Download 4-Biochemical structure of Hemoglobin

HEMOGLOBIN
Dr. Shumaila Asim
Lecture # 4
1
Introduction
• The main function of red blood cell
• Transfer of O2 from lungs to tissue
• Transfer of CO2 from tissue to lungs
• To accomplish this function red cells has
hemoglobin (Hb)
• Each red cell has 640 million molecules of Hb
2
Introduction
• Hemoglobin (Hb), protein constituting 1/3 of the red
blood cells
• Synthesis begins in proerythroblast
• 65% at erythroblast stage
• 35% at reticulocyte stage
• Two parts
• Heme
• Globin
3
4
Synthesis of Hemoglobin (Hb)
• Heme & globin produced at two different sites
in the cells
• Heme in mitochondria
• Globin in polyribosomes
• Well synchronized
5
Synthesis of Hemoglobin
6
Synthesis of Heme
• Protoporphyrin ring with an iron atom in
centre
• The main site is mitochondria as it contains
ALAS
• Mature red cell does not contain mitochondria
7
Structure of Heme
8
HEME-CONTAINING PROTEINS
 Hemoglobin
 Myoglobin
 Cytochromes
 Catalase
 Some peroxidases
9
Synthesis of globin
10
Synthesis of globin
• Various types of globin combines with heme to
from different hemoglobin
• Eight functional globin chains, arranged in two
clusters the
• β- cluster (β, γ , δ and ɛ globin genes) on the short arm
of chromosome 11
• α- cluster (α and ζ globin genes) on the short arm of
chromosome 16
11
Synthesis of globin
Globin synthesis, starts at 3rd week of gestation
• Embryonic
Haemoglobin Gower I ( ζ2ɛ2)
Haemoglobin Portland ( ζ2γ2)
Haemoglobin Gower II (α2ɛ2)
• Fetal : HbF (α2γ2), HbA (α2β2)
• Adult : HbA, HbA2 ( α2δ2), HbF.
12
13
Globin gene clusters
14
Globin chain switch
15
Alpha & beta chains
16
Chain interaction
• The chains interact with each other via
hydrophobic interactions
– Therefore, hydrophobic amino acids are not only
present in the interior of the protein chains, but also on
the surface.
Electrostatic interactions
(salt bridges) and hydrogen
bonds also exist between
the two different chains.
17
Types of Hb
Normal:
Abnormal:
HbA (97%)
HbA2 (2%)
HbF (1%)
HbA1c
Carboxy Hb
Met Hb
Sulf Hb
18
HbA structure
Oxygen binding to hemoglobin
19
Types of hemoglobin
Fetal hemoglobin (HbF):
• Major hemoglobin found in the fetus and
newborn
• Tetramer with two a and two g chains
• Higher affinity for O2 than HbA
• Transfers O2 from maternal to fetal circulation
across placenta
20
Types of hemoglobin
HbA2:
• Appears ~12 weeks after birth
• Constitutes ~2% of total Hb
• Composed of two α and two δ globin chains
21
Types of hemoglobin
HbA1c:
• HbA undergoes nonenzymatic glycosylation
• Glycosylation depends on
plasma glucose levels
• HbA1c levels are high in
patients with diabetes
mellitus
22
Abnormal Hbs
Unable to transport O2 due to abnormal
structure:
• Carboxy-Hb: CO replaces O2 and binds 200X
tighter than O2 (in smokers)
• Met-Hb: Contains oxidized Fe3+ (~2%) that
cannot carry O2
• Sulf-HB: Forms due to high sulfur levels in
blood (irreversible reaction)
23
Hemoglobin structure
• Hemoglobin is tetrameric hemeprotein (fur protein chains known as globins
with each bound to heme.
• In adults, the four globin proteins are of two different types known as α and
β, so a hemoglobin protein is an α2β2 globin protein.
• The α and β chains contain multiple α-helices where α contains 7 α-helices
and β contains 8 α-helices (similar to myoglobin).
24
Hemoglobin is allosteric
• Hemoglobin is an allosteric protein (from Greek "allos" =
"other", and "stereos" = "shape").
– An allosteric protein: a protein where binding of a molecule
(ligand) to one part of the protein affects binding of a similar
or a different ligand to another part of the protein.
• Hemoglobin exists in two forms, T-state and R-state
• The T-state is also known as the "taut" or "tense" state and it has
a low-binding affinity to oxygen.
• The R-state is known as the "relaxed" state and it has 500 times
higher affinity to oxygen than as the T conformation .
• Binding of O2 causes conformational changes in hemoglobin,
converting it from the low affinity T-state to the high affinity Rstate .
25
How does the structure change?
• When heme is free of oxygen, it has a domed structure
and iron is outside the plane of the heme group.
• When oxygen binds to an iron atom, heme adopts a
planar structure and the iron moves into the plane of the
heme pulling proximal histidine (F8) along with it.
26
How does the structure change?
• This movement triggers
– changes in tertiary structure of individual hemoglobin
subunits and
– breakage of the electrostatic bonds at the other oxygenfree hemoglobin chains.
27
The saturation curve is sigmoidal because…
• Conformational changes lead to
cooperativity among binding
sites.
• Binding of the first O2 breaks
some salt bridges with the other
chains increasing the affinity of
the binding of a second
molecule.
• Binding of the second O2
molecule breaks more salt
bridges increasing the affinity
towards binding of a third O2
even more, and so on.
28
29
Derivatives of hemoglobin
 Oxyhemoglobin (oxyHb) = Hb with O2
 Deoxyhemoglobin (deoxyHb) = Hb without O2
 Methemoglobin (metHb) contains Fe3+ instead of Fe2+ in heme groups
 Carbonylhemoglobin (HbCO) – CO binds to Fe2+ in heme in case of CO
poisoning or smoking. CO has 200x higher affinity to Fe2+ than O2.
 Carbaminohemoglobin (HbCO2) - CO2 is non-covalently bound to globin chain of
Hb. HbCO2 transports CO2 in blood (about 23%).
 Glycohemoglobin (HbA1c) is formed spontaneously by nonenzymatic reaction
with Glc. People with DM have more HbA1c than normal (› 7%). Measurement of
blood HbA1c is useful to get info about long-term control of glycemia.
30
Another significance of distal histidine
31