The way in which the protein chain is folded in the myoglobin molecule is shown (right). Myoglobin has 153 amino acids in one continuous chain, and a molecular weight of 17,000. It is a relatively small protein. For simplicity, only the alpha carbons of the main chain are shown, and the -CO-NH- amide groups connecting them are represented by a straight line. The chain is coiled into eight segments of cylindrical a helix, identified by the letters A through H. A more schematic diagram of the myoglobin molecule is shown above. The corners or bends between helices are given the two letters of the helices that they connect-corner AB between helices A and B, and so on. Only by such abrupt elbow bends can an essentially linear fibrous structure - the a helix - be fitted into a globular protein of finite dimensions. The a helix occurs in myoglobin and many other globular proteins because it is an efficient way to fold a protein chain, but the price that must be paid is irregular bends every so often along the chain.
(The myoglobin illustrations on this page are based on those from R. E. Dickerson and I. Geis, The Structure and Action of Proteins, W. A. Benjamin, Inc. Copyright © 1969 Dickerson and Geis)

Above: The representation of the myoglobin molecule emphasizes the a-helical framework and the positioning of the heme in a pocket. Histidine side chains, which have five-membered rings, extend from the E and F helices and interact with the heme iron and the O₂ molecule.