Transcript Amino Acids

UNIT I:
Protein Structure and Function
Globular Proteins
Overview
• This chapter examines relationship b/w
structure and function for the clinically
important globular hemeproteins, and
mainly Hb.
II. Globular hemeproteins
• Hemeproteins: group of specialized proteins,
contain heme as a tightly bound prosthetic
group
• Role of heme group is dictated by environ.
created by 3D structure of protein e.g.,
– Heme of a cytochrome functions as an electron
carrier
– Heme of catalase is part of active site of the enzyme
 catalyzes breakdown of H2O2
– In Hb and myoglobin, the 2 most abundant heme
proteins, heme serves to reversibly bind oxygen
A. Structure of heme
• A complex of protoporphyrin IX and ferrous iron
(Fe2+)
• Iron is held in center of heme molecule by bonds
to 4 nitrogens of porphyrin ring
• Heme Fe2+ can form 2 additional bonds, one on
each side of the planar porphyrin ring e.g., in
myoglobin and Hb, one of these positions is
coordinated to side chain of His residue of globin
molecule, the other is available to bind oxygen
Figure 3.1.
A. Hemeprotein (cytochrome c).
B. Structure of heme.
B. Structure and function of Myoglobin
• Myoglobin, a hemeprotein present in heart & skeletal
muscle
• Functions as a reservoir for oxygen & as oxygen carrier
that increases rate of transport of oxygen within muscle
cell
• Consists of a single polyp structurally similar to individual
subunit polyp of Hb molecule, making myoglobin useful
model for interpreting some complex properties of Hb
1.
•
•
2.
•
•
α-helical content:
Myoglobin a compact molecule, ~ 80% of its polyp
folded into 8 stretches of α-helix
These α-helical regions are terminated either by Pro
(its 5-membered ring cannot be accommodated in α–
helix), or by β-bends and loops stabilized by H-bonds
and ionic bonds
Location of polar and non-polar aa residues:
Interior of myoglobin molecule is composed of almost
entirely non-polar aa’s. Packed together forming a
structure stabilized by hydrophobic interactions
Charged aa’s located almost exclusively on surface,
forming H-bonds with each other and with water
Figure 3.2.
A. Model of myoglobin showing
helices A to H.
3. Binding of heme group:
–
–
–
–
Heme group sits in a crevice lined with non-polar
aa’s. Notable exceptions are 2 His residues.
One, proximal His, binds directly to iron of heme,
2nd distal His, does not directly interact with heme,
but helps stabilize binding of oxygen to ferrous iron
The protein, or globin, portion of myoglobin creates
a microenviron. for heme that permits reversible
binding of one oxygen molecule (oxygenation).
Simultaneous loss of electrons by ferrous iron
(oxidation) occurs only rarely.
B. Schematic diagram of the oxygenbinding site of myoglobin.
C. Structure and function of hemoglobin
•
Hb is found exclusively in RBC’s, its main function is
transport of oxygen from lungs to capillaries of tissues.
HbA, major Hb in adults, is composed of 4 polyps. 2 αand 2 β-chains –held together by non-covalent
interactions.
Each subunit has stretches of α-helical structure, and a
heme binding pocket.
Tetrameric Hb is more complex structurally and
functionally than myoglobin e.g.,
•
•
•
–
–
–
Hb can transport CO2 from tissues to lungs, and
carry 4 O2 from lungs to cells of the body, further
Oxygen-binding properties of Hb are regulated by interaction
with allosteric effectors
Figure 3.3
A. Structure of hemoglobin showing the polypeptide backbone. B. Simplified
drawing showing the helices.
1. Quaternary structure of Hb
• Hb tetramer can be envisioned as composed of 2
identical dimers (αβ)1 and (αβ)2.
• The 2 polyp chains in each dimer held tightly together
primarily by hydrophobic interactions (in this case
hydrophobic aa residues are localized not only in interior
of molecule but also in a region on surface of each
subunit. Interchain hydrophobic interactions form strong
associations b/w α- and β-subunits in dimers)
• Ionic and H-bonds also occur b/w members of the dimer
• The two dimers, in contrast, are able to move wrt each
other, being held primarily by polar bonds. The weaker
interactions b/w these mobile dimers result in the 2
dimers occupying different relative positions in deoxy-Hb
as compared with oxy-Hb
Figure 3.4.
