Myoglobin Structure and Function
Oxygen-Hemoglobin Dissociation Curve | How pH, CO and CO2 Affect it
The Oxygen-Hemoglobin Dissociation curve describes the oxygen saturation of hemoglobin against the partial pressure of O2 in the blood. It is affected by several influences like an acidic or alkaline environment, CO and CO2.
- As O2 enters the vial of blood, the plasma pO2 increases and more O2 binds with hemoglobin.
Hb + O2 <———–>HbO2
- The reaction also causes the color of the red blood cells (RBCs) in the vial to change from purple to red.
- As the pO2 approaches 100 torrs (or mmHg), the hemoglobin molecules become nearly fully saturated.
Hb + 1 O2 = 25% saturation
Hb + 2 O2 = 50% saturation
Hb + 3 O2 = 75% saturation
Hb + 4 O2 = 100% saturation
The O2-Hb relationship is sigmoidal (or s-shaped) and not linear. (see the image below)
The upper end of the curve is flatter than the lower end.
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This indicates that the first three O2 bind to hemoglobin molecules at relatively low pO2 (= 0 – 40 torr). In contrast, adding the 4th O2 to hemoglobin moleculess requires a relatively high pO2 (= 40 – 100 torr).
How pH affects the Oxygen-Hemoglobin Dissociation Curve:
As blood plasma pH decreases (= becomes more acidic), H+ ions increasingly bind to hemoglobin amino acids, which lessens hemoglobin’s affinity for O2. This is referred to as the Bohr effect.
The situation reverses as plasma pH increase (= becomes more alkaline; basic).
Normal pH Blood:
A low (= acidic) blood plasma pH of 7.2 causes the O2-Hb saturation curve to shift about 15% to the right of normal (= pH 7.4).
In contrast, an elevated (= alkaline or basic) blood plasma pH of 7.6 causes the O2-Hb saturation curve to shift about 15% to the left of normal.
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How CO Affects Oxy-Hemoglobin Saturation:
Each of hemoglobin’s four heme groups can also bind to carbon monoxide (CO). If this occurs, O2 cannot bind and carbon monoxide poisoning results.
As shown in the animation below, carbon monoxide association with hemoglobin is directly related to the plasma partial pressure of CO (= pCO).
In this simulation, pCO is allowed to increase to from 0.0 – 0.4 mmHg while the pCO2 is maintained at 40 mmHg, which is normal.
At pCO = 0.4 mmHg, the hemoglobin is almost fully saturated with CO. This pressure is approximately 250 X less than the pO2 needed to fully saturate hemoglobin with O2.
These data indicate that heme has a much greater affinity for CO than for O2. Therefore, if an individual breathes in a relatively small amount of CO, it will saturate the hemoglobin and prevent O2 from binding. As a result, O2 cannot be distributed as needed to the body’s tissues.
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How pCO2 Affects Oxy-Hemoglobin Dissociation Curve:
The animations show how the concentration of carbon dioxide in the plasma (partial pressure of CO2 or pCO2) affects oxygen-hemoglobin dissociation curve (O2-Hb saturation).
As the graphs reveal, high pCO2 has the same effect on the O2-Hb dissociation curve as low plasma pH and low pCO2 has the same effect as high plasma pH (= Bohr effect).
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- High pCO2 lessens hemoglobin’s affinity for O2 in two ways.
- First, carbon dioxide is converted to H+ and bicarbonate ion in red blood cells via the enzyme carbonic anhydrase.
- The H+ ions bind to hemoglobin amino acids, and the alteration makes it more difficult for O2 to also associate.
- Secondly, some of the carbon dioxide binds directly to hemoglobin amino acids. This also causes alteration to the hemoglobin that make it more difficult for O2 to bind.