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2. The a-amino acid building blocks Page 5
structures 2.3 How can we group a-amino acids?
We can group a-amino acids acids by collecting together those with R groups that have similar features (Figure 3).

We can shorten the names of each a-amino acid using a one or three letter abbreviation. These abbreviations are given in Figure 3.

The side chain plays a large part in controlling the chemical and physical properties of an a-amino acid. Its solubility in water will depend on the polarity of the side chain group. A polar group will encourage solubility, a non-polar group will discourage it. For this reason a-amino acids are often divided into two groups, those with polar and those with non-polar side chain groups.

Figure 3. shows you the general shapes of the a-amino acids.
When you cover a molecule with your mouse, its name will appear in the status bar at the bottom.
You can see a bigger diagram of a group of molecules by clicking on that group

1. Which a-amino acids have polar side chain groups?
2. Which
a-amino acids have non-polar side chain groups?
Figure 4
Two unusual a-amino acids.
Proline is an unusual a-amino acid as the amino nitrogen is part of a secondary amino group (N-H) instead of the usual primary amino group (-NH2).

A few a-amino acids, not mentioned in Figure 3, occur in some proteins in a variety of organisms. The organism’s biochemistry produces these by modifying the R groups of a-amino acids after they have been built into a protein. Proline can be modified to produce hydroxyproline and lysine can be modified to produce hydroxylysine (see Figure 4).

Figure 5
Cystine is a dimer of cysteine.
Cystine is an unusual a-amino acid in that it is a dimer formed by two cysteine side chain groups joining together (see Figure 5).

These unusual a-amino acids have important parts to play in the structures of some proteins.

Unilever Education Advanced Series: Proteins
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