Haemoglobin Structure and Synthesis Flashcards
1
Q
Main function of red cells
A
- carry O2 to tissues and return CO2 from tissues to lungs
2
Q
What does each molecule of Hb A consist of?
A
- 4 polypeptide chains, alpha2 and beta2, each with its own haem group
3
Q
What other 2 Hb’s does normal human blood contain?
A
- Hb F and Hb A2
4
Q
What do these other 2 Hb’s contain?
A
- alpha chains with gamma and delta chains
5
Q
Where does Haem synthesis occur?
A
- mitochondria
6
Q
How does Haem synthesis commence?
A
- condensation of glycine and succinyl coenzyme A using key rate-limiting enzyme delta-ALA synthase
7
Q
What is the coenzyme for this reaction?
A
- pyridoxal phosphate (vitamin B6)
8
Q
What does protoporphyrin combine with to form haem?
A
- iron in the ferrous state
9
Q
Haemoglobin molecular structure
A
- tetramer of 4 globin chains each w/ own haem group
10
Q
What percentage of RBC volume and dry weight does Hb occupy?
A
- 33% RBC volume
- 90% dry weight
11
Q
What 2 major components does a Hb molecule contain?
A
- globin chain
- prosthetic group Haem, comprising a tetrapyrolle ring structure w/ Fe(II) at its centre
12
Q
Step 1 of Haem synthesis (ALA formation)
A
- succinyl CoA + glycine –> alpha amino beta ketoadipic acid
- CO2 is removed to form ALA
13
Q
Step 2 of Haem synthesis (PBG/porphobilinogen formation)
A
- 2 ALAs
- ALA dehydratase is catalyst
- H20 removed to form PBG
14
Q
Step 3 of Haem synthesis (polymerisation of PBG)
A
- 2 enzymes (PBG deaminase, Uroporphyrinogen III Consynthetase) work in unison
- 4 PBG form tetrapyrolle ring structure (Urogen III)
15
Q
Step 4 of Haem synthesis (decarboxylation; acetyl to methyl)
A
- acetyl side chains converted to methyl groups through CO2 loss
- 4 CO2 liberated
- cytoplasmic structure: uroporphyrinogen III decarboxylase
16
Q
Step 5 of Haem synthesis (conversion of proprionyl to vinyl)
A
- oxidative decarboxylation and dehydrogenation of 2 proprionyl groups, converts them to vinyl groups
- produces protoporphyrinogen IX
- enzyme: coproporphyrinogen oxidase
17
Q
Step 6 of Haem synthesis (oxidation, removal of 6H+ atoms)
A
- mitochondrial enzyme; protoporphyrinogen oxidase
- responsible for removal of 6 H atoms
- produces protoporphyrin IX
18
Q
Step 7 of Haem synthesis (insertion of Fe(II) into ring)
A
- insertion of ferrous, Fe2+ ion into centre of protoporphyrin IX
- inner mitochondrial enzyme; ferrochelatase or haem synthetase
19
Q
Iron absorption and circulation
A
- Fe2+ readily absorbed by DMT1 by enterocytes
- Fe transported by transferrin in plasma to BM or liver
- transferrin; 76-80kD, carries 2 atoms of Fe3+
- ferroportin transports Fe out off cells
- carried more efficiently as Fe3+
- other carriers include albumin and lactoferrin
- Fe-Tr complex can only enter developing RBC by binding Transferrin receptor (TfR)
20
Q
Hepcidin
A
- Movement of Fe into plasma by ferroportin regulated by it
- Produced by liver
- 25 amino acid peptide, HAMP gene, chromosome 19
- Controls export of Fe from; enterocytes, macrophages, Kupffer cells, hepatocytes, placental cells
- Raised hepcidin- anaemia of chronic disease
21
Q
What is the Embden-Meyerhof pathway?
A
- glycolysis which converts glucose into pyruvate
- Free energy released forms ATP
- Oxygen free
- Due to lack of mitochondria within mature erythrocytes
22
Q
What stage of red cell development does Hb synthesis occur?
A
- ~65% during the late normoblast stage
- 35% during the reticulocyte stage after loss of nucleus.
