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Myoglobin
and
Hemoglobin Quaternary structure and allosteric properties of proteins
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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”
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)
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
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)
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
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
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
= pO2
pO2P50 +
Assume P50 = 10
pO2
1 0.092 0.175 0.33
10 0.5020 0.67
100 0.91
= pO2
pO2P50 +
n
n
n = number of binding sites on the protein
What about Hemoglobin?
For Hb, n = 4
100
50
pO2
Hemoglobin
P50
Region of Cooperativity
Sigmoidal Curve
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
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
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
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
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
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
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
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)
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