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 —CONH— units (see Figure 4, below) are central to the backbone (see Figure 5, below) of the peptide chain. Figures 4 and 5 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 residues in each peptide subunit in 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. Recall from the "Hemoglobin and the Heme Group: Metal Complexes in the Blood" tutorial that an alpha helix is formed when there is a regular pattern of side chains that form hydrogen bonds with one another. Figure 6 shows the hydrogen-bonding interactions between amino-acid residues that give rise to the helical structure shown in the ribbon representation.
Below are two representations of the peptide subunit in ferritin. The first representation (Figure 7) is a CPK model of the peptide chain. The CPK representation gives an approximate volume of the subunit. Figure 8 shows a ribbon representation of the peptide. The ribbon representation is useful for showing the alpha helices in the peptide.
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This page created by Matt Traverso, Washington University in St Louis.
© 2004, Washington University.
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