Biochem Lecture 9

Hemoglobin carries/transports

two end products of cellular respiration: H+ and CO2

Structural effects of H+ and CO2

binding to hemoglobin favor the T state (stabilize T state → lower the binding affinity to O2)

• the binding of H+ and CO2 is inversely related to the binding of O2

Bohr Effect

describes the effect of pH and [CO2] on the binding and release of O2 by hemoglobin

The Effect of pH on O2 Binding to Hemoglobin

when [O2] is high, hemoglobin binds O2 and releases H+

when [O2] is low, hemoglobin releases O2 and binds H+

• Lower pH (in tissue) further reduces binding affinity of Hb to O2 (pH 7.2 vs 7.6), facilitating release of O2.

Actively metabolizing tissues

generate H+, lowering the pH of the blood near the tissues relative to the lungs

Hemoglobin affinity for oxygen

depend on pH

H+ binds to Hb and stabilizes the T state.

• protonates His146, which then forms a salt bridge with Asp94

• leads to the release of O2 (in the tissues)

The pH difference between lungs and metabolic tissues

increases efficiency of the O2 transport. known as Bohr effect of how H+ and CO2 affects Hb

Hemoglobin binds and delivers

CO2

CO2 is produced

by metabolism in tissues and must be exported

15–20% of CO2 is exported in the form of a carbamate on the amino terminal residues of each of the polypeptide subunits.

Production of H+ by hydration of CO2 and carbamate

forms additional salt bridges

• stabilizing the T state, decreasing the binding affinity for oxygen

2,3-Bisphosphoglycerate regulates

O2 binding

• “Negative heterotropic” regulator of Hb function

• Present at mM concentration in erythrocytes

• small negatively charged molecule binds to the positively charged central cavity of Hb

• Stabilizes the T states and binds to a site distant from O2 binding site which greatly reduces the affinity of hemoglobin for oxygen

Effect of BPG

with O2 binding to hemoglobin, BPG increases at high altitudes

• hypoxia = lowered oxygenation of peripheral tissues so BPG increases

Binding of BPG

Binds to deoxyhemoglobin

BPG binds to the cavity between the B subunits in the T state

• cavity is lined with positively charged residues

• BPG stabilizes the T state

Binding of CO to Hb

CO has similar size and shape to O2; it can fit to the same binding site

• CO binds “heme” (protein-free heme) over 20,000 times better than O2 because the carbon in CO has a filled lone electron pair that can be donated to vacant d-orbitals on the Fe2+

• The protein pocket surrounding heme (distal histidine) decreases affinity for CO, but it still binds about 250 times better than oxygen (steric hindrance).

CO is

highly toxic, as it competes with oxygen. It blocks the function of myoglobin, hemoglobin, and mitochondrial cytochromes that are involved in oxidative phosphorylation.

CO Poisoning

exposure to large quantities of CO (from gas appliances, running

automobiles) will lead to CO poisoning

• Hb bound to CO is called carboxyhemoglobin which could be fatal - CO poisoning (asphyxiation)

• Hypoxia (blood cannot deliver O2 effectively)

CO Poisoning is treated by

• administration of 100% oxygen, often at pressures greater than atmospheric pressure (hyperbaric oxygen therapy)

• Increases partial pressure of oxygen in blood substantially to displace CO from Hb

Why is carboxyhemoglobin (CO poisoning) fatal?

• High affinity of CO for heme will displace oxygen from heme preventing its delivery

• CO binding to one site in Hb increases the affinity for oxygen, preventing its dissociation at tissues (left shift in binding curve)

CO poisons not only just by taking up space that would normally be used to transport O2, but also by preventing the O2 that is transported from being released where it is needed.

Fetal Hemoglobin (HbF)

fetus synthesizes α2γ2 hemoglobin (gamma subunit instead of beta subunit

which has a higher affinity for O2 than normal adult hemoglobin

• helps fetal blood extract oxygen from maternal blood across the placenta

• Higher affinity for CO than normal adult hemoglobin and is more sensitive to CO posioning

Fetal hemoglobin has a lower affinity for

BPG than normal adult hemoglobin

Drugs like

hydroxyurea are used to induce gamma-globin expression to increase HbF levels