Hemoglobin and hemoglobinpathies Srbová M., Průša R.
Hemoproteins
Consist of hem
– cyclic tetrapyrrole
– 1 iron cation Fe2+ bound in the middle of tetrapyrrole scelet by coordination covalent bonds
– conjugated system of double bonds
methine bridge
pyrrole ring
Types of hemoglobin Adult HbA: 2α and 2β subunits (98%HbA)
Adult HbA2: 2α and 2δ subunits (2% HbA) Fetal HbF: 2α and 2γ have higher O2 affinity than HbA – take up oxygen
from the maternal circulation Embryoinic: 2and 2 2 and 2 2 and 2 have higher O2 affinity than HbA
Hemoproteins
Hemoglobin (transports O2 to the tissues)
Myoglobin (stores O2 in the muscles)
Cytochromes (e- carriers in ETC)
Catalase + peroxidases (decomposition of peroxides)
Cytochrome P-450 (hydroxylation)
Desaturasases FA (desaturation FA)
Redox state Fe 2+ Fe 3+
Redox state Fe 2+
Structure of Hemoglobin
• 4 polypetide subunits (globins)
• Hb A (adults) heterotetramer 2α a 2β
• Each subunit contains 1 hem group
• 8 helices (A-H) β subunit
• 7 helices α subunit
• Hydrofobic pocket
- protect hem against oxidation
• Hem binding to globin – Fe 2+ is coordinated by N atom from proximal histidin F8
• Binding of O2 – distal histidin E7 hydrogen bonds to the O2
Structure of Hemoglobin
• Quaternary structure
Interactions between subunits 1) hydrofobic ( between α-β)
2) electrostatic (between α-α; β-β, α-β)
– O2 binding – loss of these interactions
Structure of Hemoglobin
α1
α2
β1 β2
• 1 polypeptide chain (153 AA)
• 1 heme
• Tertiary structures of the α and β subunits are remarkably similar, both to each other and to that of Mb
• Skeletal and heart muscles
Structure of Myoglobin
Binding of O2 (oxygenation)
• Oxygenation changes the electronic state of the Fe2+ - heme
• Color change of blood from dark purplish (venous) to the brilliant scarlet color (arterial)
• The binding of the first O2 to Hb enhances the binding futher O2 molecules
• O2 affinity of Hb increases with increasing pO2
• Sigmoidal saturation curve
• Hyperbolic curve for Mb - no cooperative behavior
Mechanism of oxygen-binding cooperativity
Satu
rati
on
O2
•Hb loads O2 to about 90% saturation under the arterial partial pressure • Hb travels to the tissue where the O2 partial pressure is 20 torr, most of Hb´s bound O2 is released
• The diference in oxygen affinity between Mb and Hb is greatest between 5 and 30 torr, where Mb binds much more O2 than does Hb. This difference allows O2 to be released at the tissues from O2 - loaded Hb, and transported to Mb
Satu
rati
on
O2
• The movement of Fe 2+ into the heme plane triggers the T→R conformational shift
•The loss of electrostatic interactions induce conformational changes in all other subunits
Conversion of T form→R form
T form (tense)
R form (relaxed)
The binding of the first O2 molecule to subunit of the T-form leads to a local conformational change that weakens association between the subunits R-form
Allosteric effectors
• CO2
• H+
• 2,3-bisphosphoglycerate
Decrease O2 affinity of Hb
Influence the equilibrium between T and R forms
2,3 - bisphosphoglycerate
•binds selectively to deoxy-Hb •stabilizes T form •lowers the affinity of Hb for oxygen •oxygen is more readily released in tissues
2,3 - bisphosphoglycerate Clinical aspects:
In people with high-altitude adaptation or smokers the concentration of 2,3-BPG in the blood is increased increases the amount of oxygen that Hb unloads in the capilaries
Fetal hemoglobin (HbF α2γ2), has low BPG affinity – the higher O2 affinity – facilitates the transfer of O2 to the fetus via the placenta
Bohr effect
• The binding of protons H+ by Hb lowers its affinity for O2
• Increasing pH, that is, removing protons,stimulates Hb to bind O2
• pH of the blood decreases as it enters
tissues because CO2 produced by
metabolism is converted to H2CO3
• Dissociation of H2CO3 produces protons
• Promote the release of oxygen
In the tissues
Bohr effect
In the lungs
Oxygen binds to Hb, causing a release protons, which combine with bicarbonate to form H2CO3
Carbonic anhydrase cleaves H2CO3 to H2O and CO2 CO2 is exhaled
Total Hb and Free Hb
• Reference values of total Hb – age and sex dependent, about 150 g/l
• Free Hb: 125 – 300 mg/l
Derivatives of hemoglobin
Deoxyhemoglobin – Hb without O2
Oxyhemoglobin – Hb with O2
Carbaminohemoglobin – Hb with CO2
– CO2 is bound to globin chain
– about 15% of CO2 is transported in blood bound to Hb
Carbonylhemoglobin – Hb with CO
– CO binds to Fe2+ 200x higher affinity to Fe2+ than O2 – poisoning, smoking
Methemoglobin – (metHb) contains Fe3+ instead of Fe2+
Autooxidation of hemoglobin
3% of hemoglobin undergoes oxidation every day
Hem – Fe2+- O2 Hem - Fe3+ + O2•-
Methemoglobin reductase
reduces methemoglobin
FAD, cytochrom b5 a NADH
Methemoglobinemia
1. Hereditary deficit of methemoglobin reductase
2. Abnormal hemoglobin HbM (Hb mutation)
3. Exposure to exogenous oxidizing drugs (sulfonamides, aniline)
Clinical aspects: cyanosis (10% Hb forms metHb) treatment: administration of methylene blue or ascorbic acid
Glycohemoglobin (HbA1c)
Formed by Hb‘s exposure to high levels of glucose
Nonenzymatic glycation of terminal NH2 group (Val) β-chain
Normally about 4 % of Hb is glycated (proportional to blood Glc concentration)
People with DM have more HbA1c than normal ( 5%)
Measurement of blood HbA1c is useful to get information about long-term control of glycemia
β1
β2
α1
α2
α1
α2
α1
α2
γ1
γ2
δ1
δ2
Hb A > 96,5% Hb F < 1% Hb A2 < 3,5%
HbA1c: What are we looking for?
