Haemoglobin

Question 1

Haemoglobin structure

Answer 1

• 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

Question 2

Globin

Answer 2

• 4 polypeptide chains
• 2α, 2β
• 140 AAs

Question 3

Haem

Answer 3

• non-protein prosthetic group

* an Fe2+ ion enclosed in a ring structure

Question 4

Haemoglobin binding with oxygen

Answer 4

• 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)

Question 5

Describe oxygen dissociation curves

Answer 5

• 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)

Question 6

partial pressure of oxygen

Answer 6

• p(O2)
• measure of concentration
• relatively to other gases in the air
• how much does the gas in question contribute to atmospheric pressure?
• kPa

Question 7

Where is p(O2) low?

Answer 7

• capillaries of respiring tissues

* oxygen is readily unloaded from Hb

Question 8

Where is p(O2) high?

Answer 8

• alveolar capillaries

* oxygen readily combines with Hb in the lungs

Question 9

Explain oxygen dissociation curves

Answer 9

• 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

Question 10

Co-operative binding

Answer 10

• 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

Question 11

Unloading

Answer 11

Occurs in capillaries

Question 12

The more H+ produced…

Answer 12

… the more O released

This is because Hb has a stronger affinity for H+ than it does for O

Question 13

H+ production

Answer 13

Caused by a higher concentration of CO2 (from cellular respiration)

Question 14

When does CO2 diffusé out of the alveoli?

Answer 14

When there is less CO2 in the lungs, because reactions are reversible

Question 15

Carbon dioxide…

Answer 15

… reduces the affinity of Hb for oxygen (because of the low pH of H+ ions)

Question 16

The process of the Bohr shift

Answer 16

• 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

Question 17

H2CO3

Answer 17

Carbonic acid

Question 18

Chloride shift

Answer 18

Cl- diffuse inwards from the blood to maintain electrical neutrality

Question 19

HHb

Answer 19

Haemoglobinic acid

Question 20

Foetal Hb graph description

Answer 20

• 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!)

Question 21

Foetal Hb graph explanation

Answer 21

• 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

Question 22

HPFH

Answer 22

• hereditary persistence of fetal Hb

* does not usually cause symptoms, as there is enough Hb to limit oxygen uptake

Question 23

Lugworm Hb

Answer 23

• need a high affinity (graph moves left)

* so saturation can be higher at lower p(O2)

Question 24

Myoglobin

Answer 24

• oxygen store
• only has one haem group
• graph is not sigmoid as only one oxygen molecule can be gained
• higher affinity (graph moves left)