Notes
Hemoglobin Allosteric Effects
Sections
Overview
Definitions
Bohr effect
- Low pH enhances hemoglobin oxygen dissociation
2, 3 Bisphosphoglycerate (BPG)
- Molecule localized in red blood cells
- Decreases hemoglobin's oxygen affinity
Dissociation curve
- % oxygen saturation vs. oxygen partial pressure (torr)
- Cooperative binding produces sigmoidal binding curve
Bohr Effect
- Decrease in blood pH shifts curve to the right
- Hemoglobin requires greater pO2 in peripheral tissues to reach 50% saturation
- Lowering pH decreases hemoglobin's oxygen affinity
2,3 BPG
- Hemoglobin without 2,3 BPG: shifts curve to the left (hyperbolic like myoglobin)
- Adding 2,3 BPG: shifts curve back to the right.
BOHR EFFECT
Beta subunit
- Aspartate (-) and histidine (+)
T-form: histidine pKa = 8.0 (side chains close to each other)
- T-form favored when blood pH --> has a high affinity for H+
R-form: histidine pKa drops to 7.1 (side chains move apart)
- Histidine loses H+
- R-form favored when blood pH is high and H+ concentration is low
Carbon dioxide
- CO2 + H2O --> H2CO3 (carbonic acid) --> HCO3- (bicarbonate) + H+ (reversible)
- Carbonic anhydrase catalyzes carbonic acid formation
- Carbonic acid spontaneously loses proton to form bicarbonate
- Bicarbonate = blood buffer
- Increase in CO2 --> lowers blood pH --> favors T-form hemoglobin
Carbaminohemoglobin: CO2 binds N-terminal amino acids in hemoglobin
2,3 BPG
- Has strong negative charge: binds central cavity in hemoglobin
- Stabilizes the T-form (only binds T-form)
CARBON MONOXIDE
- Binds iron center with 220 times the affinity of O2 (irreversible)
- Permanently increases oxygen affinity of remaining heme groups for oxygen
- Decreases oxygen release in peripheral tissues
CLINICAL CORRELATION
Acetazolamide
- Carbonic anhydrase inhibitor used to treat altitude sickness
- Increases bicarbonate excretion by kidneys
- Makes blood more acidic, promotes oxygen release in peripheral tissues
High altitude conditions
- Individuals adapted to high altitude produce more 2,3 BPG
- Favors T-form hemoglobin and O2 release: more efficient O2 delivery
Tobacco smoke
- Smokers have elevated blood CO: hinders O2 delivery
- Can produce tissue hypoxia
Full-Length Text
- Here we will learn about the Bohr effect, and discuss the effects of 2,3 bisphosphoglycerate and carbon monoxide on hemoglobin.
- To begin, start a table to list out each of the key topics we'll learn.
- Bohr effect, which describes the effects of pH and carbon dioxide on hemoglobin binding.
- 2,3 Bisphosphoglycerate (BPG), a molecule localized in red blood cells that decreases hemoglobin's affinity for oxygen.
- Carbon monoxide, which increases hemoglobin's oxygen affinity and can produce toxic effects in the body.
- As a review, draw hemoglobin's sigmoidal dissociation curve.
- Label the x-axis partial pressure and the y-axis percent saturation.
Now, let's illustrate the first allosteric effect: the Bohr Effect.
- Draw another sigmoidal curve to the right of the first one.
- Show that a decrease in blood pH, shifts hemoglobin's dissociation curve to the right.
- What does this mean?
- Extend the horizontal line that demarcates 50% saturation.
- Show that it intersects the second curve at a greater partial pressure of oxygen: lowering pH decreases hemoglobin's affinity for oxygen.
- To better understand this, draw the three-dimensional structure of hemoglobin: with two alpha subunits and two beta subunits.
Let's take a closer look at a beta-subunit.
- Within it draw two functional groups: that of aspartate (negative) and histidine (positive).
- Indicate that in T-form hemoglobin, these side chains are close to each other, which raises the pKa of histidine to 8.0.
- Thus, hemoglobin has a high affinity for protons in the deoxygenated state (high pKa means a high proton affinity).
Now, let's draw these groups in R-form hemoglobin.
