Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins:- Myoglobin Has a Single Binding Site for Oxygen
Myoglobin (Mr 16,700; abbreviated Mb) is a relatively simple oxygen-binding protein found in almost all mammals, primarily in muscle tissue. As a transport protein, it facilitates oxygen diffusion in muscle. Myoglobin is particularly abundant in the muscles of diving mammals such as seals and whales, where it also has an oxygen storage function for prolonged excursions undersea. Proteins very similar to myoglobin are widely distributed, occurring even in some single-celled organisms. Myoglobin is a single polypeptide of 153 amino acid residues with one molecule of heme. It is typical of the family of proteins called globins, all of which have similar primary and tertiary structures. The polypeptide is made up of eight -helical segments connected by bends (Fig. 5–3). About 78% of the amino acid residues in the protein are found in these helices. Any detailed discussion of protein function in evitably involves protein structure. To facilitate our treatment of myoglobin, we first introduce some structural conventions peculiar to globins. As seen in Figure 5–3, the helical segments are named A through H. An individual amino acid residue is designated either by its position in the amino acid sequence or by its location within the sequence of a particular -helical segment. For example, the His residue coordinated to the heme in myoglobin, His93 (the 93rd amino acid residue from the amino-terminal end of the myoglobin polypeptide sequence), is also called His F8 (the 8th residue in α helix F). The bends in the structure are designated AB, CD, EF, FG, and so forth, reflecting the -helical segments they connect.
FIGURE 5–3 The structure of myoglobin. (PDB ID 1MBO) The eight-helical segments (shown here as cylinders) are labeled A through H. Nonhelical residues in the bends that connect them are labeled AB, CD, EF, and so forth, indicating the segments they interconnect. A few bends, including BC and DE, are abrupt and do not contain any residues; these are not normally labeled. (The short segment visible between D and E is an artifact of the computer representation.) The heme is bound in a pocket made up largely of the E and F helices, although amino acid residues from other segments of the pro tein also participate.
