Hemoglobin and myoglobin are proteins that have evolved to carry out the specialized functions of oxygen transport and storage in animals. Animals must ensure a steady supply of oxygen to body cells and remove metabolic waste products such as carbon dioxide. Diffusion transport through tissues is generally not fast enough.

Almost all animals except insects pump oxygen in the blood through arteries to the tissues and return carbon dioxide via venous blood to lungs or gills (Figure 7.1). All higher organisms have oxygen transport proteins, which allow the blood to carry a much greater load of oxygen than would be permitted by solubility alone. Oxygen transport proteins may be either dissolved in the blood (as in some invertebrates) or concentrated in specialized cells, like the human erythrocytes.

In all vertebrates the oxygen transport protein is hemoglobin, a protein that can pick up oxygen in lungs or gills and deliver it to tissues. Myoglobin, by contrast, is an oxygen storage protein. Oxygen transported to tissues must be released for utilization. In tissues, such as muscle, with high oxygen demands, myoglobin provides large oxygen reserves.

Myoglobin and hemoglobin are built on a common structural motif, as shown in Figure 7.3. Myoglobin contains a single polypeptide chain folded about a prosthetic group, the heme, which contains the oxygen binding site. Hemoglobin is a tetrameric protein. Each polypeptide subunit closely resembles myoglobin. Note, for example that myoglobin and each subunit of hemoglobin consists of eight helical segments, which are labeled A through H in Figure 6.1. The multiple subunit structure of hemoglobin gives it important oxygen binding properties that are different than myoglobin's, consistent with hemoglobin's role in oxygen transport.