Schematic diagram showing structural changes resulting from oxygenation and
deoxygenation of hemoglobin.
a) T-form:
•
•
•
deoxy form of Hb “T”, or taut (tense) form.
The 2 αβ dimers interact through a network of ionic
and H-bonds  constrain movement of polyp
chains
Low oxygen-affinity form of Hb
b) R-form:
–
–
–
Binding of oxygen to Hb causes rupture of some of
the ionic and H-bonds b/w αβ dimers
This leads to a structure called “R” or relaxed form,
here polyp chains have more freedom of movement
R-form is high oxygen-affinity form of Hb
D. Binding of oxygen to myoglobin and Hb
•
•
•
Myoglobin can bind 1 O2 molecule, it contains
only 1 heme group
Hb can bind 4 O2 molecules, one at each of its
4 heme groups
Degree of saturation (Y) of these oxygenbinding sites on all myoglobin or Hb molecules
can vary b/w zero (all sites are empty) and
100% (all sites are full)
1. Oxygen dissociation curve
• A plot of Y measured at different pO2
• Curves for myoglobin & Hb show important
differences. Myoglobin has a higher oxygen
affinity than Hb. Partial pressure of oxygen
needed to achieve half-saturation of binding
sites (P50) is ~ 1mm Hg for myoglobin & 26 mm
Hg for Hb
• Note: the higher the oxygen affinity (i.e., the
more tightly oxygen binds), the lower P50
Figure 3.5
Oxygen dissociation curves for
myoglobin and hemoglobin.
a. Myoglobin
•
•
•
The oxygen dissociation curve for myoglobin has a
hyperbolic shape. This reflects myoglobin reversibly
binds a single molecule of oxygen
Thus, oxygenated (MbO2) and deoxygenated (Mb)
exist in a simple equilibrium:
Mb + O2 ↔ MbO2
Mb is designed to bind oxygen released by Hb at the
low pO2 found in muscles. Mb releases oxygen within
muscle cell in response to oxygen demand
b. Hb
•
•
•
The oxygen dissociation curve for Hb is
sigmoidal in shape. This reflects that subunits
cooperate in binding oxygen.
Cooperative binding of O2 by the 4 subunits of
Hb means binding of O2 to one heme group
increases the oxygen affinity of remaining
heme groups in the same Hb molecule, this
effect is heme-heme interaction
Although binding of 1st O2 is difficult,
subsequent binding of O2 occurs with high
affinity, shown by steep upward curve in the
region 20-30 mm Hg
Figure 3.6
Hemoglobin binds oxygen
with increasing affinity.
E. Allosteric effects
•
Ability of Hb to reversibly bind oxygen is affected by
pO2 (through heme-heme interaction), pH of environ.
pCO2 and availability of 2,3-bisphosphoglycerate.
•
Collectively called allosteric (“other site”) effectors, as
their interaction at one site on Hb molecule affects
binding of oxygen to heme groups at other locations on
the molecule
•
Binding of oxygen to myoglobin is not influenced by
allosteric effectors of Hb
1. Heme-heme interactions: sigmoidal oxygen-binding
curve reflects specific structural changes that are
initiated at one heme and transmitted to other heme
groups in Hb tetramer. Net effect  affinity of Hb for
last oxygen ~ 300x greater than affinity for 1st oxygen
a. Loading and unloading oxygen: cooperative binding of oxygen
allows Hb deliver more oxygen to tissues in response to relatively
small changes in pO2. Figure 3.5 indicates pO2 in alveoli of lung
and capillaries of tissues
- e.g., in lung conc. oxygen is high and Hb becomes saturated
(loaded) with oxygen.
- in peripheral tissues, oxy-Hb releases (unloads) much of its
oxygen for use in oxidative metabolism
b. Significance of sigmoidal O2-dissociation curve: Steep slope of O2dissociation curve over the range of oxygen conc. b/w lungs and
tissues permits Hb to carry and deliver oxygen efficiently from sites
of high to sites of low pO2
A molecule with hyperbolic O2-dissociation curve, e.g. myoglobin
could not achieve the same thing. Instead, it would have max
affinity for oxygen throughout this oxygen pressure  would deliver
no oxygen to tissues
Figure 3.7.
Transport of oxygen and CO2
by hemoglobin.
