Hemoglobin, An Allosteric Protein

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Hemoglobin, An Allosteric Protein

• Hemoglobin and heme
• Myoglobin and Hemoglobin
• Hill Coefficient
• Cooperative binding of oxygen
• BPG effect and Bohr effect
• Variants of homoglobin

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Protoporphyrin and Heme

• Vampire
• Chlorophyll
• Cancer

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Myoglobin and Hemoglobin

• Both proteins contain hemes
• Myoglobin, storage for oxygen monomeric protein
• Hemeglobin, oxygen transporter four subunits
  with 2a and 2b subunits.

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Hemoglobin
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Hill Coefficient

• A measure of cooperativity in a binding process.
• For Myoglobin (Mb), the Hill Coefficient is 1.
• Mb(O2) Mb O2
• Y 1-Y pO2
• K (1-Y)pO2/Y (K 2 Torr)
• At Y 0.50, K p(50)
• p(50) (1-Y)pO2/Y
• Y/(1-Y) PO2/P(50)

One torr, unit of pressure, is defined as the
pressure exerted by a millimeter depth of mercury
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Hill Coefficient

• For Hemoglobin (Hb), the Hill Coefficient is gt1.
• Hb(O2)n Hb nO2
• Y 1-Y pO2
• K (1-Y)(pO2)n/Y (K 26 Torr)
• At Y 0.50, K p(50) n
• P(50)n (1-Y)(pO2)n/Y
• Y/(1-Y) (PO2/P(50))n
• Log Y/(1-Y) n Log(pO2/p50)

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Hill Coefficient
Myoglobin n1
Hemoglobin n2.8
Log (ratio of Y/(1-Y))
Log (pO2)
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Myoglobin and Hemoglobin Have Different Oxygen
Binding Properties
Hyperbolic
Sigmoid
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Allosteric Effects

• Homotropic effect
• Oxygen binding induces a dramatic structural
  change
• Heterotropic effect
• 2,3-bisphosphoglycerate
• pH
• CO2

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The iron center in heme is outside the plane of
the porphyrin
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Oxygen Binding Pulls the Iron to the Plane of the
Porphyrin
NO, CO
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Quaternary Structure of Hemoglobin
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T State and R State
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Oxygen Induced Conformational Changes in
Hemoglobin
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Concerted or Sequential?

• Concerted model
• First three oxygen binding changes the T-state to
  R-state, and increases the binding affinity of
  the last site by 20 fold.
• Sequential model
• First oxygen binding does not change the T-state
  to R-state but the binding affinity of the
  second site increases by 3 fold.

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Concerted Model
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Sequential Model
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Heterotrophic Effect 2,3-BPG

• In vivo, the K 26 torrs
• In vitro, the K 1 torr
• In 1967, Benesch discovered 2,3-BPG

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Heterotrophic Effect 2,3-BPG
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Heterotrophic Effect 2,3-BPG
2,3-BPG
2,3-BPG stabilizes T state
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Bohr Effect
In 1904, Bohr discovered High CO2 and H
stimulate O2 release from Hb. Late, Haldane
discovered High O2 stimulates CO2 and H
release from Hb
In lung, High concentration of O2 helps release
CO2 from Hb. In muscle. High concentration of
CO2 and H helps release O2 from Hb
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Bohr Effect
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Bohr Effect
Low pH
Negative charge in Asp 94
Stabilize T-state
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Bohr Effect
Negative charge
Stabilize interface of a/b
Stabilize T-state
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Bohr Effect
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Variants of Hemoglobin

• Hb-A a2b2
• Hb-F a2g2, which has low affinity for 2,3-BPG

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Variants of Hemoglobin

• Hb-S sickle-cell hemoglobin
• A single amino acid mutation in b-chain (Glu6 ?
  Val6)

One in twelve African Americans has sickle-cell
trait.
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Malaria and Sickle Cell Disease

• Malaria and sickle cell disease are almost
  overlapped in Africa population
• Hb Malaria
• Hb-A homozygotes Vulnerable
• Hb-S heterozygotes More resistance
• Hb-S homozygotes Sickness of Hb-s

http//sickle.bwh.harvard.edu/scd_background.html
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Malaria Disease

• Malaria is caused by a Protozoan parasite (e.g.
  Plasmodium falciparum) living in blood cells
• One of the most serious diseases
• Kills 1-3 million each year.

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Malaria Disease
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Selection of Sickle Trait in Endemic Malarial
Areas
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Natural Selection
1. Infection of parasites depletes oxygen in
red blood cells, resulting sickle cells. 2.
Sickle cells generate more free radicals to
kill parasites.