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) groups. 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 joined together using a peptide bond (green) and is shown in the figure below.

Figure 2.214 Peptide bond formation¹

Amino acids can also come together to form tripeptides (three amino acids), oligopeptides (3-10 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/Glossary/index.cfm?id=149