Channels in Ferritin
Once the iron is soluble (as Fe(H2O)62+), how does it leave the ferritin shell? Recall that ferritin has two types of channels, three-fold and four-fold, in the shell. The soluble Fe2+ ion exits through the three-fold channels (Figure 12, below). These channels have a special property, known as polarity, that enables the passage of Fe2+ ions through these channels.
Polarity refers to significant differences in electronegativity between adjacent atoms in a molecule. For instance, the hydroxyl (-OH) functional group consists of an oxygen atom, which is highly electronegative, covalently bound to a hydrogen atom, which is much less electronegative. The highly electronegative oxygen atom draws the (negatively-charged) electrons in the bond to itself more than the less electronegative hydrogen atom does. Hence, the oxygen atom has a slight negative charge relative to the hydrogen atom. The bond between the oxygen and hydrogen atoms is then said to be polar, because it contains a negative pole (the oxygen) and a positive pole (the hydrogen). Because opposite charges attract one another, polar molecules interact well with other polar molecules and charged particles. The negative poles attract positive ions or the positive poles of other polar molecules, while the positive poles attract negative ions or the negative poles of other polar molecules.
Some amino acids have side chains that contain polar groups; these amino acids are known as polar amino acids. (Examination of the amino-acid structure in Figure 3 shows that all amino acids have an amino group and a carboxylic-acid group, which are polar. However, these polar groups form part of the backbone and do not contribute to the polarity of an amino-acid residue in a peptide. Only the side chain determines whether or not the amino acid is considered polar.) The three-fold channel in ferritin is lined with the polar amino acids aspartate (Asp) and glutamate (Glu) (Figure 13, below). Because it is lined with polar amino-acid side chains, the three-fold channel is also said to be polar. The channel's polarity allows it to interact favorably with the Fe2+ ion and with water, because the positive charge of the ion (or the positive pole of water) attracts the negative poles of the side chains (Figure 11). The favorable interaction allows Fe2+ to pass through the channel.
Fe(II) is probably not accompanied by all six water molecules of the hydrated complex as it passes through the channel, because this entire complex would be too large to fit through the channel. Most likely, Fe(II) is coordinated to some water molecules and to some of the polar side chains lining the channel as it passes from the inside to the outside of the ferritin shell. Once outside the shell, the Fe(II) then regains the six water molecules and is again solvated as Fe(H2O)62+.
What about the other type of channels in ferritin, the four-fold channels (Figure 14) These channels are lined with the nonpolar amino acid leucine (Leu, Figure15). The side chain of leucine contains only carbon and hydrogen atoms, which have similar electronegativities. Hence, the four-fold channel is considered to be nonpolar. Because it is nonpolar, this channel does not interact favorably with the Fe2+ ion, and Fe2+ does not leave the ferritin shell through these channels. Rather, it is thought that these channels function as the site of electron transfer, whereby the Fe(III) in the mineral lattice is reduced to Fe(II). However, the mechanism of this electron transfer is not well understood.
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This page created by Matt Traverso, Washington University in St Louis.
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