Authors: Rachel Casiday and Regina Frey
Key Concepts: Graphical molecular representations; protein structural components; crystal-lattice mineral structure; oxidation states; polarity.
Iron is necessary for oxygen transport in the blood, supplying the body with a reliable source of energy, and for maintaining several other important structures and systems in the body. However, too much free iron in the blood can lead to the dangerous disease known as hemochromatosis. How does the body regulate the amount of iron? Fortunately, most of us are able to maintain appropriate levels of available iron in the body (enough available iron to ensure an adequate supply of hemoglobin, but not so much as to produce toxic effects), even if our iron consumption does not always exactly match the body's iron loss. Ferritin is the key to this important control of the amount of iron available to the body. Ferritin is a hollow, spherical protein that stores iron and releases it in a controlled fashion. Hence, the body has a "buffer" against iron deficiency (if the blood has too little iron, ferritin can release more) and, to a lesser extent, iron overload (if the blood and tissues of the body have too much iron, ferritin can help to store the excess iron). Ferritin stores iron as Fe(III) in a crystal-lattice mineral structure inside the hollow sphere of the protein. Iron is released from the protein as water-soluble Fe(II).
The tutorial uses a variety of molecular representations to describe the structure of ferritin, and how the structural components and features (e.g., polar and nonpolar amino acids, peptides, and channels) allow the protein to store iron and release it in a controlled fashion. Hence, a major segment of the tutorial introduces students to different types of molecular representations, describing the advantages and limitations of each, so that students appreciate that different types of representations give different information about a molecule. The tutorial describes protein structure in detail appropriate for students with little or no background in biology, and applies the information about protein structure to the ferritin protein. A final section describes the mechanism by which iron is released from the protein. This tutorial is one of four blood-chemistry tutorials available for General Chemistry from Washington University in St. Louis.
At Washington University in St. Louis, this tutorial is used in conjunction with a first-semester laboratory experiment in which the students use the spectrophotometer to perform a trace analysis of the iron content in ferritin and to observe the kinetics of the iron-release process. First, the total amount of iron in a ferritin sample is determined by breaking apart the protein shell, and then the time course of iron released from the intact protein by the reduction of the iron-mineral core is determined. In this manner, the mechanism of the in-vitro iron-release process is examined.
An earlier molecular-graphics tutorial on ferritin, designed for second-year students, and detailed information about the ferritin experiment are also available.
