Tutorial developed by Ross Feldberg, Dept. of Biology, Tufts University
Introduction to Myoglobin and Hemoglobin
Myoglobin and hemoglobin are proteins designed to carry oxygen. Hemoglobin carries oxygen from the lungs to the tissues. Myglobin is the oxygen storage protein of the muscle. The richest source of myoglobin is the muscle of aquatic diving mammals, such as seals or whales, since these tissues need a very rich store of oxygen to see them through long anoxic periods during a dive. It serves as an oxygen reservoir, picking up O2 from the hemoglobin and delivering it to the cells as O2 is used up in metabolism. Sperm whale myoglobin consists of 153 amino acids (Mw = 17,199). This tutorial will use hemoglobin and myoglobin to illustrate some general aspects of secondary, tertiary and quaternary structure in proteins.
This shows the backbone of the single peptide chain that makes up myoglobin. You can see that the peptide twists and turns to form a relatively compact globular structure. In addition to the protein components, both myoglobin (Mb) and hemoglobin (Hb) require a non-amino acid component to actually complex with the bound oxygen. Such non-amino acid components are called "cofactors" or "coenzymes" or "prosthetic groups", more or less interchangeably. The prosthetic group of both Hb and Mb is the heme group, also found in cytochromes and in enzymes such as catalase. The heme group is made up of four fused five member rings to form a planar structure with four nitrogens projecting toward the center and various side groups projecting away from the ring. The nitrogen ring structure is just the right size and shape to form a coordination complex with iron in its +2 oxidation state (ferrous iron).
Residues close together in the primary structure often take on regular configurations in space known as secondary structure. These are shown by a cartoon representation to emphasize this aspect. Myoglobin (and hemoglobin) are unusual in that they contain only alpha helix secondary structure (shown here as helical loops in red) linked together by stretches of random coil. How many separate helical segments are present in this molecule?
Hemoglobin consists of 2 alpha subunits and 2 beta subunits to give a four chain structure. This first button shows a single alpha subunit. Although the alpha subunit sequence is quite different from the sequence of myoglobin, you should note that this structure also only contains alpha helical segments in the same number and relative orientation as in myoglobin.
This shows a dimer of a Hb alpha1 subunit and a Hb beta1 subunit. Hemes are shown in red, with the iron in orange and the bound oxygens in white. The interaction of separate peptides allows for the formation of noncovalent complexes and is known as tertiary structure.
While there are a number of interactions between the alpha and beta subunits, the two alpha subunits are far enough apart that there are few interactions. Can you identify which amino acids probably do form a bond between the two alpha subunits? What sort of bond is formed?
Here the four Hb subunits are shown. The two alpha subunits are dark blue and green while the two beta subunits are light blue and yellow. Here again, you can see that the two alpha subunits show very little interaction, while strong interactions occur between alpha and beta subunits. The overall pattern of interchain patterns makes up the quaternary structure. Although the spacefilling representation (in which each atom is shown as a sphere with radius equal to van der Waals radius) is a more realistic representation, it is difficult to extract information from this view. In deoxyhemoglobin the quaternary structure is such that a relatively large central cavity is formed between the four subunits. When oxygen binds to the hemes, there is a relatively small conformational change in the tertiary structure of each subunit which results in a larger change in the interactions between the individual subunits (i.e the quaternary structure), narrowing the central cavity. This is a crucial factor in the ability of hemoglobin to bind oxygen in the lungs and to release it in the tissues. (covered in more detail in Bio 152).