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Human erythrocytes
(red blood cells)
Erythrocytes are small disk-shaped
cells in the blood.
They have lost their intracellular
organelles, can not reproduce and
survive about 4 months.
Erythrocytes contain 34%
hemoglobin by weight.
Blood is oxygenated at the lungs,
where Hemoglobin is 95%+ saturated
with O2.
As blood passes through the tissues,
hemoglobin releases O2 and is now as
low as 35% saturated with O2.
Lung pH = 7.6
Tissue pH = 7.2
Hemoglobin releases
CO2 at lungs and binds O2.
Hemoglobin releases O2 and binds CO2, H+ in extremities
Bohr Effect
Hemoglobin carries oxygen from the lungs to the tissues for
cellular respiration.
Hemoglobin carries CO2 and H+, byproducts of cellular respiration,
back to the lungs and kidneys for excretion.
In erythrocytes CO2 is converted to HCO3- by carbonic anhydrase.
CO2 + H2O
H+ + HCO3-
This results in a decrease in pH in the tissues.
At low pH hemoglobin binds H+ and CO2 and the affinity for O2 decreases.
O2 is therefore released in the tissues as CO2 and H+ are bound.
Regulation of O2 binding by 2,3-Bisphosphoglycerate
The molecule 2,3-bisphosphoglycerate
(BPG) is bound by hemoglobin and lowers
the affinity for O2.
If there is an increase in BPG in the blood,
hemoglobin will have a lower O2 affinity
and will release more O2 in the tissues.
At high altitudes where O2 is depleted we
produce more BPG to deliver more oxygen
to the tissues.
BPG binds to hemoglobin at a site distant from
the oxygen binding site of hemoglobin.
Fraction of hemoglobin with bound oxygen
BPG lowers
the oxygen
affinity of
Fetal Hemoglobin
A fetus must extract O2 from the mother’s blood.
A fetus has g hemoglobin subunits instead of b subunits.
The a2g2 fetal hemoglobin has a lower affinity for BPG than
a2b2 adult hemoglobin
Therefore a2g2 fetal hemoglobin has a higher affinity for O2.
Normal Erythrocytes
Sickle-cell anemia
Glu 6
Glu 6
Val 6
Val 6
In sickle-cell anemia
hemoglobin S molecules
aggregate and form
insoluble crystalline fibers.
This gives the erythrocyte a
“sickle” shape
State whether each of the following will increase or decrease the
affinity of hemoglobin for oxygen.
Change in blood pH from 7.2 to 6.8
Change in pCO2 from 36 torrs to 12 torrs
Change in the blood 2,3-bisphosphoglycerate [BPG] concentration
from 2 x 10-4 M to 8 x 10-4 M
Climbing up Mount Everest from 1,000 meters above sea level to
4,000 meters above sea level
Change in blood proton [H+] concentration from
1.8 x 10-7 M to 2.1 x 10-6 M
Indicate whether each of the following statements about
hemoglobin and myogobin is true or false.
Hemoglobin is present in red blood cells (erythrocytes)
Myoglobin is abundant in mammalian muscle tissue.
Myoglobin is a tetrameric protein with four protein subunits.
Hemoglobin is a structural protein whose function is to give erythrocytes
their disk shape.
Hemoglobin has four identical protein subunits, each designated as alpha (a).
Hemoglobin and myoglobin both contain a prosthetic group called a heme.
Hemoglobin utilizes a bound zinc (Zn2+) ion to bind oxygen.
Hemoglobin has a higher affinity for oxygen at pH 7.2 than at pH 7.6.
Hemoglobin has a lower affinity for oxygen when CO2 is present in high
concentrations in the blood.
Challenge Question
Myoglobin and the individual subunits of hemoglobin are
similar in size, shape, and function.
Would you expect a molecule of myoglobin or hemoglobin to
have a greater ratio of nonpolar to polar amino acids?