1st Step: Unstable, reversible reaction between Glucose and the N-terminal valine of the β-chain (Schiff base)
2nd Step: During red blood cell circulation, some of the labile A1C is
converted to form a stable HbA1c (Amadori rearrangement)
HbA1c is currently defined as:
Hemoglobin A which is irreversibly glycated at one or both N-terminal Valines of the chains in the tetramer.
Glycation elsewhere on the or chains is irrelevant.
G
G
G
G
G
G
G
G
G
N
N
N
N
N
All of these are HbA1c
The nature of the problem – what is HbA1c?
Glycohemoglobin, or GHb, or Total GHb, is defined as:
Hb having one or more sugars irreversibly attached at any point in any
of the globin chains.
(This also includes all forms of HbA1c).
G
G
N
G
G
G
N
G
N
G
G
G
G N
All of these are GHb (but not HbA1c)
The nature of the problem – what is HbA1c?
Hb A0 93-95%
Hb A1 = GHb
Glycated Hbs
5-7%
Hb A
+ + +
Hb A1a 0,5%
Fructose-1,6-diphosphate Glucose-6-phosphate
Hb A1b 0,5%
pyruvate
Hb A1c 4-6%
glucose
Hb F Hb A2
HbA1c: What are we looking for?
Glycation at the N-terminal Valin
of the β-globin chain
The Pros and the Cons of using HbA1c for Diabetes Diagnosis David B.Sacks; AACC Webinar April 10th 2012
Fructosamine 3- kinase
• AGE (advanced glycation endproducts)
• Deglycation protective enzyme
• Brain, erytrocytes, lens – high activity
• D.M. and fasting – no influence on activity
• Polymorphisms of gene – lower or higher activity
Hemoglobinopathies
mutation → abnormal structure of the hemoglobin
Large number of haemoglobin mutations, a fraction has deleterious effects: sickling, change in O2 affinity, heme loss or dissociation of tetramer
hemoglobin M and S, thalassemias
1. Hemoglobin M
• Replacement of His E7α by Tyr (Hb Boston) or
• Replacement of Val E11β by Glu (Hb Milwaukee)
• the iron in the heme group is in the Fe3+ state (methemoglobin) stabilized by the tyrosine or by glutamate
• Methemoglobin reductase cannot reduce Fe3+
• methemoglobin can not bind oxygen
2. Thalassemias
• Mutation that results in decreased synthesis of α or β-chains
• thalassemia mutations provide resistence to malaria in the heterozygous state
α- thalassemias – complete gene deletion
4 α globin genes per cell:
1 copy of gen is deleted: without symptoms
2 copies are deleted: RBC are of decreased size (microcytic) and reduced Hb concentration (hypochromic), individual is usually not anemic
3 copies are deleted: moderately severe microcytic hypochromic anemia with splenomegaly
4 copies are deleted: hydrops fetalis: fatal in utero
Excess β chains form homotetramer HbH which is useless for delivering oxygen to the tissues (high oxygen affinity)
• β+ – some globin chain synthesis
• β0 – no globin chain synthesis
Heterozygotes: microcytic hypochromic RBC, mild anemia
Homozygotes β0 β0 : severe anemia
Excess α chains precipitate in erythroid precursor – their destruction-ineffective erythropoiesis
β- thalassemias
3. Hemoglobin S (sickle-cell)
• Causes a sickle-cell anemia
• Replacing Glu A3β with the less polar amino acid Val - forming „an adhesive region“ of the β chain
• HbS proteins aggregate into a long rodlike helical fiber
Sickle-cell anemia Red blood cells adopt a sickle shape in a consequence of the forming haemoglobin S fibers The high incidence of sickle-cell disease coincides with a high incidence of malaria Individuals heterozygous in HbS have a higher resistance to malaria; the malarial parasite spends a portion of its life cycle in red cells, and the increased fragility of the sickled cells tends to interrupt this cycle