2. Bohr effect:
–
–
–
Release of oxygen from Hb is enhanced when pH is
lowered or when Hb is in pressure of an increased
pCO2. Both result in decreased oxygen affinity 
shift to the right in O2-dissociation curve.
This change in oxygen binding is called Bohr
effect.
Conversely, raising pH or lowering conc. of CO2 
greater affinity for oxygen, and a shift to the left in
O2-dissociation curve.
Figure 3.8.
Effect of pH on the oxygen
affinity of hemoglobin.
a. Source of the protons that lower the pH:
• Conc. of both CO2 and H+ in capillaries of metabolically active
tissues is higher than that observed in capillaries of lung, where
CO2 is released into expired air
• Note: organic acids e.g., lactic acid, are produced during anaerobic
metabolism in rapidly contracting muscle
• In tissues, CO2 is converted by carbonic anhydrase to carbonic
acid,
CO2 + H2O ↔ H2CO3
which spontaneously loses a proton becoming bicarbonate, the
major blood buffer
H2CO3 ↔ H+ + HCO3• The proton produced contributes to lowering pH. This differential pH
gradient (lungs having higher, tissues lower pH) favors unloading
oxygen in peripheral tissues, and loading of oxygen in lung.
• Thus, oxygen affinity of Hb responds to small shifts in pH b/w lungs
and oxygen-consuming tissues, making Hb a more efficient
transporter of oxygen.
b. Mechanism of the Bohr effect:
Deoxy form of Hb has a greater affinity for protons than does oxyHb. This fact is caused by ionizable groups, e.g., N-terminal αamino groups, & specific His side chains that have higher pKa’s in
deoxy-Hb than in oxy-Hb.
An increase in conc. of protons causes these groups to become
protonated (charged) and able to form ionic bonds (a.k.a salt
bridges), which stabilize deoxy form of Hb, producing a decrease
in oxygen affinity
Bohr effect schematically:
HbO2 (oxy-Hb) + H+ ↔ HbH (deoxy-Hb) + O2
where an increase in protons (or a lower pO2) shifts equilibrium to
right, whereas an increase in pO2 (or decrease in protons) shifts
equilibrium to left
3. Effect of 2,3 bisphosphoglycerate on oxygen affinity
• 2,3 BPG an important
regulator of binding of
oxygen to Hb
• It is the most abundant
organic phosphate in
RBC, where its conc. ~
that of Hb.
• 2,3 BPG is synthesized
from an intermediate of
glycolytic pathway
Figure 3.9. Synthesis of 2,3-BPG.
[Note:
is a phosphoryl group.]
a.
Binding of 2,3 BPG to deoxy-Hb
2,3 BPG decreases oxygen affinity
of Hb by binding to deoxy-Hb but not
to oxy-Hb.
This preferential binding stabilizes
the “taut” conformation of deoxy-Hb.
HbO2 + 2,3-BPG ↔ Hb-2,3-BPG + O2
b.
Binding site of 2,3 BPG
One molecule of 2,3 BPG binds to a
pocket, formed by the two β-globin
chains, in center of deoxy-Hb
tetramer.
This pocket contains several
positively charged aa’s that form
ionic bonds with the negatively
charged phosphate groups of 2,3
BPG
A mutation of one of these residues
can result in Hb variants with
abnormally high oxygen affinity
2,3 BPG is expelled on oxygenation
of Hb.
Figure 3.10. Binding of 2,3-BPG by
deoxyhemoglobin.
c. Shift of oxygen dissociation curve
• Hb from which 2,3 BPG
removed, has a high
affinity for oxygen.
In RBC, presence of 2,3
BPG significantly reduces
affinity of Hb for oxygen,
shifting oxygendissociation curve to the
right.
This reduced affinity
enables Hb to release
oxygen efficiently at
partial pressures found in
tissues
Figure 3.11. Effect of 2,3-BPG on the
oxygen affinity of hemoglobin.
d. Response of 2,3-BPG levels to chronic hypoxia or anemia
• Conc. of 2,3 BPG in RBC increases in response
to chronic hypoxia, e.g., that observed in
obstructive pulmonary emphysema, or at high
altitudes, where Hb may have difficulty receiving
sufficient oxygen.
Intracellular 2,3 BPG also elevated in chronic
anemia in which fewer than normal RBCs are
available to supply body’s oxygen needs.
