Download p(O 2 )

Oxygen binding by myoglobin (Mb)
Since O2 is a gas, we can replace the concentration [O2] by partial pressure p(O2)
[ Mb] p(O2 )
Kd 
[ MbO2 ]
p(O2 )
Y
[ K d  p(O2 )]
Exercise 1
Calculate the saturation curve for oxygen binding to myoglobin. Disociation
constant of the MbO2 complex at 37 °C, pH = 7, p = 760 Torr: Kd = 2.8 Torr.
p(O2) [Torr]
0.5
1
2
3
5
10
20
30
40
50
60
70
80
90
Y [%]
Y [%]
15.2
26.3
41.7
51.7
64.1
78.1
87.7
91.5
93.5
94.7
95.5
96.2
96.6
97.0
100
80
60
Exercise 2: Calculate the slope of the
saturation curve at p(O2) = 0.
Plot the slope dY/d[p(O2)] as a
function of p(O2)
40
20
0
0
30
60
p(O2) [Torr]
90
p(O2 )
Y 
[ K d  p(O2 )]
Kd
dY
1
p(O2 )



dp(O2 ) [ K d  p(O2 )] [ K d  p(O2 )]2 [ K d  p(O2 )]2
At p(O2) = 0
dY
1

dp(O2 ) K d
 Čím silnější afinita mezi Mb a O2, tím strmější křivka v bodě 0;0
-1
dY/dp(O ) [Torr ]
2
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0
20
40
60
p(O2) [Torr]
80
 Slope of the saturation curve decreases with p(O2)
100
Saturation curve for
hemoglobinu does not
correspond to a single
reversible reaction
Binding of O2 to one subunit
of hemoglobin increases the
affinity for O2 of the other
subunits
„Cooperative effect“
Cooperativity of oxygen binding by the 4 subunits of hemoglobin:
In deoxygenated form, the 4 subunits stabilize mutually the domed conformation.
The oxygen affinity of unloaded hemoglobin is smaller than that of individual
subunits. Oxygen binding to one subunit of hemoglobin favors the planar form
at neighboring subunits  fully loaded hemoglobin has an affinity similar to that
of an individual subunit.
http://www.chemistry.wustl.edu/~edudev/LabTutorials/Hemoglobin/MetalComplexinBlood.html
Effect of CO2 on oxygen afinity of hemoglobin: „Bohr-Effect“
In muscles, where metabolic activity produces CO2, amino groups
of certains amino acids are transformed to carbamate:
NH2
+
O
C
O
amino acid
amino acid
HN
N
+
H+
+
H+
O
The liberated H+ protonates histidine residues:
HN
O-
NH
N+
H
At subunit interfaces salt bridges are formed:
O
HN
N+
amino acid
H
O-
NH
These salt bridges favor the domed conformation  favor O2 release
 CO2 favors release of O2 which is then taken up by myoglobin
In muscles:
High CO2 concentration favors domed
conformation  favors O2 release
In bronchi:
Low CO2 concentration favors planar
conformation  favors O2 binding
http://www.chemistry.wustl.edu/~edudev/LabTutorials/Hemoglobin/MetalComplexinBlood.html