Myoglobin and Hemoglobin

 
Both of the oxygen-binding proteins myoglobin and hemoglobin have a heme prosthetic group.

INTRODUCTION

Myoglobin and hemoglobin are globular proteins that serve to bind and deliver oxygen. These globins dramatically improve the concentration of molecular oxygen that can be dissolved in the biological fluids of vertebrates and some invertebrates.

Both of these globins contain an organic prosthetic group for binding oxygen and they contain many similar structural characteristics. Yet the differences in function between these two proteins can be explored from a structural perspective, leading to a discourse on ligand binding and allosteric regulation.

Myoglobin and hemoglobin are of obvious critical importance to vertebrate life, yet they are also studied because they serve as excellent, well-characterized models for illustrating the principles of protein structure, dynamics, and function. Keep in mind that the concepts you will be learning in this exercise, especially regarding oxygen binding and allostery, are also directly applicable to the study of enzyme kinetics, more specifically substrate binding and allosteric regulation of enzyme activity.

Myoglobin

Myoglobin is found in vertebrate muscle cells. Muscle cells, when put into action, can quickly require a large amount of oxygen for respiration because of their incredible demand for energy. Therefore, muscle cells use myoglobin to accelerate oxygen diffusion and act as localized oxygen reserves for times of intense respiration.

Myo - from the Greek word for "muscle"

Hemo - from the Greek word for "blood"

Hemoglobin

In vertebrates, hemoglobin is found in the cytosol of red blood cells in the bloodstream. Hemoglobin is sometimes referred to as the oxygen transport protein, in order to contrast it with its stationary cousin myoglobin, although its function and mechanism are more complex than this name would suggest.

In vertebrates, oxygen is taken into the body by the tissues of the lungs, and passed to the red blood cells in the bloodstream. Oxygen is then distributed to all of the tissues in the body and offloaded from the red blood cells to respiring cells. Hemoglobin then picks up carbon dioxide to be returned back to the lungs. Thus, hemoglobin binds and offloads both oxygen and carbon dioxide at the appropriate tissues, serving to deliver the oxygen needed for cellular metabolism and removing the resulting waste product, CO2.

Learning objectives

  • Review myoglobin's and hemoglobin's secondary, tertiary, and quaternary structures.
  • Become familiar with globin tertiary structure:
    • Compare myoglobin, Hb a, and Hb b chains;
    • Identify heme, His F8, and His E7 in all three chains;
    • Locate positions of hydrophobic and hydrophilic residues; and
    • Learn how the heme group binds oxygen.
  • Understand how the structural differences between these two oxygen carrier proteins lead to their different functions within the body.
  • Learn how conformational changes in hemoglobin lead to allostery.
  • Understand how BPG binding changes hemoglobin's affinity for oxygen.