4B: Cooperativity and Allostery

Binding of oxygen to hemoglobin is cooperative. What is positive cooperativity?

binding at 1 site enhances binding at the next site.

what is the total movement of Fe2+?

• it is 0.029 nm - 0.29 A triggers a large global conformation change.

• small movement of the iron, less than a C-C bond causes leverage to take place.

Oxygen binding to hemoglobin results in a what?

• changes the structure of hemoglobin, and that makes it easier for oxygen to bind. Additional oxygen binding further increases the affinity.

• Subunit motion in hemoglobin when the molecule goes from the deoxy form to the oxy form.

When is hemoglobin able to bind to oxygen in the lungs?

When must hemoglobin be able to release oxygen in capillaries?

• When there is high pO2 (partial pressure about 100 torr) in the alveoli

• When there is low pO2 (-50 torr) in the venous blood

What happens when hemoglobin behaves like myoglobin?

very little oxygen would be released in the capillaries

where does gas exchange takes place?

Takes place in the alveoli in the capillaries. The capillaries are small enough that a single red blood cell can pass through it, and have gas exchange happen along its surface.

The sigmoidal nature of hemoglobin-oxygen binding allows for what?

large change in saturation within a small range of pO2. It is the partial pressure of oxygen range of 15% to 40%.

What is the pO2 (partial pressure) in the venous blood?

30 torr

What is the of arterial pO2 (partial pressure)?

100 torr

In addition, the entire oxygen binding curve can shift in response to physiological signals. What are these signals?

• As proton increases, pH goes down (H+) (pH)

• CO2

• Cl-

• Bisphosphoglygerate (BPG)

Shifting the curve, changing affinity

In the lungs what happens when hemoglobin binding curve shifts to the left?

This leads to hemoglobin having a higher affinity for O2 which results in oxygen binding.

Shifting the curve, changing affinity

In the lungs what happens when hemoglobin binding curve shifts to the right?

This leads to hemoglobin having a lower affinity for O2 which results in oxygen being released.

When it comes to the oxygen saturation curves for myoglobin and for hemoglobin at different pH values. What happens to proton concentration as we shift to the right?

Proton concentration increases and pH steadily drops.

What is the Bohr Effect?

• H+ promotes the dissociation of oxygen from hemoglobin.

• describes the decrease in the oxygen affinity of hemoglobin in the presence of low pH or high CO2.

The effect of H+ on O2 binding was discovered by?

Christian Bohr

What is the effect of the binding of protons?

It diminishes oxygen binding. The more proton the lower the affinity we have for O2 binding.

What is the effect of binding of oxygen?

It diminishes proton binding. If we have more O2, we will have less proton binding.

CO2 has a double effect.

what happens when CO2 shift the hemoglobin curve to the right?

• it lowers the affinity. It decreases the affinity for oxygen causing more release of O2

CO2 can decrease the affinity of hemoglobin for O2 in two ways: What are these ways?

• CO2 binds directly to hemoglobin form “carbaminohemoglobin” result in the release of oxygen.

• CO2 reacts with H2O to form carbonic acids, which dissociate to release H+, enhancing the Bohr effect results in oxygen release.

Chloride ion can also reduce the affinity for O2.

What happen when we have a high Cl-?

High Cl- leads to hemoglobin having a low affinity for oxygen which leads to O2 release.

What is the physiological significance of Cl- reducing affinity for O2?

High [H+], [Cl-] and pCO2 all results of metabolism

CO2 diffuses from tissue to RBC. Discuss CO2 transport in RBC.

1. CO2 binds to Hb

2. Reacts with H2O to make carbonic acid

3. HCO3- is exchange with Cl-

• This results in low O2 affinity for Hb

Allostery

So how do these molecules promote oxygen release?

• H+, Cl- and CO2 do not compete with the O2 binding site

• Instead, they bind far away from the heme center

What effect can binding of H+, Cl- and CO2 have?

It can change the conformation of Hemoglobin to a low-affinity conformation, releasing oxygen.

binding of H+, Cl-, and CO2 changes the conformation of Hb to a low-affinity conformation.

This is known as the Allosteric modulation. What is it?

Binding at one site, affects binding at a far away site.

What is negative allostery?

• binding at one site, decreases affinity at far away site.

• H+, Cl- and CO2 are all neg allosteric effectors

  • they decrease the affinity of Hb for O2

What is positive allostery?

Binding at one site, increases affinity at a far away site.

What does 2,3-Bisphosphoglycerate (BPG) do?

• Decrease hemoglobin affinity for oxygen. releasing O2

• facilitates the supply of oxygen to the tissues by binding to hemoglobin.

• BPG is produced when O2 demand is high.

Why is BPG (bisphosphoglygerate) considered a negative allosteric effector?

When is BPG produced?

• It binds at a site distant from the Fe where oxygen binds.

• Produced when O2 demand is high (high altitude)

What does BPG have a higher affinity for?

• It has a higher affinity for deoxyHb

• pulls the equilibrium towards deoxy form.

Ionic interactions between the phosphates of BPG and Hb are found where?

• Found in the deoxy-conformation of Hb

• Interaction: between the beta subunit.

Unlike adult hemoglobin (2α, 2β subunits) fetal hemoglobin have?

• 2α, 2y subunits (y pronounced gamma)

• Lower affinity for BPG which increase O2 binding

The only source of oxygen for the fetus is through what?

What must the fetal hemoglobin have a higher affinity for?

• Through the mother’s hemoglobin

• Fetal hemoglobin must have a higher affinity for O2 than the mother’s hemoglobin.

Lower affinity of fetal hemoglobin for BPG means?

less of the negative allosteric effect of BPG. As a result, oxygen moves from the mother to fetal hemoglobin.

2,3-BPG is less likely to bind to fetal hemoglobin than adult hemoglobin.

what is the structure of fetal hemoglobin?

Fetal hemoglobin lower affinity for BPG means?

• Fetal hemoglobin has four subunits 2 alpha and 2 gamma.

• higher affinity for O2

Why does fetal hemoglobin has a higher affinity for O2?

Fetal hemoglobin must pull oxygen from the adult hemoglobin of the mother. So, in order to do that, fetal hemoglobin must have a higher affinity for oxygen compared to adult hemoglobin.

What are some features of Sickle cell anemia?

• Mutation in beta chain of hemoglobin

• Position 6: Glu to Val (E6V mutation)

• Mutant hemoglobin is known as HbS

• Change from polar charged amino acid to non-polar (hydrophobic amino acid)

What is mutant hemoglobin known as?

Known as HbS

There is a hydrophobic pocket exposed only in the deoxy-Hb form. What fits in that pocket, and what is deoxy-Hb prone to?

• valine side chain fits into that hydrophobic pocket

• Deoxy-Hb is prone to polymerization.

What are some characteristics of sickle-cell anemia?

• Hb S molecules aggregate into long, chainlike polymeric structures

• Sickle-cell anemia patients have abnormally-shaped red blood cells.

• The erythrocytes are crescent-shaped instead of disc-shaped.

• The sickle cells pass less freely through the capillaries, impairing circulation and causing tissue damage.

What can sickle cell mutation provide protection against?

Where is sickle cell disease most common?

• Against malaria

• Even heterozygous individuals are protected.

• Most common in sub-Saharan African due to natural selection