Download Hemoglobin

Hemoglobin
Structure & Function
HMIM224
Objectives of the Lecture
1- Understanding the main structural & functional details
of hemoglobin as one of the hemoproteins.
2- Identify types & relative concentrations of normal
adult hemoglobin with reference to HBA1c with its
clinical application.
3- Recognize some of the main genetic & biochemical
aspects of methemoglobinopathies with some
implications on clinical features (with focusing on
thalassemias).
Hemoglobin is a globular hemoprotein
• Hemeproteins are a group of specialized proteins that contain heme as
a tightly bound prosthetic group.
• Heme is a complex of protoporphyrin IX and ferrous iron (Fe2+) .
• The iron is held in the center of the heme molecule by bonds to the four
nitrogens of the porphyrin ring.
• The heme Fe2+ can form two additional bonds, one on each side of the
planar porphyrin ring.
In myoglobin and hemoglobin, one of these positions is coordinated to
the side chain of a histidine residue of the globin molecule, whereas the
other position is available to bind oxygen
Globin of hemoglobin is a globular protein with a
quaternary structure
Structure of heme
Heme is a complex of protoporphyrin IX and ferrous iron (Fe2+).
The iron is held in the center of the heme molecule
by bonds of the four nitrogens of the protoporphrin ring.
Heme F2+ can form two additional bonds, one on each side of
the porphyrin ring. One of these positions is coordinated to the
Side chain of histidine residue of the globin molecule, whereas
the other position is available to bind oxygen.
Structure & function of hemoglobin
• Hemoglobin is found exclusively in RBCs.
• Each day 6-7 g of Hb is synthesized to replace the heme lost through
the normal turnover of erythrocytes .
• Its main function is to transport oxygen from lungs to the tissues &
carbon dioxide & hydrogen protons from tissues to lungs.
• Hemoglobin A is the major hemoglobin in adults, is composed of four
polypeptide chains, 2 alpha (a) & 2 beta (b) chains, held together by
non-covalent interactions
• Each subunit has stretches of a-helical structure & a heme binding
pocket.
Structure & function of hemoglobin (cont.)
Structure & function of hemoglobin (cont.)
Quaternary structure of hemoglobin:
•
The hemoglobin tetramer can be envisioned as being composed of two identical dimers, (αβ)1 and
(αβ)2, in which the numbers refer to dimers one and two.
•
The two polypeptide chains within each dimer are tightly held together, primarily by hydrophobic
interactions
•
In contrast, the two dimers are able to move with respect to each other, being held together primarily
by polar bonds.
•
The weaker interactions between these mobile dimers result in the two dimers occupying different
relative positions in deoxyhemoglobin as compared with oxyhemoglobin
oxygenation & deoxygenation of hemoglobin
(oxyhemoglobin & deoxyhemoglobin)
Deoxyhemoglobin
Taut structure
Oxyhemoglobin
Relaxed structure
Types of adult hemoglobin
3–6 %
HBA:
HBA2:
HBF:
HBA1C :
the major hemoglobin in humans
first appears 12 weeks after birth- a minor component of normal adult HB
normally synthesized only during fetal development
has glucose residues attached to b-globin chains – increased amounts in DM
Hemoglobin A1c (HBA1c)
Some of hemoglobin A is glycosylated
Extent of glycosylation depends on the
plasma concentration of a particular
hexose (as glucose).
The most abundant form of glycosylated
hemoglobin is HBA1c which has a
glucose residues attached to b-globin
chains in hemoglobin RBCs.
Increased amounts of HBA1c are found
in RBCs of patients with diabetes
mellitus (DM).
HbA1c could be used as a monitor for the control of the blood glucose level
during the last 2 months for diabetic patients
Hemoglobinopathies
Hemoglobinopathies are members of a family of genetic disorders caused
by:
1- Production of a structurally abnormal hemoglobin molecule
(Qualitative hemoglobinopathies)
Or: 2- Synthesis of insufficient quantities of normal hemoglobin
(Quantitative hemoglobinopathies)
Or: 3- both (rare).
Thalassemias
• Thalassemias are hereditary hemolytic diseases in which an
imbalance occurs in the synthesis of globin chains.
• They are most common single gene disorders in humans.
• Normally, synthesis of a- and b- globin chains are coordinated,
so that each a-globin chain has a b-globin chain partner.
This leads to the formation of a2b2 (HbA).
•
In thalassemias, the synthesis of either the a- or b-globin chain
is defective.
Thalassemias (cont.)
Thalassemia can be caused by a variety of mutations, including:
1- Entire gene deletions (whole gene is absent)
Or: 2- Substitutions or deletions of one or more nucleotides in the DNA.
Each thalassemia can be classified as either:
1- A disorder in which no globin chains are produced
(ao- or bo -thalassemia)
Or: 2- Some b-chains are synthesized, but at a reduced rate.
(a+- or b+- thalassemia).
Thalassemias (cont.)
1- b-thalassemias:
 Synthesis of b-globin chains are decreased or absent, whereas aglobin chain synthesis is normal.
 a-globin chains cannot form stable tetramers, and therefore a-chain
precipitate causing premature death of RBCs.
 Accumulation of a2g2(HbF), a22 (Hb A2) also occurs.
 These factors end result in development of chronic anemia
(hemolytic).
Thalassemias (cont.)
Some genetic aspects of thalassemia:
There are only two copies of the b -globin gene in each cell (one on each
chromosome 11).
So, individuals with b -globin gene defects have either:
1- b-thalassemia minor (b -thalassemia trait):
if they have only one defective b-globin gene.
