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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.
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 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). |
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 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. |
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Unilever Education Advanced Series: Proteins | ||||||||||
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