haemoglobin structure and function

Question 1

what are the three main haemoglobin types and what are their subunits

Answer 1

-HBA is made of two alpha and 2 beta global subunits
-HBF is made of two alpha and two gamma
- HBA2 is made of two alpha and 2 delta (<3.5% in adult blood)

Question 2

what is the globin structure

Answer 2

• primary structure (9 conserved amino acids for oxygen binding)
• secondary (75% alpha helix)
• tertiary (non polar resides face inwards)
• quaternary ( 1 dimer (alpha 1 and beta 1) on top of second dimer)

Question 3

how man amino acids is in the alpha globin subunit

Answer 3

• 141 amino acids

Question 4

how many amino acids is in the beta, delta and gamma globin structure

Answer 4

• 146 aa

Question 5

what is the Haem structure

Answer 5

• protoporphyrin ring with iron atom at centre
• iron binds to 6 coordinating valencies with 4 linked to nitrogen which is found in the ring and 2 linked to histidine residues in globin
• bond between distal histidine and oxygen is unstable (hydrogen bonding) and easily replaced by oxygen

Question 6

haemolytic synthesis overview and steps

Answer 6

• 1 rate controlling step- involves ALA synthase
• 3 enzyme controlled steps
• first step condensation of glycine and succinate which forms d-ALA
• intermediate step forms three intermediates
• final step- insertion of iron into proton-porphyrin

Question 7

what is the genetic control of globin snythesis

Answer 7

• two chromosomes which have genes coding for specific haemoglobin subunits
• chromsome 16 and 11
• chromsome 16 has alpha 1, alpha 2 and zeta
• chromsome 11 has beta, gamma, delta, epsilson, pseudogenes (non functional)

Question 8

how many exons and introns does the globin genes have, and what do the introns begin and end with

Answer 8

• 3 exons and 2 introns
• introns begin with G-T and end with A-G

Question 9

what is the binding of oxygen to haemoglobin like

Answer 9

• positive co-orperativity
• binding of first oxygen makes it easier for subsequent oxygen to bind

Question 10

what reduces affinity for oxygen

Answer 10

• low ph
• high temperatures
• carbon dioxide (Bohr effect)
• 2,3-DPG

Question 11

what happens to hb structure when oxygen is bound

Answer 11

• when deoxygenated it is open and taut (T)
• bound it is compact and relaxed (R)
• rotation of dimer Paris over each other 15 degrees
• movement of histidine residue

Question 12

how does DPG reduce oxygen affinity

Answer 12

• hb exist in two forms T and R
• during the T phase hb is open and taut and has a lower affinity for oxygen, when oxygen bind to hb it moves from the t state to the r state however the binding of dog during the t state stabilises the t state and prevents the transition to the r state and therefore oxygen affinity is reduced and makes hb more prone to release oxygen

Question 13

what is the effect of DPQ on oxygen dissociation curve

Answer 13

causes it to shift to the right
more DPG formed with anaemia and hypoxia

Question 14

what is the p50

Answer 14

the partial pressure of oxygen when haemoglobin is 50% saturated
increases in dpg causes increase in p50

Question 15

what are the two main categories of haemoglobin disorders

Answer 15

• structural globin defects e.g sickle cell
• decreased globin synthesis e.g. thalassaemias

Question 16

what is the distribution of thalassemia

Answer 16

• mediteranian e.g. greeks, Cypriots etc
• middle east
• south asia
• south east asia
• common in places were malaria was historically present

Question 17

distribution of sickle cells

Answer 17

• African and Caribbean
• specifically west Africa

Question 18

what is the pathophysiology of thalassemia

Answer 18

1. unpaired globin
2. haemolsis or ineffective erythropoiesis
3. haemolytic goes to splenomegaly than to anaemia
4. ineffective erythropoiesis goes to anaemia
5. anaemia goes to either transfusionn which leads to iron overload or erythropoietin
6. erythropoietin goes to erythroid marrow expansion
7. bone marrow expansion goes to skeletal deformities or iron overload

Question 19

what is the classification of alpha thalassemia

Answer 19

• alpha thalassemia is based on deleted genes not really mutations
• so if you are alpha thalassemia zero you have deletion of both alpha genes on same chromosome
• alpha thalassemia plus you have deletion of one of the alpha globin genes in one chromosome
• not there’s two alpha globin genes on one chromosome (16)