2- b- thalassemia major (Cooley anemia):
if both genes are defective.
Thalassemias (cont.)
Mutation in one of
b-globin genes
Mutation in both
b-globin genes
b-thalassemia
minor
b-thalassemia
major
Thalassemias (cont.)
Some clinical aspects of thalassemias:
1- As b-globin gene is not expressed until late fetal gestation, the physical
manifestations of b- thalassemias appear only after birth.
2- Individuals with b - thalassemias minor, make some b-chains, and usually
require no specific treatment.
3- Infants born with b - thalassemias major seem healthy at birth, but
become severely anemic during the first or second years of life.
They require regular transfusions of blood.
In these cases, bone marrow replacement therapy is recommended.
Thalassemias (cont.)
2- a-thalassemia:
Synthesis of a-globin chains is decreased or absent.
Each individual's genome contains four copies of the a-globin (two
on each chromosome 16), there are several levels of a-globin
chain deficiencies
Thalassemias (cont.)
Types:
If one of the four genes is defective
the individual is termed a silent carrier of a- thalassemia as no physical manifestations of the
disease occur.
If two a-globin genes are defective,
the individual is designated as having a-thalassemia trait.
If three a-globin genes are defective;
the individual has hemoglobin H (HbH) disease which is a mildly to moderately hemolytic
anemia.
Synthesis of unaffected g- and then b- globin chains continues, resulting in the accumulation of
g tetramer in the newborn (g4, Hb Bart's) or b-tetramers (b4, HbH).
The subunits do not show heme-heme interactions. So, they have very high oxygen affinities.
Thus, they are essentially useless as oxygen carriers to tissues
If four a-globin genes are defective,
Hydrops fetalis & fetal death (death at birth), occurs as a-globin chains are required for the
synthesis of HbF
Thalassemias (cont.)
Types of a-thalassemias
Biochemistry of hemoglobin II
Sickle cell anemia
Definition:
Sickle cell anemia is a genetic disorder of the blood caused by
a single nucleotide alteration (a point mutation) in the b-globin gene.
Inheritance of sickle cell anemia:
Homozygous recessive disorder:
Occurs in individuals who have inherited two mutant genes (one from each
parent) that code for synthesis of the b chains of the globin molecule.
RBCs of homozygous is totally HB S (a2bs2 )
Heterozygotes individuals:
Have one normal and one sickle cell gene.
RBCs of heterozygotes contain both HB S (a2bs2 ) & HB A (a2b2 )
Clinical manifestations of sickle cell anemia
Homozygous individuals
Clinical manifestations begin early in childhood
With
- Chronic hemolytic anemia
- Lifelong episodes of pain & increased
- Susceptibility to infection.
- Acute chest syndrome
- Stroke
- Splenic & renal dysfunction
- Bone changes due to bone marrow hyperplasia
Heterozygote individuals
Usually do not show clinical symptoms
Amino acid substitution in HB S b chains
HB S contains two mutant b-globin chains (bs ).
In mutant chains, glutamate (polar) at position 6 is
replaced with valine (nonpolar) resulting in:
Formation of a protrusion on the b-globin that fit into
a complementary site on the a chain of another
hemoglobin molecule in the cell.
In low oxygen tension, deoxy HB S polymerize inside
the red blood cell leading to stiffening & distorting of
the cell ending in production of rigid misshapen RBCs.
Sickle cells block the flow of blood in narrow
capillaries resulting in interruption of oxygen supply
(localized anoxia) in tissues causing pains.
Finally, cell death occurs due to anoxia (infarction)
Also, RBCs of HB S have shorter life span than normal
RBCs (less than 20 days, compared to 120 of normal)
Hence, anemia is a consequence of HB S.
Factors that increase sickling
The extent of sickling is increased by any factor that increases the
proportion of HB S in the deoxy state as in cases of
1- Decreased oxygen tension:
in high altitudes
flying in a nonpressurized plane
2- Increased pCO2
3- Decrease pH
4- Increased 2,3- BPG in RBCs
Diagnosis of HB S
Hemoglobin
Electrophoresis
HB S migrates more slowly
towards anode (+ve electrode) than
normal hemoglobin
due to absence of negatively charged
glutamate resulting in decrease of
negativity of hemoglobin
Selective advantage of the heterozygote state
Heterozygotes individuals for sickle cell anemia are less susceptible to
malaria caused by the parasite Plasmodium falciparum as their RBCs
have shorter lifespan than normal , the parasite cannot complete its
natural stage of development in RBCs.
HB S gene is highly frequent in Africa in which malaria is also
highly frequent.
Methemoglobinemia
•
•
•
Methemoglobin results from oxidation of the ferrous ion (Fe2+) of heme of hemoglobin
to ferric (Fe3+) ion.
Methemoglobinemia is characterized by “chocolate cyanosis” i.e. brown-blue
coloration of skin & membranes & chocolate colored blood.
Causes of oxidation of ferrous ions:
1- Drugs as nitrates
2- Endogenous products (as reactive oxygen species )
3- Inherited defects (as in certain mutations of a or b chains)
4- Deficiency of NADH-Met HB reductase :enzyme for reduction of Fe3+ of Met HB
•
RBCs of newborns have ½ capacity of adults to reduce Met HB & therefore they are
more susceptible to Met HB formation by previous factors.
•
•
Clinically, symptoms are due to tissue hypoxia
Treatment: Methylene blue (to reduce the ferric ions)
Assignments
 Methemoglobin
 Sickle cell anemia (Genetic, Biochemical
& Clinical Aspects)