Oxygen Transport Proteins: Myoglobin and Hemoglobin

Overview

• Proteins Involved: Myoglobin and Hemoglobin
• Function: Transport oxygen to cells for aerobic cellular respiration to produce ATP.

Structures and Properties

• Myoglobin:
  • Single polypeptide chain → single heme group.
  • High oxygen affinity, stores oxygen in muscle cells.
• Hemoglobin:
  • Four polypeptide chains → four heme groups.
  • Can bind four times as many oxygen molecules as myoglobin.
  • Lower oxygen affinity, transports oxygen from lungs to tissues.

Oxygen Binding Curves

• Oxygen Binding Curve:
  • Graphical representation of oxygen binding properties.
  • Y-axis: Fractional saturation of protein (0 to 1).
  • X-axis: Partial pressure of oxygen (PO2), measured in mmHg or Torr.

Myoglobin Curve (Blue Curve)

• Type: Simple binding curve.
• High affinity for oxygen.
• Sharp increase in binding with small oxygen concentration.
• Physiological Meaning:
  • Effective for oxygen storage in muscle cells.
  • Releases oxygen only at very low partial pressures.

Hemoglobin Curve (Red Curve)

• Type: Sigmoidal (S-shaped) curve.
• Lower affinity for oxygen than myoglobin.
• Binds oxygen cooperatively:
  • Binding at one site increases likelihood of binding at other sites.
  • Release at one site promotes release at others.
• Physiological Meaning:
  • Efficient oxygen carrier from lungs to tissues.
  • Releases oxygen more readily than myoglobin.

Cooperative vs Non-cooperative Binding

• Myoglobin:
  • Non-cooperative binding due to single polypeptide chain.
  • Ideal for storing oxygen until needed.
• Hemoglobin:
  • Cooperative binding due to multiple polypeptide chains.
  • Ideal for oxygen transport and release in tissues.

Conclusion

• Role of Myoglobin: Stores oxygen in muscle cells; releases under low oxygen conditions.
• Role of Hemoglobin: Transports oxygen via blood; releases oxygen in tissues.
• Cooperative behavior of hemoglobin enhances its role as an oxygen carrier.

Next Steps

• Next lecture will explore in more detail why hemoglobin, not myoglobin, is the primary oxygen carrier in red blood cells.