Question 20

what is the 4 main types of alpha thalassemia

Answer 20

• alpha thalassemia minor or ‘trait’= deletion of one alpha globin gene (positive) in either one homologous chromosome or both homologous chromosome
• alpha thalassemia minor zero- both alpha globin genes deleted in one homologous chromosome
• HbH disease- 3 alpha globin genes deleted
• hydrops foetalis or alpha Thal major - 4 genes deleted no alpha globin chains produced at all

Question 21

what are the clinical features of alpha thalassemia trait positive or zero

Answer 21

• microcytic red blood cells

Question 22

what are the clinical features of Hbh

Answer 22

• splenomegaly
• anaemia

Question 23

what are the clinical signs of alpha Thal major or also known as hydros foetalis

Answer 23

• death in utero (30-34 weeks)
• formation of hb bart (4 gamma chain)
• hb bart cause oedema and hepatospelnomegaly

Question 24

what are the 3 types of beta thalassemia

Answer 24

• minor beta thalassemia or trait (heterozygous)
• beta thalassemia intermedia (homozygous)
• beta thalassemia major (homozygote)

Question 25

what are the classification of beta thalassemia based off

Answer 25

- based on mutations not gene deletions - more severely mutated genes means no beta globin is synthesised - more minor mutated genes results in less beta globin chains being produced - so if you are beta thalassemia positive the mutations are less severe and have reduced beta globin synthesis - if zero means severe mutations and no globin chain synthesis

Question 26

what are the clinical sign of beta thalassemia minor or trait

Answer 26

- microcytic not anaemia - raised hba2 - genetic counselling

Question 27

what is beta thalassemia minor or trait

Answer 27

- one defected/mutated gene can be positive or zero

Question 28

what is beta thalassemia intermedia

Answer 28

- 2 defected genes but genes are positive so not severe mutation and therefore reduced beta goblin chain synthesis - mutation can be also one severe and one mild so one positive and one negative

Question 29

what is beta thalassemia major

Answer 29

- 2 mutated genes - severely mutated - both zero

Question 30

clinical signs of beta Thal major

Answer 30

- severe anaemia dependant on transfusion - present 1-2 months after birth - bone marrow expansion - cardiac death room iron overload

Question 31

what organ system are affected by iron overload and how

Answer 31

- heart, pituitary gland, liver, pancreas, gonadal - iron over load means non transferring bound iron and therefore increased iron uptake from selective organs and thus generation of free hydroxyl radicals which cause tissue damage

Question 32

Thal major standard therapy

Answer 32

- transfusion - chelation -monitoring

Question 33

examples of chelate

Answer 33

- desferrioxamine - deferasirox - deferiprone

Question 34

other therapies for Thal major

Answer 34

- bone marrow transplantation - gene therapy - screening

Question 35

what type of hb disorder is ss

Answer 35

- structural hb disorder

Question 36

what causes ss/ pathophysiology

Answer 36

- point mutation from glutamic acid to valine - when deoxygenated the hb molecules undergo polymerisation to form crystals - this causes the red blood cell to be sickle cell shaped

Question 37

what is the inheritance of ss

Answer 37

- co dominance - if heterozygous would have 50% hbs and 50% hba - if homozygous with mutation you would have no hba

Question 38

what can sickle cell red blood cells cause

Answer 38

- haemolysis they are more prone to breaking leading to haemolytic anaemia - vaso-occclusion ( vaso-occlusive episodes)= acute crisis and chronic vasodilation-occlusion can cause organ damage - vaso occlusion can lead to sequestration e.g. of spleen or chest

Question 39

what would you find on peripheral blood film of sickle cell

Answer 39

- sickle cells - hypo chromatic cells - neutrophils - howell jolly body cells

Question 40

clinical features of sickle cell

Answer 40

- haemolytic anaemia - sequestration crises - infracrtion of organs such as spleen- risk of infection - acute crises - stroke in 10% o children - acute chest syndrome

Question 41

what is sickle chest syndrome or acute chest syndrome and causes

Answer 41

- complication of sickle cell causes: - ischaemia to lung/ reduced blood flow - infection e.g. streptococcus pneumonia - fat embolism- necrosis of spleen leads to fat deposited in blood which travel to lung

Question 42

treatment of scd

Answer 42

- preventative measures and treat acute complications -prevent organ damage - anti sickling options e.g. transfusion or increase hbf - curative such as stem cell transplantation or gene therapy