- Show that they are farther apart, and that the pKa of histidine drops to 7.1.
- As we have seen, the transition between T-form and R-form hemoglobin causes shifts in amino acid conformations throughout the entire protein.
- Show that because its pKa decreases, histidine loses a proton.
- Thus, write that R-form is favored when blood pH is high, and the proton concentration is low.
- Now, write that T-form is favored when the blood pH is low, and the proton concentration is high.
- Thus, a low pH enhances oxygen dissociation and shifts hemoglobin's dissociation curve to the right.
Now, we know that a low pH enhances oxygen dissociation. But what produces low blood pH in the first place?
- Carbon dioxide!
- Write out the following equation:
- Carbon dioxide plus water reversibly converts to carbonic acid.
- Show that the enzyme carbonic anhydrase catalyzes this reaction.
- Next, indicate that carbonic acid spontaneously loses its proton to form bicarbonate.
- Thus bicarbonate releases protons into the blood stream.
- Indicate that bicarbonate functions as a buffer in the blood.
- As a clinical correlation, write that acetazolamide is a carbonic anhydrase inhibitor that is often used to treat altitude sickness.
- How does it work? It produces an increase in bicarbonate excretion by the kidneys, via a mechanism we will not cover here.
- As a result, it makes the blood more acidic, and promotes the release of oxygen in the peripheral tissues.
- Finally, write that an increase in carbon dioxide in the body lowers blood pH and favors T-form hemoglobin.
- This facilitates hemoglobin's physiologic function in the body.
- Note that carbon dioxide can also form carbaminohemoglobin by binding to N-terminal amino acids in hemoglobin. We will not discuss this, here.
Now that we have learned the Bohr effect, let's move on to 2,3-bisphosphoglycerate (BPG), a molecule synthesized by red blood cells.
Our current sigmoidal dissociation curve already accounts for the allosteric effects of normal 2,3-BPG levels in red blood cells.
- To visualize 2,3-BPG's effects, draw a hyperbolic curve to the left of the hemoglobin curve.
- It should resemble myoglobin's dissociation curve.
- Indicate that this curve represents hemoglobin without 2,3-BPG.
- Thus, without 2,3-BPG, hemoglobin's oxygen affinity increases dramatically.
- Show that adding 2,3-BPG shifts the curve back to the right.
How does 2,3-BPG lower hemoglobin's oxygen affinity? Let's illustrate this, now.
- To do this, take a closer look at the center of our hemoglobin molecule: the cavity created at the intersection of all four subunits.
- Draw histidine residues at the periphery of each of the beta subunits.
- Show that these histidines are positively charged; they repel each other.
- Now, importantly, label this diagram the T-form (deoxygenated hemolgobin).
Now, let's add 2,3-BPG.
- Draw a 2,3-BPG molecule within this central cavity.
- Indicate that it has a strong negative charge, which stabilizes the T-form.
- Other positively charged amino acid side chains also bind 2,3 BPG here, but we won't draw all of them.
- Now, write that 2,3-BPG only binds the T-form.
- It encourages oxygen dissociation, and facilitates oxygen delivery!
- As a clinical correlation, indicate that individuals that have adapted to high altitude conditions produce more BPG.
- Why? More BPG favors T-form hemoglobin and oxygen release; it allows hemoglobin to deliver more oxygen to the peripheral tissues.
Finally, let's illustrate carbon monoxide.
- Draw a simplified hemoglobin iron center.
- Show that carbon monoxide irreversibly binds it.
- We draw it at an angle because a distal histidine causes it to bend.
- Write that carbon monoxide binds iron with an affinity 220 times greater than oxygen!
- Indicate that by irreversibly binding iron, it permanently increases the oxygen affinity of the remaining heme groups for oxygen.
- Thus, it pushes the hemoglobin dissociation curve to the left.
What are the physiological consequences?
- Indicate that carbon monoxide leads to decreased oxygen release in the peripheral tissues.
- As a clinical correlation, write that people who regularly smoke tobacco have elevated levels of carbon monoxide in their blood, which hinders hemoglobin's ability to deliver oxygen.
- Thus, a consequence of elevated carbon monoxide is tissue hypoxia.