24-Subunit Structure

To make the ferritin protein, 24 peptide subunits (Figures 7-8) are assembled into a hollow spherical shell (Figure 9). The sphere that is formed is approximately 80 Angstroms in diameter, and the walls are approximately 10 Angstroms thick. The molecular weight of ferritin (i.e., with all 24 subunits combined) is 474,000 g/mol.

Channels (i.e., small holes through which certain ions or molecules can travel) in the sphere are formed at the intersections of three or four peptide subunits. As we shall see, these channels are critical to ferritin's ability to release iron in a controlled fashion. Two types of channels exist in ferritin. Four-fold channels (such as the one shown in the center of Figure 9) occur at the intersection of four peptide subunits. Three-fold channels (such as those shown on the outskirts of Figure 9) occur at the intersection of three peptide subunits. The two types of channels have different chemical properties, and hence perform different functions, as we shall see later ("Release of Iron" section).

Figure 9

This is a molecular model of ferritin in the CPK representation. CPK pictures represent the atoms as spheres, where the radius of each sphere is equal to the van der Waals radius of the atom. Hence, CPK representations are a good way to show the approximate volume occupied by a molecule.

All of the 24 subunits are identical, but they have been color coded to help illustrate the structure. Dark blue subunits are closest to you, magenta subunits are farther away, and light blue subunits are the farthest away from you. The four (4) subunits colored in dark blue form the walls of a 4-fold channel. The 3-fold channels occur at the intersections of the light blue, dark blue, and magenta-colored subunits. The locations of 3-fold channels are indicated on the figure, but the channels themselves are obscured from this viewing angle.

Note: This Figure shows the same view of Ferritin as Figure 1, but in a different representation. (Figure 1 uses the ribbon representation for the closest peptide subunits, the stick representation for the other subunits, and the CPK representation for the iron core (not shown in Figure 9).) Compare Figures 1 and 9 to see how these representations provide different information about the structure of ferritin.

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Ferritin: Assembly of 24 Peptide Subunits

Figure 9