Haemoglobin Structure and Function

Question 1

describe the structure of Hb

Answer 1

• it is a globular protein which makes up 1/3 of the RBC
• a specialised protein that contain a ham prosthetic group
• haem is a complex of protoporphyrin IX and Fe2+
• Fe2+ held in centre of haem molecule by bonds to the 4 nitrogens of a porphyrin ring
• Fe2+ has 6 coordinations: 4 to the porphyrin, 1 to the histidine and 1 the free O2

Question 2

how is Hb synthesised?

Answer 2

• 65% synthesised in erythroblasts, 35% at the reticulocyte stage
• [Hb] in the blood is 13.5-16.5 g/dl in adult male, 12.5-15 g/dl in adult females
• synthesis stimulated by hypoxia, where the kidney increases EPO production, which leads to increase in RBC production and Hb production

Question 3

where and how does haem synthesis occur?

Answer 3

• occurs largely in the mitochondria
• haem synthesis starts with glycine and succinyl CoA (w B6 as cofactor)
• occurs in 3 stages:
  1) Fe delivery and supply: Fe delivered to reticulocyte by transferrin
  2) synthesis of protoporphyrins in the mitochondria of RBC precursors, mediated by EPO and vitamin B6
  3) protoporphyrin and Fe combine to form haem

Question 4

where and how does globin synthesis occur?

Answer 4

• globin is a protein so synthesis occurs in the RER
• proper globin synthesis depends on genes
• the precise order of aa in globin chain is critical to structure and function of Hb
• proteinsynthesis!

Question 5

various types of globin will combine w haem to form different types fo Hb.

how many functional globin chains are there and what are the 2 clusters that form?

Answer 5

-there are 78 functional globin chains and are arranged in 2 clusters

• B cluster (β, γ, δ, ε globin genes: present in the short arm of chromosome 11
• A cluster (α, ζ globin genes) are present in the short arm of chromosome 16
• β globin genes are expressed at low level early life and replaced 3-6 months after birth
• α globin genes have 2 types: 1 or 2

Question 6

when does globin synthesis begin?

Answer 6

3rd week of gestation

Question 7

what type of Hb is present in embryos?

Answer 7

• Hb Gower I: ζ2ε2
• Hb Portland: ζ2γ2
• Hb Gower II: α2ε2

Question 8

what Hb is present in foetal Hb?

Answer 8

HbF: α2γ2

Question 9

what type of Hb do adults have?

Answer 9

• HbA (96-98%): α2β2
• HbA2 (1.5-3.2%): α2δ2
• HbF (0.5-0.8%): α2γ2

Question 10

how does Hb deliver O2 to tissues?

Answer 10

• 1 Hb molecule can bind to 4 O2 molecules
• less than 0.01secs required for oxygenation
• when oxygenated, 2,3-DPG pushed out and β chains move closer
• when β chains move apart, O2 is unloaded and 2,3-DPG enters, lowering the Hb’s affinity to O2

-the amount of O2 bound to Hb and released from the tissues depends of the partial pressure of O2, partial pressure of CO2 and Hb affinity to O2

Question 11

what is O2 affinity?

Answer 11

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

as O2 affinity increases, Hb affinity for O2 also increase so they bind more strongly

Question 12

what is the Bohr effect?

Answer 12

• alterations I the blood pH shifts O2 dissociation curve
• in acidic pH, cirve shifts to right as there is an enhanced capacity to release O2
• normal position depends on [2,3-DPG], [H+], [CO2] in RBC, and structure of Hb

Question 13

what is base excess (BE)?

Answer 13

the amount go H+ required to return pH of blood to 7.35 if pCO2 weer adjusted to normal

Question 14

what is p50?

Answer 14

the partial pressure of O2 when Hb is 50% saturated

Question 15

how does Hb aid in CO2 transport?

Answer 15

• CO2 dissolves in the plasma and forms H2CO3

- H2CO3 binds to carbaminohaemoglobin

Question 16

how does Hb act as a buffer?

Answer 16

maintains the blood pH by changing from oxyhemoglobin to deoxyhaemoglobin

Question 17

what are the Hb derivatives?

Answer 17

• oxyhaemoglobin (oxyHb): Hb + O2
• deoxyhaemoglobin (deoxyHb): Hb - O2
• methaemoglobin (metHb): Hb + Fe3+
• carboxyhaemoglobin (HbCO): CO binds to Fe2+ in Hb (CO poisoning)
• carbominohaemoglobin (HbCO2): CO2 binds non covalently to globin chain (transports 20% of CO2)

Question 18

what causes the changes in structure of Hb?

Answer 18

• inherited disorders of Hb like thalassaemia or sickle cell
• globin disorders: mutations or deletion may lead to abnormal synthesis of globin chain (sick cell) or reduced rate of synthesis (thalassaemia)

Question 19

what are the 2 types of thalassaemia and elaborate on them

Answer 19

1) α-thalassaemia: clinically silent until the feral-adult switch, transfusion dependent from 3-6 months
- caused by the loss of 1, 2, 3 or 4 α chains
- loss of 1 or 2 α chains leads to mild microcytic anaemia
- loss of 3 α chains leads to moderate anaemia (HbH disease)
- loss of 4 α chains causes death in utero (hydrops fetalis)

2) β-thalassaemia
- caused by the loss of 1 β chain leads to mild microcytic anaemia
- loss of both β chains leads to thalassaemia major
- excess α chains precipitate in erythroblasts which leads to haemolysis and ineffective erythropoiesis

Question 20

what is sickle cell disease?

Answer 20

group of Hb disorders w inherited sickle β globin gene

Question 21

what is sickle cell anaemia (HbSS)?

Answer 21

• homozygous HbSS is most common
• there is a change in the codons

normal: CCT GAG GAG
sickle β globin: CCT GTG GAG

-in heterozygous conditions, patients have either: Hbs/βthal, HbSC, HbSD