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2.21 Amino Acids
Similar to carbohydrates, proteins contain carbon (C), hydrogen (H), and oxygen (O). However,
unlike carbohydrates (and lipids) proteins also contain nitrogen (N). Proteins are made up of
smaller units called amino acids. This name amino acid signifies that each contains an amino
(NH2) and carboxylic acid (COOH) group. The only structural difference in the 20 amino acids is
the side group represented by the R below.
Fig 2.211 Structure of an amino acid
To illustrate the differences in the side group we will consider glycine and alanine, the two
simplest amino acids. For glycine the R group is hydrogen (H), while in alanine the R group is a
methyl (CH3). The structures of these two amino acids are shown below.
Figure 2.212 Structure of glycine
Figure 2.213 Structure of alanine
Individual amino acids are bonded together through a dehydration reaction (-H2O) forming a
dipeptide (2 amino acids). This bond between amino acids is known as a peptide bond and is
shown in the two figures below. The following video also shows the formation of a peptide
bond.
Figure 2.214 Peptide bond formation
Figure 2.215 Peptide bond formation part 21
Check Yourself
What are the three components of an amino acid? Which is structurally different among the
amino acids?
Amino acids can also come together to form tripeptides (three amino acids), oligopeptides (310 amino acids), and polypeptides (10 or more amino acids). A polypeptide is a chain of amino
acids as shown below.
Figure 2.216 A polypeptide chain2
References & Links
1. http://en.wikipedia.org/wiki/File:Peptidformationball.svg
2. http://www.genome.gov/Pages/Hyperion/DIR/VIP/Glossary/Illustration
/amino_acid.cfm?key=amino%20acids