Haemoglobin Flashcards
Haemoglobin structure
- globular conjugated protein
- globin (protein)
- haem
- held together by hydrogen bonding
- outer hydrophilic R-groups/side chains for maintaining 3D shape
- primary structure important in forming shape and binding to haem
Globin
- 4 polypeptide chains
- 2α, 2β
- 140 AAs
Haem
- non-protein prosthetic group
* an Fe2+ ion enclosed in a ring structure
Haemoglobin binding with oxygen
- each haem group combines with one molecule of oxygen (oxygenation)
- iron in haem remains as Fe2+ when oxygen is bound
- globin cooperates in binding oxygen; allostéric effect
- binding is reversible (based on partial pressure)
Describe oxygen dissociation curves
- degree of Hb oxygenation is determined by p(O2) of the immediate surroundings
- if p(O2) is low, Hb carries a relatively small amount of oxygen)
- if p(O2) is high, Hb becomes saturated with O2
- show the degree of Hb saturation with O2 plotted against different values of p(O2)
- the curve is sigmoid - non-linear
- the range of p(O2)s in the body is very narrow, and yet the curve is steep: % saturation of Hb in this range varies greatly for small changes in p(O2)
partial pressure of oxygen
- p(O2)
- measure of concentration
- relatively to other gases in the air
- how much does the gas in question contribute to atmospheric pressure?
- kPa
Where is p(O2) low?
- capillaries of respiring tissues
* oxygen is readily unloaded from Hb
Where is p(O2) high?
- alveolar capillaries
* oxygen readily combines with Hb in the lungs
Explain oxygen dissociation curves
- at p(O2) close to zero there is no oxygen bound to the Hb
- at low p(O2) the polypeptide chains are tightly bound together, making it difficult for an oxygen to gain access to the iron ions; curve rises gently
- co-operative binding
- at very high p(O2), the Hb becomes saturated and the curve levels off
Co-operative binding
- as one oxygen molecule binds to a haem group, the polypeptide chain opens up
- exposes the other 3 haem groups
- makes oxygenation easier
- curve rises more steeply
Unloading
Occurs in capillaries
The more H+ produced…
… the more O released
This is because Hb has a stronger affinity for H+ than it does for O
H+ production
Caused by a higher concentration of CO2 (from cellular respiration)
When does CO2 diffusé out of the alveoli?
When there is less CO2 in the lungs, because reactions are reversible
Carbon dioxide…
… reduces the affinity of Hb for oxygen (because of the low pH of H+ ions)
The process of the Bohr shift
- increased cellular respiration causes greater p(CO2)
- CO2 diffusés into RBCs where it is converted to H2CO3; catalysed by carbonic anhydrase
- the H2CO3 dissociates, forming H+ and HCO3-
- HCO3- diffuse out of the cell and are transported in solution in the plasma
- causes chloride shift
- H+ combines with Hb to form HHb, forcing Hb to release its oxygen load
- by taking up excess H+, Hb acts as a buffer - prevents blood from becoming too acidic
H2CO3
Carbonic acid
Chloride shift
Cl- diffuse inwards from the blood to maintain electrical neutrality
HHb
Haemoglobinic acid
Foetal Hb graph description
- foetal Hb affinity needs to be high (graph further left)
- mother’s Hb affinity needs to be low (graph further right)
- can’t stay this way, because a foetus may eventually become a mother
- on birth, the gene changes (epigenetics!)
Foetal Hb graph explanation
- the foetus has a higher affinity; loads more oxygen at the same p(O2) ; oxygen moves from mother to foetus
- after birth Hb oxygen affinity decreases, causing easier oxygen dissociation to respiring cells in need of oxygen for growth
HPFH
- hereditary persistence of fetal Hb
* does not usually cause symptoms, as there is enough Hb to limit oxygen uptake
Lugworm Hb
- need a high affinity (graph moves left)
* so saturation can be higher at lower p(O2)
Myoglobin
- oxygen store
- only has one haem group
- graph is not sigmoid as only one oxygen molecule can be gained
- higher affinity (graph moves left)