Elevated 2,3 BPG levels lower oxygen affinity of
HB, permitting greater unloading of oxygen in
capillaries of tissues
e. Role of 2,3 BPG in transfused blood:
• 2,3 BPG is essential for normal oxygen
transport function of Hb.
• e.g., storing blood in acid-citrate-dextrose 
decrease of 2,3 BPG in RBCs. Such blood
displays an abnormally high oxygen affinity,
and fails to unload its bound oxygen properly
in the tissues.
• Hb deficient in 2,3 BPG thus acts as an
oxygen “trap” rather than as an oxygen
transport system.
• Transfused RBCs are able to restore
depleted supplies of 2,3 BPG in 24-48 h.
However, severely ill patients may be
seriously compromised if transfused with
large quantities of such 2,3 BPG –”stripped”
blood.
• Decrease in 2,3 BPG can be prevented by
adding substrates e.g., inosine
(hypoxanthine-ribose) to storage medium.
• Inosine, uncharged molecule can enter RBC,
its ribose moiety is released, phosphorylated,
and enters hexose mosophosphate pathway,
eventually converted to 2,3 BPG
4. Binding of CO2:
• Most CO2 produced in metabolism is hydrated
and transported as bicarbonate ion.
• However, some CO2 is carried as carbamate
bound to uncharged α-amino groups of Hb
(carbamino-Hb):
Hb-NH2 + CO2 ↔ Hb-NH-COO- + H+
• Binding of CO2 stabilizes T (taut) or deoxy form
of Hb, resulting in decrease in its affinity for
oxygen. In lungs, CO2 dissociates from Hb,
released in breath.
5. Binding of CO:
• CO binds tightly (but reversibly) to Hb iron 
carbon monoxyhemoglobin (HbCO).
• When CO binds to one or more of the 4 heme
sites, Hb shifts to relaxed conformation, causing
remaining sites bind oxygen with high affinity.
• This shifts oxygen saturation curve to the left,
and changes normal sigmoidal shape toward a
hyperbola.
• As a result affected Hb is unable to release
oxygen to tissues
• Affinity of Hb to CO is 220x greater than for
oxygen.
Figure 3.12. Effect of carbon monoxide on the oxygen affinity
of hemoglobin. CO-Hb = carbon monoxyhemoglobin.
• Even minute quantities of CO in environ. can
produce toxic conc’s of HbCO in blood.
• CO toxicity appears to result from a
combination of tissue hypoxia and direct COmediated damage at cellular level.
• CO poisoning is treated with 100% oxygen
therapy, which facilitates dissociation of CO
from Hb.
F. Minor hemoglobins
• HbA is one member of a functionally &
structurally related family of proteins, the
hemoglobins
• Each of these oxygen-carrying proteins is a
tetramer, composed of 2 α-like and 2 ß-like
polyps
• Certain Hbs e.g., HbF, are normally synthesized
only during fetal development, others e.g.,
HbA2, are synthesized in the adult although at
low levels compared to HbA
• HbA can also become modified by covalent
addition of a hexose e.g., addition of glucose 
glucosylated Hb derivative HbA1c.
Figure 3.13
Normal adult human hemoglobins. [Note: The a-chains in these
hemoglobins are identical.]
1. Fetal hemoglobin (HbF):
•
HbF is a tetramer of two α, plus 2 γ chains
(α2γ2). γ chains are members of ß-globin gene
family.
a. HbF synthesis during development:
–
–
–
–
In 1st few wks after conception, embryonic Hb (Hb
Gower 1), composed of 2 zeta and 2 epsilon chains
(ζ2ε2), synthesized by embryonic yolk sac
Within few wks, fetal liver begins to synthesize HbF
in developing bone marrow
HbF is major Hb found in fetus and newborn,
accounting for ~60% of total Hb in RBCs during last
months of fetal life
HbA synthesis starts in BM at about 8th month of
pregnancy and gradually replaces HbF
Figure 3.14
Developmental changes in
hemoglobin.
b. Binding of 2,3 BPG to HbF:
- Under physiologic conditions, HbF has a higher
affinity for oxygen than does HbA, as a result
HbF’s binding only weakly to 2,3 BPG
- The γ-globin chains lack some of the positively
charged aa’s responsible for binding 2,3 BPG in
ß-globin chains
- Because 2,3 BPG serves to reduce affinity of
HbA for oxygen, weaker interaction b/w 2,3 BPG
and HbF  higher oxygen affinity for HbF
relative to HbA.
