Ferritin: Assembly of 24 Peptide Subunits
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).
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