Myoglobin and Hemoglobin

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• Myoglobin and Hemoglobin

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Lecture Outline

• Transport of O2 by myoglobin and hemoglobin
• Cooperativity of O2 binding by Hb
• Allosteric regulation

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Transport of O2 by myoglobin and Hb

• Problem Oxygen has a low solubility in H2O

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• Solution Use proteins to transport oxygen from
  the lungs to other parts of the body

Hb is used to transport O2 in the blood stream
Myoglobin is used to transport O2 in muscle
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The heme prosthetic group binds iron
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Iron has two oxidation states
Ferrihemoglobin (Fe3), also known as
methemoglobin, does not bind O2
Ferrohemoglobin (Fe2) binds O2
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Oxygen binds to Fe2 in heme
A cross-sectional view of the heme group
(shown in gray), showing the binding of His 93
and O2 to Fe2.
Binding of O2 to Fe2 in heme, looking down
the O2-Fe2 bond
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Why doesnt free heme transport O2?
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• When heme is free in solution, it can bind
  O2, but in the presence of a second heme
  molecule, O2 will oxidize Fe2 to Fe3, so the
  first heme no longer binds O2. This process
  continues until all of the heme iron molecules
  have been oxidized.
• One function of myoglobin and hemoglobin is
  to physically separate the heme molecules, so
  oxygen does not oxidize the Fe2.

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The heme binding sites in myoglobin and
hemoglobin prevent oxidation of Fe2 by O2
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The distal histidine reduces the affinity of heme
for carbon monoxide
Carbon monoxide binding to heme in the absence of
the distal histidine
Oxygen binding to heme in the presence of
the distal histidine
Carbon monoxide binding to heme in the presence
of the distal histidine
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• In H2O, the binding of carbon monoxide (CO)
  to heme is 25,000 times stronger than the binding
  of oxygen to heme. In myoglobin and hemoglobin,
  carbon monoxide binds 200 times tighter to heme
  than does oxygen.
• The decreased affinity of heme in myoglobin
  and Hb for carbon monoxide is physiologically
  important, since CO is produced endogenously in
  cells during degradation of the heme groups. In
  addition, this is a good illustration of how a
  protein can alter the properties of a group (in
  this case heme) by altering its surrounding
  environment.

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Cooperativity of O2 binding by hemoglobin
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Hemoglobin is composed of four polypeptide chains
a2b2

• Hb is composed of two a chains a 1 (red)
  and a 2 (green) and two b chains b1 (orange)
  and b2 (blue).
• One can think of hemoglobin as a dimer of
  dimers a 1 b 1 and a 2 b 2.

a1b1
(a1b1)(a2b2)
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The a and b chains of hemoglobin have structures
similar to myoglobin
myoglobin
b-chain of hemoglobin
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• Although only 24 of the 141 amino acids are
  identical in myoglobin and the a and b chains of
  hemoglobin, the structures of these three
  polypeptides are very similar.
• This is a good example of how different primary
  sequences can encode very similar
  three-dimensional structures.

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Oxygen binds to hemoglobin in a cooperative manner
The binding of oxygen to myoglobin follows a
hyperbolic curve, while the binding of oxygen to
Hb follows a sigmoidal curve.
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• The sigmoidal nature of the oxygen binding
  curve for Hb suggests that the binding of one
  oxygen molecule to Hb increases its affinity for
  binding additional oxygen molecules. This effect
  is known as cooperative binding and is often
  observed in multisubunit proteins.
• The oxygen binding sites in Hb talk to one
  another, so when one site binds O2, it makes it
  easier for the other sites to bind O2.

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Oxygen binding induces a conformational change in
Hb
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• In the absence of bound O2, the Hb subunits
  are in a low affinity state (also known as the
  taut, or T state).
• High affinity state of hemoglobin (also known
  as the relaxed or R state).

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Each subunit in hemoglobin can exist in either a
high affinity (R) or low affinity (T) state
T state
R state

• T state no O2, low affinity binding of O2
• R state high affinity binding of O2

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What is the physiological significance of
cooperativity in hemoglobin?
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• Case 1 no cooperativity
• In alveoli of lungs, pO2100 torr
• 79 of the O2 binding sites in Hb are filled
• In muscle, pO220 torr
• 43 of the O2 binding sites in hemoglobin are
  filled
• Fraction of O2 delivered to muscle79-4336

• Case 2 cooperativity
• In alveoli of lungs, pO2100 torr
• 98 of the O2 binding sites in Hb are filled
• In muscle, pO220 torr
• 32 of the O2 binding sites in hemoglobin are
  filled
• Fraction of O2 delivered to muscle98-3266

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From the previous slide, we see that
The cooperative binding of O2 allows Hb to
deliver 1.83 times more O2 to the muscle cells
under physiological conditions than would be
delivered if the O2 binding sites in Hb were
independent of each other.
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The Bohr Effect H and CO2 promote the release
of O2 from hemoglobin
In contracting muscle, both CO2 and H are
produced. Elevated levels of CO2 and H in the
capillaries of muscle promote release of O2 from
Hb. In lungs, high O2 levels promote the release
of CO2 and H from Hb. The Bohr effect is
specific for Hb, it is not observed in myoglobin.
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CO2 binds to the terminal amino groups in
hemoglobin
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CO2 binds outside the O2 binding site in Hb
CO2 lowers the O2 affinity of Hb by binding
to a site outside of the oxygen binding site.
This type of regulation is known as allosteric
regulation.
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2,3-bisphosphoglycerate lowers the affinity of Hb
for oxygen
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Fetal Hb (a2g2) has a higher affinity for O2 than
maternal (adult) Hb
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Summary

• Transport of O2 by myoglobin and Hb
• Role of heme prosthetic group
• Oxygen binding curves
• Cooperativity of O2 binding by Hb
• Physiological significance

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Summary

• Allosteric activation and inhibition
• Bohr effect
• 2,3-bisphosphoglycerate
• Other types of hemoglobin
• Fetal hemoglobin