- If both HbF and HbA stripped of their 2,3 BPG
they then have similar affinity for oxygen
- The higher oxygen affinity of HbF facilitates
transfer of oxygen from maternal circulation
across placenta to RBCs of fetus.
HbA2:
• Is a minor component of normal adult Hb, appearing ~ 12 wks
after birth  ~ 2% of total Hb. composed of α2δ2.
HbA1c:
• Under physiologic conditions, HbA slowly & non-enzymatically
glycosylated
• Extent of glycosylation dependent on plasma conc. of a particular
hexose
• Most abundant form of glycosylated Hb is HbA1c: has glucose
residues attached predominantly to NH2 groups of the N-terminal
Val of ß-globin chains
• Increased amounts found in RBCs of diabetes mellitus, as their
HbA has contact with higher glucose concs during the 120 d
lifetime of these cells
Figure 3.15
Nonenzymic addition of
glucose to hemoglobin.
III. Organization of the globin genes
To understand diseases resulting from genetic alterations in
structure or synthesis of Hbs.
A. α-Gene family
Genes for α- and β-globin-like subunits of Hb chains
occur in 2 separate gene clusters (families) located on
two different chr’s
α-gene cluster on chr 16 contains 2 genes for αglobin chains. It also contains zeta (ζ) gene,
expressed early in development as a component of
embryonic Hb, & a number of globin-like genes that
are not expressed (pseudogenes)
B. β-Gene family
- A single β-globin gene located on chr 11, plus
additional 4 β-globin-like genes: epsilon (ε)
gene, two gamma (γ) genes (Gγ & Aγ that are
expressed in fetal, HbF), and the δ gene that
codes for globin chain found in minor adult
HbA2.
Figure 3.16
Organization of the globin gene families.
C. Steps in globin chain synthesis
• Begins in nucleus of RBC precursors, where
DNA sequences encoding gene is transcribed.
• RNA produced by transcription is a precursor of
mRNA
• Before translation, two non-coding stretches of
RNA (introns) must be removed, and remaining
3 fragments (exons) reattached in a linear
manner.
• Resulting mature mRNA enters cytosol, its
genetic info. translated  globin chain
Figure 3.17
Synthesis of globin chains.
IV. Hemoglobinopathies
• Family of disorders caused by production of
structurally abnormal Hb, synthesis of
insufficient quantities of normal Hb, or rarely
both.
• Sickle-cell anemia (HbS), Hb C disease (HbC),
and thalassemia syndromes are examples that
can have severe clinical consequences.
• HbS & HbC result from altered aa sequences,
thalassemia caused by decreased production
A. Sickle cell disease (HbS disease)
• a.k.a sickle cell anemia, a genetic disorder of
blood, caused by a single nt alteration (point
mutation) in β-globin gene.
• Most common inherited blood disorder in US,
affecting ~80,000 Americans (primarily, AfricanAmerican population, affecting 1/500 newborn
infants)
• Homozygous recessive disorder. Occurs in
individuals inherited 2 mutant genes that code
for β-chains
• Mutant β-globin chain = βS, resulting Hb = α2βS2
 HbS
• Sickle cell disease is characterized by lifelong
episodes of pain (“crises”), chronic hemolytic
anemia, and increased susceptibility to
infections, usually beginning in early childhood.
• Other symptoms include: acute chest syndrome,
stroke, splenic & renal dysfunction
• Lifetime of an erythrocyte homozygous for HbS
is ~20 days, compared with 120 d for normal
RBCs
• Heterozygotes (1 of 10 African-Americans, have
one normal and one sickle cell gene). Their
RBCs contain both HbS and HbA. These
individuals have sickle cell trait  usually do
not show clinical symptoms & can have normal
life span
Figure 3.18. Sickle cell disease in the United States.
Figure 3.19
Amino acid substitutions in HbS and HbC.
Figure 3.20.
A. Photograph of a gel prior to electrophoresis.
B. Diagram of hemoglobins A, S, and C after
electrophoresis.
2. Sickling causes tissue anoxia:
- Substitution of non-polar Val for a charged Glu
forms a protrusion on the β-globin that fits into a
complementary site on the α-chain of another Hb
molecule in the cell.
- At low oxygen tension, HbS polymerizes inside
RBCs 1st forming a gel, then assembling into a
network of fibrous polymers that stiffen and distort
cell, producing rigid, misshapen RBCs.
