Oxygen binding by myoglobin (Mb)

Since O2 is a gas, we can replace the concentration [O2] by partial pressure p(O2)

)]([

)(

2

2

OpK

OpY

d

][

)(][

2

2

MbO

OpMbKd

0

20

40

60

80

100

0 30 60 90

Y [%]

p(O2) [Torr]

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)

p(O2) [Torr] Y [%]0.51235102030405060708090

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.

15.226.341.751.764.178.187.791.593.594.795.596.296.697.0

)]([

)(

2

2

OpK

OpY

d

22

22

2

22 )]([)]([

)(

)]([

1

)( OpK

K

OpK

Op

OpKOdp

dY

d

d

dd

dKOdp

dY 1

)( 2

At p(O2) = 0

Čím silnější afinita mezi Mb a O2, tím strmější křivka v bodě 0;0

Slope of the saturation curve decreases with p(O2)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0 20 40 60 80 100p(O2) [Torr]

dY/dp(O2) [Torr

-1]

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 individualsubunits. Oxygen binding to one subunit of hemoglobin favors the planar format neighboring subunits fully loaded hemoglobin has an affinity similar to thatof 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 groupsof certains amino acids are transformed to carbamate:

amino acidNH2 O C O+

amino acidNH O-

O

+ H+

The liberated H+ protonates histidine residues:

HNN + H+

HNN+ H

At subunit interfaces salt bridges are formed:

amino acidNHO-

O

HNN+ H

These salt bridges favor the domed conformation favor O2 release CO2 favors release of O2 which is then taken up by myoglobin

http://www.chemistry.wustl.edu/~edudev/LabTutorials/Hemoglobin/MetalComplexinBlood.html

In muscles:High CO2 concentration favors domed conformation favors O2 release

In bronchi:Low CO2 concentration favors planar conformation favors O2 binding