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Hemoglobin & Myoglobin: 4. Dissociation Curves

Hemoglobin & Myoglobin: 4. Dissociation Curves

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Dissociation Curves
We can compare compare the binding properties of both myoglobin and hemoglobin by drawing their dissociation curves. These curves measure their relative affinities for oxygen.
  • We draw a graph and label the x-axis oxygen partial pressure (torr). Number it 0 to 120.
– 30 torr is ~ the partial pressure of oxygen in the body's tissues. – 100 torr is ~ the partial pressure in the lungs.
  • The y-axis % oxygen saturation: it's numbered 0 to 100.
We will use it to compare the oxygen binding patterns of both hemoglobin and myoglobin.
oxygen binding curve for hemoglobin and myoglobin
Hemoglobin
  • We draw a sigmoidal curve that plateaus just below 100% saturation.
  • Cooperative binding produces this sigmoidal shape.
– As one oxygen molecule binds, hemoglobin's affinity for additional oxygen increases, and its percent saturation rapidly increases.
  • We show that hemoglobin reaches half saturation in the peripheral tissues – it responds to oxygen availability and releases it when partial pressure is low.
Myoglobin
  • We draw a hyperbolic curve to the left of the hemoglobin curve, a much simpler binding pattern that corresponds to myoglobin's single heme group.
– Myoglobin has a high affinity for oxygen, and does not release it until the partial pressure is very low. – These binding properties correspond to myoglobin's role in oxygen storage. – We label the early portion of the curve "Exercising muscle" and the plateau "Muscle at rest."
Myoglobin vs. Hemoglobin
  • Myoglobin releases oxygen in response to the muscle's immediate needs.
  • Hemoglobin's cooperative binding allows it to respond to changes in oxygen availability.
Fetal hemoglobin dissociation curve
  • As a clinical correlation, we show that the fetal hemoglobin dissociation curve is to the left of the adult hemoglobin curve.
  • This is because it has a greater affinity for oxygen to facilitate oxygen transfer from the maternal hemoglobin to the fetus; fetal oxygen supplies come from the mother.

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