Iron in the Body
The Body as a Chemical System: Chemical Elements that Make Up the Body
Of the more than 100 chemical elements known to scientists today, only a relatively small number of these elements are found in the human body. In fact, only 24 different elements are thought to be essential for humans. The largest elemental components of the body, by mass, are oxygen (65%), carbon (18%), hydrogen (10%), and nitrogen (3%). The other elements in the body, such as calcium, phosphorus, iron, and copper, are known to physiologists as mineral elements and trace elements. Although these elements make up a much smaller percentage of the mass of the body, they are vital to the body's proper functioning. They must be present in the body in the proper amounts, and they must also be available to react with other elements to form critical molecules and participate in important chemical reactions. In this tutorial, we will describe the importance of one essential mineral in the body, iron. Although iron comprises only 0.008% of the body's mass (approximately 6 g for a 160-lb (75-kg) adult male), it is necessary for our survival.
Iron complexed with the protein hemoglobin is necessary for oxygen transport in the blood. Recall that iron is the central atom of the heme group, a metal complex that binds molecular oxygen (O2) in the lungs and carries it to all of the other cells in the body that need oxygen to perform their activities (e.g., muscle cells). In addition to hemoglobin, other important proteins in the body contain heme groups including myoglobin, which takes oxygen from hemoglobin and delivers it to muscle cells, and the cytochromes, which are important for generating energy. Other proteins, such as those needed for DNA synthesis and cell division, also rely on iron. Furthermore, iron is used to help produce the connective tissues in our body, some of the neurotransmitters in our brain, and to maintain the immune system.
Because iron plays such a crucial role in the body, it is important for us to maintain an adequate supply of iron. Our bodies continually lose iron through everyday processes such as urination, defecation, sweating, and sloughing off skin cells. Bleeding contributes to further loss of iron from the body. To compensate for these losses and to maintain an adequate supply of iron, we should consume approximately 18 mg of iron daily. Certain conditions, including heavy bleeding and pregnancy, further increase the requirement for iron consumption. Good dietary sources of iron include red meat, liver, egg yolk, beans, nuts, and fortified cereals.
When the body's supply of available iron is too low, a condition known as iron deficiency results. People with iron deficiency cannot produce an adequate amount of hemoglobin to meet their body's oxygen-transport needs. When the deficiency becomes severe such that there are too few circulating red blood cells or the hemoglobin content of these cells is very low, the condition is diagnosed as iron-deficiency anemia. The most common symptoms of iron-deficiency anemia are tiredness and weakness due to the inadequate oxygen supply to the body's cells, and paleness in the hands and eyelids due to the decreased levels of oxygenated hemoglobin, which is red-colored. Iron-deficiency anemia can be treated with iron supplements.
It is also possible to have too much iron deposited in the body tissues. This condition is known as iron overload. If the iron overload becomes severe (usually when the total amount of iron in the body exceeds 15 g), the condition is diagnosed as hemochromatosis. Hemochromatosis can result in serious damage to the body's tissues, including cirrhosis of the liver, heart failure, diabetes, abdominal pain, and arthritis. A recessive genetic mutation can put some people, particularly those of Irish or Celtic descent, at a higher risk for developing hemochromatosis. Treatment for the condition consists of removing blood from the patient to decrease the amount of iron in the body.
Ferritin: The Iron-Storage Protein
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, even if our iron consumption does not always exactly match the body's iron loss. This regulation of blood-iron levels is mediated by the protein ferritin (Figure 1). Ferritin can release iron if the blood has a low iron concentration, and it can help to store excess iron if the blood and tissues have a high iron concentration. Hence, ferritin functions as a "buffer" against iron deficiency and, to a lesser extent, against iron overload.
How does ferritin store iron? Ferritin has the shape of a hollow sphere. Inside the sphere, iron is stored in the Fe(III) oxidation state. It is incorporated in the mineral ferrihydrite, [FeO(OH)]8[FeO(H2PO4)], which is attached to the inner wall of the sphere. To release iron when the body needs it, the iron must be changed from the Fe(III) to the Fe(II) oxidation state. Then, the iron leaves through channels in the spherical structure. To understand how ferritin controls storage and release of iron, we need to first study its structure.
