hydrophobic, can stick to a hydrophobic pocket in deoxy Hb. This forms an insoluble crystalline structure.
Vaso-occlusion can result.
