hb.ppt

Structure and function of
hemoglobin
GIT | 1 Lecture | Dr. Usman Ghani

Hemoglobin (Hb)
 A hemeprotein found only in red blood cells
 Oxygen transport function
 Contains heme as prosthetic group
 Heme reversibly binds to oxygen

The heme group
 A complex of protoporphyrin IX and ferrous
iron (Fe2+)
 Fe2+ is present in the center of the heme
 Fe2+ binds to four nitrogen atoms of the
porphyrin ring
 Forms two additional bonds with:
 Histidine residue of globin chain
 Oxygen

The heme group: Fe2+– porphyrin complex with bound O2

Types of Hb
Normal: HbA (97%)
HbA2 (2%)
HbF (1%)
HbA1c
Abnormal: Carboxy Hb
Met Hb
Sulf Hb

Hemoglobin A (HbA)
 Major Hb in adults
 Composed of four polypetide chains:
 Two α and two β chains
 Contains two dimers of ab subunits
 Held together by non-covalent interactions
 Each chain is a subunit with a heme group in
the center that carries oxygen
 A Hb molecule contains 4 heme groups and
carries 4 moelcules of O2

T-form of Hb
 The deoxy form of Hb
 Taut form
 The movement of dimers is
constrained
 Low-oxygen-affinity form

R-form of Hb
 The oxygenated form of Hb
 Relaxed form
 The dimers have more
freedom of movement
 High-oxygen-affinity form

Hemoglobin function
 Carries oxygen from the lungs to tissues
 Carries carbon dioxide from tissues back to
the lungs
 Normal level (g/dL):
• Males: 14-16
• Females: 13-15

Factors affecting oxygen binding
 Three allosteric effectors:
 pO2 (partial oxygen pressure)
 pH of the environment
 pCO2 (partial carbon dioxide pressure)
 Availability of 2,3-bisphosphoglycerate

Oxygen Dissociation Curve
 The curve is sigmoidal
 Indicates cooperation of
subunits in O2 binding
 Binding of O2 to one heme
group increases O2 affinity
of others
 Heme-heme interaction

P50
 Indicates affinity of Hb to O2
 P50(mm Hg): the pressure at which Hb is 50%
saturated with O2
 High affinity  slow unloading of O2
 Low affinity  fast unloading of O2
 Lung pO2 is 100 mm  Hb saturation 100%
 Tissue pO2 is 40 mm  Hb saturation reduces
 Hence O2 is delivered to tissues

The Bohr effect
 Effect of pH and pCO2 on:
 Oxygenation of Hb in the
lungs
 Deoxygenation in tissues
 Tissues have lower pH
(acidic) than lungs
 Due to proton generation:
 CO2 + H20  HCO3
- + H+
 Protons reduce O2 affinity
of Hb

The Bohr Effect
 Causing easier O2 release into the tissues
 The free Hb binds to two protons
 Protons are released and react with HCO3 – to form
CO2 gas (HCO3
- + H+CO2 + H20 )
 The proton-poor Hb now has greater affinity
for O2 (in lungs)
 The Bohr effect removes insoluble CO2 from
blood stream
 Produces soluble bicarbonate

Availability of 2,3 bisphosphoglycerate
 Binds to deoxy-hb and stabilizes the T-form
 When oxygen binds to Hb, BPG is released
At high altitudes:
-RBC number increases
-Hb conc. increases
-BPG increases

High altitude and O2 affinity
 In hypoxia and high altitude
 2,3 BPG levels rise
 This decreases O2 affinity of Hb
 Thus increases O2 delivery to tissues

High O2 affinity
High O2 affinity is due to:
 Alkalosis
 High levels of Hb F
 Multiple transfusion of 2,3 DPG-depleted
blood

Low affinity to O2
High affinity to O2

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

HbA2
 Appears ~12 weeks after birth
 Constitutes ~2% of total Hb
 Composed of two a and two d globin chains

HbA1c
 HbA undergoes non-
enzymatic glycosylation
 Glycosylation depends on
plasma glucose levels
 HbA1c levels are high in
patients with diabetes
mellitus

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)

hb.ppt

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