We will use the different types of Molecular Representations to study the structure of ferritin. We will begin at the smallest level of protein structure by using 2D-ChemDraw representations to show how atoms are combined to make amino acids and how amino acids come together to form the protein subunits known as peptides . We will then examine how the sequence of amino acids determines the shape of the peptide. Finally, we will use the ribbon and CPK representations to show how 24 peptide subunits are combined to make the hollow spherical shape and channels of ferritin. The basic structural features of proteins that you will learn about in this tutorial will provide a foundation for understanding the structure and function of any protein, a tremendously important class of biological molecules.
Amino Acids: The Building Blocks of Proteins
All proteins consist of chains of amino acids. An amino acid (Figure 3) is a molecule containing a central carbon atom and three special functional groups: a carboxylic acid group (-COOH), an amino group (-NH2), and variable side chain (generically denoted by "R"). (Note: The "-" in "-COOH" and "-NH2" indicates a bond to another atom in the rest of the molecule.) There are 20 different amino acids that are available to be incorporated into proteins. The side chains of the 20 amino acids have different properties, which in turn give different properties to the amino acids. For instance, side chains may be charged (e.g., glutamate) or electrically neutral (e.g., leucine), bulky (e.g., tryptophan) or consisting only of a hydrogen atom (e.g., glycine).
Peptides: Protein Subunits
Amino acids are linked together to form chains known as peptides. These links are formed by covalent bonds known as peptide bonds between the carbon atom of the carboxylic acid group (-CO2H) of one amino acid and the nitrogen atom of the amino group (-NH2) of an adjacent amino acid (Equation 1).
The peptide bonds in the -CO-NH- units are central to the backbone of the peptide chain. Figures 3 and 4 show the three amino acid residues leucine, alanine, and glutamate (Leu-Ala-Glu) that are bound together and form a part of the peptide subunit found in ferritin.
Peptides may be very long chains of amino acids. There are 184 amino acid residues in each peptide subunit of human ferritin. The side chains of amino acids in a peptide can interact with one another, causing the peptide to fold. The shape of the peptide depends on where the peptide is folded, which in turn depends on the sequence of amino acids in the peptide (i.e., the location of side chains whose properties enable them to interact with other side chains). One common example of folding in a peptide is the alpha helix motif, which is common in many proteins. An alpha helix is formed when there is a regular pattern of side chains that form hydrogen bonds with one another. Figure 5 shows the hydrogen-bonding interactions between amino-acid residues that give rise to the helical structure.
Below are two representations of the peptide subunit in ferritin. The first representation is a CPK model of the peptide chain, which gives an approximate volume of the subunit. The figure also shows a ribbon representation of the peptide, which is useful for showing the alpha helices in the peptide.
Ferritin: Assembly of 24 Peptide Subunits
To make the ferritin protein, 24 identical peptide subunits (Figure 6) are assembled into a hollow spherical shell (Figure 7). 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 are small holes through which certain ions or molecules can travel. In the ferritin sphere, channels 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 occur at the intersection of four peptide subunits. Three-fold channels occur at the intersection of three peptide subunits. The two types of channels have different chemical properties, and therefore they perform different functions.
Release of Iron from Ferritin
The iron in the ferritin core is stored as Fe(III) in a crystalline solid that has the chemical formula [FeO(OH)]8[FeO(H2PO4)]. The best model for ferritin's core is the mineral ferrihydrite. A crystalline solid is a three-dimensional structure in which the constituents (i.e., atoms, ions, or molecules) are arranged in a definite repeating pattern. The positions of the constituents' centers are represented by points on a three-dimensional lattice. The smallest repeating unit of the lattice is called the unit cell. This unit cell is repeated in a specific pattern to form an extended nonmolecular structure (see Figure 8).
In the mineral ferrihydrite, every Fe(III) ion is coordinated to six O(II) ions. However, in ferritin, the mineral core has approximately 10% of the Fe(III) ions coordinated to five O(II) ions and one phosphate group. Most of the phosphate groups that are coordinated to the iron ions lie on the outside of the crystalline structure and are used to bind the mineral to the residues on the inside of the ferritin shell (the protein).
