Biochemistry 410 Lecture 6 Vikas
Transcript of Biochemistry 410 Lecture 6 Vikas
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Biochemistry 410
Myoglobin and hemoglobin
Vikas Kumar
Bio-Bio Room 321
Office hours 2pm-4pmTue-wed & Fri
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Overview
• Myoglobin and Hemoglobin
• Structure
• Oxygen binding
• Cooperativity
• Allosteric effects
• Problems
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Myoglobin and Hemoglobin
• Most studied proteins in nature
• Both works as a oxygen carrier
• Myoglobin works as a oxygen storage and transport
protein in muscles
• Hemoglobin binds oxygen and transporting it throughout
the body through blood vessels
• Both are globular proteins consisting of heme as a
prosthetic group
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Structure
Myoglobin
Monomer
153 a.a
17.2kDa
Hemoglobin
Tetramer
α2β2
64.5kDa
Iron prefers to interact with 6 ligands
4 porphyrin N atoms
N from His F8
O2 is 6th
Heme
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Conformational change after O2 binding
• Without O2, Fe2+ is out of heme plane
• O2 pulls the Fe2+ into the heme plane
• Fe2+ pulls its His F8 ligand along with it
•This change means little to Mb, but lots
to Hb!
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Ligand BindingLigand = O2
We know
pO2
Y =
Kd + pO2
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The “affinity” constant, p50
50% of myoglobin is saturated with O2
pO2
Y =
p50 + pO2
• For Mb partial pressure of 2.8 torr is sufficient for half saturation
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Cooperative ligand binding in Hb
pO2n
Y =
p50 + pO2n
Hills
equation
The binding of O2 to Hb is cooperative – binding ofoxygen to the first subunit makes binding to the othersubunits more favorable.
p50=26 torr
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Hill Plot
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Allosteric effectors
• Oxygen is an allosteric effector as well as a ligand
• Binding of O2 to Hb is affected by several agents, including
• H+
• CO2
• 2,3-bisphosphoglycerate
• The effect of H+ is particularly
important. This is the Bohr effect.
• Deoxy-Hb has a higher affinity for
H+ than oxy-Hb.
• Thus, as pH decreases, dissociation
of O2 from hemoglobin is enhanced
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Allosteric effectors
CO2 has two effects:
1. CO2 diminishes O2 binding by decreasing pH.
• HCO3- is transported with the blood
back to lungs
• Hb binds O2 in the lungs releasing H+
• H+ reacts with HCO3- and CO2 is
released
2. Hb also acts as CO2 transportor
2,3-Bisphosphoglycerate as allosteric effector:
• Found in RBC’s
• Binds preferentially to deoxy-Hb form
• Binds in the central cavity of tetramer
• Negative charge interacts with 2Lys, 4His and
2N-termini
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ProblemA protein has binding affinity for its ligand (a peptide) of Ka = 2 105 M at
pH 5.0 and 25oC. At what concentration of the ligand is half of the protein
bound?
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ProblemWhat fraction of the Hb is bound at partial pressure of 1.25 torr (p50 = 5
torr)?
At what oxygen concentration will be 80% of the Hb bound?
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ProblemWhat conclusions about possible cooperativity of the binding can you
draw from these plots? What does different shape of the binding curves
tells you about the affinity of the ligand?