Haemoglobin structure and function

Question 1

what is haemoglobin and what are haemoproteins?

Answer 1

→Haemoglobin is a globular haemoprotein, making up about 1/3 of the RBC

→haemoproteins are a group of specialised proteins that contain haem as a tightly bound prosthetic group.

Question 2

what is the structure of haemoglobin?

Answer 2

→It consists of 2 α and 2 β chains, each holding its own haem molecule.

→Haem is a complex of protoporphyrin IX and ferrous iron (Fe2+).

→ Iron is held in the centre of the haem molecule by bonds to the four nitrogens of the porphyrin ring.

Question 3

at what stages is the haemoglobin synthesized?

Answer 3

→65% of the haemoglobin in RBCs is synthesised at the erythroblast stage, and 35% at the reticulocyte stage.

Question 4

how is haemoglobin synthesis regulated?

Answer 4

→ stimulated by tissue hypoxia (when it isn’t getting an adequate supply of oxygen).

→The hypoxia causes the kidneys to increase the production of Erythropoietin (EPO), which increases RBC and Hb production.

Question 5

where does haem synthesis occur?

Answer 5

→haem synthesis occurs largely in the mitochondria.

Question 6

Describe the chain of events that leads to haem production.

Answer 6

→ IRON DELIVERY AND SUPPLY:
→Iron is delivered to the reticulocyte by transferrin.

→ SYNTHESIS OF PROTOPORPHYRINS:
→This occurs in the mitochondria of RBC precursors.
→It is mediated by EPO and Vit B6.
→This is done to create Protoporphyrin IX.

→COMBINING TO MAKE HAEM:
→Protoporphyrin IX and iron combine to make a haem molecule.
→The haem can combine with globin to form haemoglobin.

Question 7

where does globin synthesis happen?

Answer 7

Globin synthesis occurs in the polyribosomes.

Question 8

how does haemoglobin synthesis happen?

Answer 8

→globin mRNA is transcribed from the genes then translated into globin by the polyribosomes.

→The proper globin synthesis depends on genes, because the precise order of amino acids in the globin chains is critical to the structure and function of haemoglobin.

→the rates of haem and globin synthesis are carefully coordinated to ensure optimal efficiency of haemoglobin assembly

Question 9

how are the globin chains arranged? ( describe the clusters)

Answer 9

→There are eight functional globin chains, arranged in two clusters:

→B-CLUSTER (β, γ, δ and ε globin genes) on the short arm of chromosome 11

→A-CLUSTER (α and ζ globin genes) on the short arm of chromosome 16

Question 10

what are the functional haemoglobins at different times of human growth?

Answer 10

EMBRYONIC:
→Gower I (ζ2 ε2)
→Gower II (α2 ε2)
→ Portland (ζ2 γ2)

FOETAL:
→ HbF (α2 γ2)
→ HbA (α2 β2)

ADULT:
→HbA
→ HbA2 (α2 δ2)
→ HbF

Question 11

what makes up adult haemoglobin (including their percentages)?

Answer 11

→It is made up of HbA, HbA2 and HbF.
→96-98% of the haemoglobin is HbA,
→HbA2 at 1.5-3.2%
→HbF at 0.5-0.8%.

Question 12

what could happen to the synthesis of globin chains if mutations occur?

Answer 12

Mutations or deletions may lead to:
→abnormal synthesis of globin chains (eg. Sickle Cell Disease)
→reduced rate of synthesis of normal α or β-globin chains (eg. Thalassaemias)

Question 13

what are the functions of haemoglobin?

Answer 13

→ to carry oxygen from the lungs to the tissues
→ to remove CO2
→buffering action (maintains blood pH as it changes from oxyhaemoglobin to deoxyhaemoglobin)

Question 14

describe the oxygen delivery to the tissues

Answer 14

→One Hb molecule can bind to four O2 molecules

→When oxygenated, 2.3-DPG is pushed out; the β-chains move closer

→β-chains are pulled apart when O2 is unloaded

→permitting entry of 2,3-DPG resulting in lower affinity of O2.

Question 15

what does O2 binding to haemoglobin depends on?

Answer 15

→ PO2
→PCO2
→ affinity of haemoglobin for O2 Oxygen affinity is the ease with which haemoglobin binds and releases oxygen.

Question 16

what is oxygen affinity? and what does increased and decreased oxygen affinity mean?

Answer 16

→It determines the proportion of O2 released to the tissues or loaded onto the cell at a given oxygen pressure.

→Increases in oxygen affinity mean that haemoglobin has an increased affinity for O2, so it can bind more and stronger.

→Decreases in oxygen affinity causes O2 to be released.

Question 17

what is the oxygen dissociation curve and what is it determined by and what is P50?

Answer 17

→The oxyhaemoglobin dissociation curve relates oxygen saturation (SO2) and partial pressure of oxygen in the blood (PO2)

→it is determined by the “haemoglobin affinity for oxygen”

→The P50 is the oxygen tension at which haemoglobin is 50% saturated.

Question 18

what is haemoglobin affinity for oxygen?

Answer 18

→ how readily haemoglobin acquires and releases oxygen molecules into the fluid that surrounds it.

Question 19

what is the Bohr effect?

Answer 19

→ haemoglobin’s oxygen binding affinity is inversely related both to acidity and to the concentration of carbon dioxide.

→in acidic pH, the curve shifts to the right; this results in an enhanced capacity to release O2 where it is needed.

Question 20

describe what happens during the right shift of an oxygen dissociation curve

Answer 20

→increased P50
→ decreased affinity to O2
→ Hb more willing to release O2 to tissue

Question 21

what are examples of situations where the oxygen curve is right shifted?

Answer 21

acidosis and anaemia (even though we may have a lowered number of RBCs, they act more efficiently to deliver O2 to the target).

Question 22

describe what happens during the left shift of an oxygen dissociation curve

Answer 22

→decreased P50
→ increased affinity to O2 -
→Hb less willing to release O2 to tissue

Question 23

what are examples of situations where the oxygen curve is left shifted?

Answer 23

alkalosis and the presence of abnormal haemoglobins.

Question 24

what does the normal position of the curve depend on?

Answer 24

→the concentration of 2,3-DPG - H+ ion concentration (pH)
→ CO2 in RBCs
→ the structure of haemoglobin
→The standard conditions are: - TEMP: 37ºC - pH: 7.4 - BASE EXCESS (BE): 0

Question 25

what are the three mechanisms for carbon dioxide transport?

Answer 25

→dissolution in plasma
→ formation of carbonic acid
→ binding to carbaminohaemoglobin