As long as a lattice remains intact, the atoms in the lattice are not soluble because they form part of the lattice's continuous structure . Thus, in order for iron to be released from ferritin, the mineral lattice must be dissolved (i.e., to allow the iron atoms to break away from the lattice structure). This is accomplished by reducing iron from the Fe(III) (ferric) oxidation state to the Fe(II) (ferrous) oxidation state. In the Fe(II) state, iron breaks away from the lattice as the Fe2+ ion. The positive charge of the Fe2+ ion attracts the electronegative oxygen atoms of water, and a water "cage" forms around the ion, with six water molecules surrounding the ion . Thus, iron becomes soluble as a hydrated Fe2+ ion , Fe(H2O)62+ , and it can be released from the ferritin protein via the channels in the spherical shell.
Channels in Ferritin
Once the iron is soluble, how does it leave the ferritin shell? Recall that ferritin has two types of channels in the shell: three-fold and four-fold channels. The soluble Fe2+ ion exits through the three-fold channels (Figure 9). These channels are polar, which enables the passage of Fe2+ ions.
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 partially charged negative pole (the oxygen) and a partially charged positive pole (the hydrogen). Opposite charges attract one another, so 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 2 shows that all amino acids have an amino group and a carboxylic-acid group, both of 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 (Figure 9) is lined with the polar amino acids aspartate (Asp) and glutamate (Glu) (Figure 10). 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. In this interaction, the positive charge of the ion (or the positive pole of water) attracts the negative poles of the side chains (Figure 11). This favorable interaction allows Fe2+ to pass through the channel.3
When the hydrated Fe(II) ions, Fe(H2O)62+, 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 through the ferritin shell. Once outside the channel, the Fe(II) then regains six water molecules and solvated again as Fe(H2O)62+.
Now we will examine the four-fold channels in ferritin (Figure 12). These channels are lined with the nonpolar amino acid leucine (Figure 13). 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.
Iron is an essential trace element that is used to form molecules in the body such as hemoglobin. Ferritin is the protein within the body that stores iron and releases it through channels in a controlled fashion. The unique structure of ferritin forms a spherical shell in which the iron is "stored" as Fe(III) in a crystalline mineral. Ferritin consists of 24 peptide subunits that form two types of channels where these subunits intersect: the 3-fold channel is polar, and the 4-fold channel is nonpolar. The residues that line the channels determine the polarity of the channel. When the Fe(III) in the crystalline mineral is reduced to Fe(II), the iron becomes solvated and ferritin releases the solvated iron, Fe(H2O)62+, through the 3-fold polar channel. Hence, ferritin can control the amount of available iron in the body, preventing iron disorders like anemia and iron overload.
The three-dimensional structure of ferritin is crucial to its function within the body. To better understand ferritin's role in the body, we used different types of molecular representations to study ferritin's three-dimensional stucture. Each representation used in this tutorial gives important information about ferritin. Only using these representations in conjunction with one another and with other information about the molecule, can we begin to understand the complex relationship between the protein's structure and its function.
1The structure of this protein was determined using x-ray crystallography. The structure of the iron core is based on a simplied model compound, and the image was rendered using the Insight II molecular-modeling system from Molecular Simulations, Inc. (see References).
2The crystal structures were drawn using PowderCell for Windows, and the images were rendered using POV-Ray.
3The hydrated Fe(II) ions, Fe(H2O)62+, 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 through the ferritin shell. Once outside the channel, the Fe(II) then regains six water molecules and solvated again as Fe(H2O)62+.
The authors thank Bill Buhro for obtaining the structural information for the iron-mineral core, Greg Noelken for creating the jmol script files, and Dewey Holten, Michelle Gilbertson, Jody Proctor and Carolyn Herman for many helpful suggestions in the writing of this tutorial.
The development of this tutorial was supported by a grant from the Howard Hughes Medical Institute, through the Undergraduate Biological Sciences Education program, Grant HHMI# 71199-502008 to Washington University.
Updated On: 7/14/2011 (G Noelken)