- Such sickled cells frequently block flow of blood in
narrow capillaries. This interruption in supply of
oxygen  localized anoxia (oxygen deprivation) in
the tissue  pain and eventually death
(infarction) of cells in vicinity of blockage.
Figure 3.21
Molecular and cellular events
leading to sickle cell crisis.
3. Variables that increase sickling:
- Extent of sickling & severity of disease is
enhanced by any variable that increases
proportion of HbS in deoxy state (i.e., reduces
affinity of HbS for oxygen)
- These variables include decreased oxygen
tension as a result of high altitude or flying in
non-pressurized plane, increased pCO2,
decreased pH, and an increased conc. of 2,3
BPG in RBCs
4. Treatment:
Involves adequate hydration, analgesics, aggressive
antibiotic therapy if infection present, & transfusions in
patients at high risk for fatal vasocclusions.
Intermittent transfusions with packed RBCs reduce
risk of stroke, but benefits must be weighed against
complications of transfusion, which include iron
overload (hemosiderosis), blood-borne infections,
and immunologic complications.
Hydroxyurea (an anti-tumor drug) decreases
frequency of painful crises & reduces mortality.
Mechanism of action not fully understood, but may
include a modest increase in fetal Hb, which
decreases sickling
5. Possible selective advantage of heterozygous state:
High freq of HbS gene among black Africans,
despite damaging effect in homozygous state,
suggests that a selective advantage for
heterozygous individuals
E.g., heterozygotes for sickle cell gene are less
susceptible to malaria, caused by Plasmodium
falciparum, which spends part of its life cycle in
RBC. As RBCs in heterozygotes, like those in
homozygotes, have shorter lifespan than normal,
parasite cannot complete the intracellular stage of
its development. This may provide selective
advantage to heterozygotes living in regions where
malaria is a major cause of death.
Figure 3.22
A. Distribution of sickle cell in Africa expressed
as percentage of the population with disease.
B. Distribution of malaria in Africa.
B. Hemoglobin C disease
• Also has a single aa substitution in 6th position of
β-globin chain; a Lys is substituted for Glu
• This substitution  HbC to move more slowly
toward anode than does HbA or HbS
• Patients homozygous for HbC generally have
mild, chronic hemolytc anemia. Do not suffer
from infarctive crises, and no specific therapy is
required
C. Hemoglobin SC disease
• Some β-globin chains have sickle cell mutation, other βglobin carry mutation found in HbC disease (compound
heterozygotes)
• Hb levels tend to be higher in HbSC disease than in
sickle cell disease, and may even be at low end of
normal range
• Clinical course of adults with HbSC disease differs from
that of sickle cell disease, in which patients generally
have painful crises beginning in childhood
• It is common for patients with HbSC to remain well (and
undiagnosed) until they suffer infarctive crisis. This crisis
often follows childbirth or surgery and may be fatal.
D. Methemoglobinemias
• Oxidation of heme component of Hb to ferric (Fe3+)
state forms methemoglobin, which cannot bind oxygen
• This oxidation may be caused by action of certain drugs,
e.g., nitrates, or endogenous products, e.g., reactive
oxygen intermediates
• The oxidation may also result from inherited defects,
e.g., certain mutations in α- or β-globin chain promote
formation of methemoglobin (HbM)
• Further, deficiency of NADH-cytochrome b5 reductase
(a.k.a NADH methemoglobin reductase), enzyme
responsible for conversion of methemoglobin (Fe3+) to
Hb (Fe2+)  accumulation of HbM
• RBCs of newborns have ~one-half capacity of
adults to reduce HbM. They are particularly
susceptible to effects of HbM-producing cpds
• Methemoglobinemias are characterized by
“chocolate cyanosis” (a brownish blue coloration
of skin and membranes) & chocolate coloredblood, as a result of dark-colored HbM.
• Symptoms are related to tissue hypoxia, and
include anxiety, headache, and dyspnea. In rare
cases, coma & death can occur.
E. Thalassemias
• Hereditary hemolytic diseases, imbalance in synthesis of
globin chains
• As a group, most common single-gene disorders in
humans
• Normally, synthesis of β- and α-globin chains are
coordinated, so each has a partner  formation of α2β2
(HbA). In thalassemia synthesis of either chain is
defective
• Can be caused by a variety of mutations, including entire
gene deletions, substitution or deletion of one to many
nt’s in DNA
• Each thalassemia can be classified as a disorder with no
globin chains produced (βº- or αº-thalassemia) or one in
which some synthesized, but at reduced rate (β+- or α+thalassemia)
β-Thalassemia:
• Synthesis of β-globin chains decreased or absent,
whereas α-globin chains synthesis is normal.
