myoglobin and hemoglobin quaternary structure and allosteric properties of proteins fe
TRANSCRIPT
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Myoglobin
and
Hemoglobin Quaternary structure and allosteric properties of proteins
Fe
FeFe
FeFe
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The importance of quaternary structure in proteins isbest illustrated by the binding of O2 to hemoglobin
Philosophy:
By comparing hemoglobin with myoglobin, (a singlesubunit protein), subunits can be seen to regulatethe strength of binding and give rise to a propertyof proteins called “cooperativity”
Cooperativity is the basis of another property of proteins called “allosterism” or “allostery”
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Binding of O2 to Myoglobin
Mb + O2 MbO2
= MbO2
MbO2 + Mb
[MbO2 ]
[Mb][O2]K =
Filled Sites
Total Sites
= the fraction of filled sites = YO2
Define (YO2)
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Derive an expression for as a function of O
We are asking: How do the sites filled vary with oxygenpressure?
[MbO2 ]
[Mb][O2]K =
MbO2 = K[Mb][O2]
= MbO2
Mb + MbO2
Mb + O2 MbO2
Sites Filled
Total Sites71% filled
10 out of 14
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Substitute the value for MbO2 into the equation
=K[Mb][O2]
K[Mb][O2] [Mb] + MbMb1
= K[O2]
1 + K[O2]KK
K
1K = or Kd = O2 pressure that half fills sites
= P50
When 1/K = [O2], = 0.5 (also called Kd)Therefore:
(P50 distinguishes the specific O2-binding protein)
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Redefining
O2 = pO2
Charles Law: Quantity of gas absorbed by a liquid is proportional to the partial pressure of thegas above the liquid.
= pO2
pO2P50 +
Note: Mb is left out of the final equation.
Note: Only connection to Mb is 1/K or P50
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P50
100
50
pO2
BINDING PARAMETERSO2
(x 100)
When O2 binds tomyoglobin, the bindingincrease is hyperbolicwith pressure increase
Two points on the binding curve are apparent:(1) the point of half saturation(2) the point of full saturation (never attained)
Full saturation
Half saturation
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P50
100
50
pO2
BINDING PARAMETERSO2
(x 100)
Weaker Binding
StrongBindingThe binding is considered weaker when it takes a greater oxygen pressure (larger P50)
to reach half-saturation
P50
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= pO2
pO2P50 +
Assume P50 = 10
pO2
1 0.092 0.175 0.33
10 0.5020 0.67
100 0.91
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= pO2
pO2P50 +
n
n
n = number of binding sites on the protein
What about Hemoglobin?
For Hb, n = 4
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100
50
pO2
Hemoglobin
P50
Region of Cooperativity
Sigmoidal Curve
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Why does Hemoglobin show Cooperative Binding?
Oxygen Transport
Inspired airAveolar air
Torrs158100
Arterial Blood 90Capillary 40Interstitial 30Cytosol 10
Binding
Release
Cooperativity is designed to RELEASE O2 at low Pressures
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EXAM 1BICH 410 (all sections) 100 pts
Tuesday, Oct 4, 11:10 – 12:00, Rm 108 Biochemistry
81/2 x 11 scantron (Blue)
Calculator
Chapters 1-5: Basics of energy and acid-base, amino acids, peptides, proteins, myoglobin, hemoglobin, allostery
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Protein-Ligand Interactions Revisited
P + L = PL
PL = P + L
Ka = [PL]
[P][L]
Kd = [P][L][PL]
Kd = 1/Ka = [L]0.5
orO = L+ L1/Ka
O = L+ LKd
Association
Disassociation
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P + nL = PLn Hb + 4O2 = Hb(O2)4
Ka = [PLn]
[P][L]n Ka = [Hb(O2)4]
[Hb] [O2]4
O = Ln
+ LnKd
O = pO24
+1/Ka pO24
Any Ligand Hemoglobin n=4
O = Ln
+ Ln1/Ka
O = pO24
+ Kd pO24
Where Kd = 1/Ka = [L]n0.5
= Pn50
Where Kd = 1/Ka = [O2]40.5
= P450
Multi-Ligand Interactions
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ALLOSTERIC BINDING
Rearranging the Equation
n log pO2 - nlog P50=
n = Hill Coefficient
pO2 1 –
=P50
n
n
log
1 –
O = pO2n
+ P50n pO2
n
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log
1 - n log pO2 - nlog P50=
y = m x + b
nH = 1.0 No Cooperativity
nH = > 1.0 Positive Cooperativity
nH = < 1.0 Negative Cooperativity
nH = n Full Cooperativity
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How is nH Determined
log
1 -
log pO2
Myoglobin
nH = 10
-3
+3
0 1.0-1.0
CooperativeTransition
nH = 3
HemoglobinLow Affinity
nH = 1
HemoglobinHigh Affinity nH = 1
Hemoglobin
R-State
T-State
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How is nH Determined
log
1 -
log pO2
Myoglobin
nH = 10
-3
+3
0 1.0-1.0
nH = 3
HemoglobinLow AffinitynH = 1
HemoglobinHigh Affinity
The higher the O2 pressure, the stronger the binding, (binding mode)The lower the O2 pressure, the weaker the binding, (release mode)