Section 5.5  

Myoglobin & Hemoglobin

Introduction

Aerobic energy (ATP) requires oxygen

Large animals need a way to continually supply oxygen to metabolically active tissues. The oxygen is needed in relatively large amounts because the highly efficient aerobic conversion of foodstuffs such as lipids and carbohydrates into available energy in the form of ATP culminates in the reduction of oxygen to water shown at right.

Oxygen Delivery and Storage

Delivery of the oxygen requires both a circulatory system and an oxygen carrier in the circulatory fluid. An oxygen carrier is needed because oxygen, which is a non-polar molecule, is only sparingly soluble in aqueous fluids such as blood (solubility is only 3 mL oxygen/L blood). The protein hemoglobin serves as the oxygen carrier in the blood. It serves to increase the oxygen carrying capacity of human blood to 200 mL oxygen/L blood. Hemoglobin provides a means of getting oxygen to metabolizing tissues, but cells also need a means to bind and store oxygen released from the hemoglobin as blood passes through the capillaries. This function is carried out by the intracellular protein myoglobin, a structural relative of hemoglobin.

The hemoglobin in the red blood cells and myoglobin inside tissues such as muscle cells have to act in tandem for effective oxygen transport. We can examine these coordinated activities using the oxygen binding curves of the two respiratory pigments. For hemoglobin the task is to become completely loaded with oxygen as the blood traverses through the capillaries of the lungs. It must then be able to efficiently off-load oxygen when the blood flows past the low oxygen environment of active tissues, such as heart muscles. The following graph illustrates the different oxygen affinities of myoglobin and hemoglobin at different concentrations of oxygen (given as partial pressure of O2)

The Bohr Effect

Note how the hemoglobin dissociation curve is S-shaped, or sigmoidal, in character. This abrupt change in oxygen affinity over a small range of oxygen concentration is almost like a switch, allowing the hemoglobin to almost fully unload bound oxygen at the tissues where it is needed. We shall see later that a number of modifiers of hemoglobin function act by altering the nature of this binding curve, changing the affinity of the hemoglobin for oxygen.

By contrast to hemoglobin, which is found in circulating red blood cells or erythrocytes, myoglobin is found intracellularly in body tissues. To perform its job, myoglobin (Mb) must effectively bind any oxygen released from hemoglobin (Hb). Myoglobin therefore needs to have a higher oxygen affinity than hemoglobin. The oxygen binding curves for myoglobin and hemoglobin are compared in the figure at left.

Relevant Units

Enzyme Specificity animation
Oxidative Phosphorylation animation

Copyright 2002, John Wiley & Sons Publishers, Inc.