• α-globin chains cannot form stable tetramers and
precipitate  premature death of cells initially destined
to become mature RBCs
• Accumulation of α2γ2 (HbF) and γ4 (Hb Bart’s) also
occurs
• There are only 2 copies of β-globin genes  individuals
with β-globin gene defects have either β-thalassemia
trait (β-thalassemia minor) if only one copy defective, or
β-thalassemia major if both genes are defective
• As β-globin gene is not expressed until late in fetal
gestation, physical manifestations of β-thalassemia
appear only after birth.
• Individuals with β-thalassemia minor make some βglobin chains and usually do not require specific
treatment.
• However, infants born with β-thalassemia major seem
healthy at birth, but become severely anemic, usually
during 1st or 2nd yr of life. These patients require regular
transfusions of blood
• Although this treatment is lifesaving, cumulative effect of
transfusions is iron overload (a syndrome known
hemosiderosis) which typically causes death b/w ages
15-25 yrs
• Increasing use of BM replacement therapy has been a
boon to these patients
Figure 3.23
A. β-Globin gene
deletions in the βthalassemias.
B. Hemoglobin
tetramers
formed in βthalassemias.
α-Thalassemia:
• Synthesis of α-globin chains decreased or absent.
• As each individual’s genome contains 4 copies of α-globin gene, there
are several levels of α-globin chain deficiencies
• If 1 of the 4 genes is defective, individual is silent carrier of αthalassemia  no physical manifestations of disease occur
• If 2 α-globin genes are defective  α-thalassemia trait
• If 3 α-globin genes are defective  individual has hemoglobin H
(HbH) disease –a mildly to moderately severe hemolytic anemia
• Synthesis of unaffected γ- and then β-globin chains continues 
accumulation of γ tetramers in newborn (γ4, Hb Bart’s) or β tetramers
(β4, HbH)
• Although these tetramers are fairly soluble, the subunits show no
heme-heme interaction. Their oxygen dissociation curves are almost
hyperbolic  they have very high oxygen affinities  essentially
useless as oxygen deliverers to tissues
• If all 4 α-globin genes are defective, hydrops fetalis and fetal death
result, because α-globin chains are required for synthesis of HbF
Figure 3.24.
A. α-Globin gene
deletions in the
α-thalassemias.
B. Hemoglobin
tetramers formed in αthalassemias.
Summary
• HbA major adult Hb : α2ß2 held together by noncovalent interactions. Subunits occupy different relative
positions in deoxy- & oxyhemoglobin
• Deoxy- form = “T” or taut (tense) form. It has a
constrained structure that limits movement of polyp
chains
• T form is low oxygen affinity form.
• Binding of oxygen  rupture of some ionic & H-bonds 
R (relaxed form), subunits have more freedom of
movement
• R form is high oxygen affinity form
• Oxygen dissociation curve for Hb is sigmoidal (that of myoglobin is
hyperbolic) indicating subunits cooperate in binding oxygen i.e.,
binding to one heme increases affinity of remaining heme groups in
same Hb molecule
• Hb’s ability to bind oxygen reversibly is affected by pO2, and
availability of 2,3 BPG e.g., release of O2 from Hb is enhance when
pH lowered or oCO2 increased (Bohr effect), e.g., in exercising
muscle, & oxygen dissociation curve of Hb is shifted to right.
• To cope with long-term effects of chronic hypoxia or anemia, conc of
2,3 BPG in RBCs increases. 2,3 BPG binds Hb and decreases
affinity to oxygen  shifts curve to right
• CO binds tightly (but reversibly) to Hb iron,  carbon monoxyHb
(HbCO)
• Hemoglobinopathies are disorders of Hb caused by production of
structurally abnormal or synthesis of insufficient quantities of normal
Hb subunits, or rarely, both.
• HbS disease, HbC disease, & thalassemia syndromes are examples
and can have severe clinical consequences
Figure 3.25. Key concept map for hemoglobin structure and function.
Figure 3.26. Key concept map for hemoglobinopathies.