23
Q
What does build up of Haem do?
A
- inhibits own synthesis and stimulates globin chain production
24
Q
How much Hb does each normal red cell contain?
A
- 27-32pg
25
What is meant by ineffective erythropoiesis?
- red cell precursor fails to acquire sufficient Hb, so its destroyed before leaving the marrow
26
Primary structure of globin (globin genes)
- derived from 2 families of polypeptides – alpha & beta.
- Various Hb molecules produced contain 2 globins from each family.
- Synthesis of the globin chains is under control of globin genes, reside on the long arms of chromosomes 11 and 16
27
Globin genes
- between the active genes in these clusters are non-functional ‘pseudogenes’
- Two are duplicated; gamma and alpha.
- One gamma gene codes for alanine at position 136, the other coding for glycine at the same position.
- a chain gene is duplicated but both are identical and active.
28
Secondary structure of globin
- 75% of a and b globin chains are in form of a-helices
- All functional Hb molecules have this same helical content – 8 helices labelled A --> H.
- Remaining 25% of residues are in linear portions which connect the helices.
29
Tertiary structure of globin
- Haem group of each globin chain sits deep in a hydrophobic pocket or crevice between E and F helices.
- Proline residues within the linear portions of the globin give the chain necessary flexibility to take up its complex globular shape.
30
What does alteration in amino acid composition in Hb?
- may make Hb less stable and liable to precipitation.
| - Precipitated Hb within a red cell renders it useless as an oxygen carrier and causes premature red cell destruction.
31
Glucose-6-phosphate dehydrogenase
- converts glucose-6 phosphate to 6-phosphoglucono-d-lactone and maintains NADPH
- G6PD deficiency can lead to lack of NADPH to maintain reduced glutathione to combat oxidant stress imposed on the cell
- Heinz Bodies; contain denatured Hb caused by oxidant damage
- Haemolytic anaemias
32
Structural contacts in a quaternary structure in globin
- Monomers held together by hydrophobic bonds between adjacent areas of the polymer.
- The monomers interact across various planes of contact.
- contacts mainly involve the B and H helices.
- These strong interactions stabilize the molecule and prevents dissociation of the a1b1 and a2b2 dimers into free monomers
33
Functional contacts in quaternary structure in globin
- The a1b2 and a2b1 contact planes are less extensive.
- Only 9 residues from either chain are involved.
- Involves contacts between C helix and the F-G corner
34
Functional contacts in O2 binding
- allow conformational changes of the molecule when the oxygenation state of Hb is changed.
- Deoxygenated Hb has the Fe(II) sitting slightly above the plane of the haem moiety.
- The haem is slightly concave due to interaction with F8
- Oxygenation leads to the movement of the Fe(II) into the plane of the porphyrin group.
- Fe(II) now sits within the plane of the porphyrin ring
- Simultaneous changes in the conformation of parts of the globins occurs.
35
What makes Hb an allosteric protein?
- Changes in shape of tetramer facilitate O2 uptake and release under physiological conditions.
- F helix and haem groups swing towards the centre of the molecule.
- Methaemoglobin (MetHb) contains Fe3+ in Haem. Cannot bind/exchange oxygen. MetHb Reductase pathway ensures MetHb is converted back into functional Hb
36
2,3-Biphosphoglycerate (2,3-BPG)
- binds to the b chains in the middle of the tetramer.
- combines with deoxyHb; reduces affinity of Hb for O2.
- Reversibly bound, altering the conformation of the Hb molecule, facilitating O2 release.
- O2 dissociation curve for Hb is shifted to the right-lower affinity.
- The greater the level of 2-3 BPG the more O2 is released from the Hb.
37
Factors that affect the release of O2 from Hb
- pH
- temperature
- PCO2
- PO2
- 2-3 BPG.
38
Foetal Hb
- Foetal Hb (HbF), has a higher affinity for O2 because it cannot bind 2-3 BPG strongly.
- Allows HbF to carry 20-30% more O2 than maternal HbA at a particular PO2
- HbF preferentially takes O2 from maternal Hb across the placental membranes.
