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Proteins |
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| 2. The a-amino acid building blocks |
Page 4 |
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2.1 The a-amino acids
Amino acids are the monomer building blocks from which proteins are made. In general an acid is an organic molecule that contains both a carboxylic acid (-COOH) and an amine (-NH2) functional group. The acids that make proteins are all a-amino acids. In these acids both functional groups are attached to the same carbon atom (the a-carbon atom) at one end of the molecule. The simplest way to draw the structure of an a-amino acids is shown on the left.
In this diagram R stands for the rest of the molecule - the side chain group. R is the part of the molecule that makes one a-amino acid different from another. There are some twenty different a-amino acidsthat can make up proteins, so there are twenty different possibilities for the structure of the side chain group (see Figure 3). |
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2.2 Amino acids in 3D
The structures you have seen so far give us only a limited picture of what a-amino acid molecules may look like; they show the atoms that the molecules contain and how these atoms are linked together. They give no information about the 3-dimensional shape of the molecule - how the atoms are arranged in space. In some cases a pair of molecules that share the same sequence of atoms can have different arrangements in space called configurations. This leads to a type of isomerism called stereoisomerism. |
 Figure 1
a-amino acids in 3D. |
Almost all a-amino acids display a particular type of stereoisomerism that comes from the molecules being chiral . This means that the two arrangements of the molecules are non-identical mirror images, in the same way that your left hand is a non-identical mirror image of your right hand (see Figure 1). Non-identical mirror images are sometimes described as non-superimposable (you can't put a left hand glove on your right hand). The two molecules are called enantiomers. |
 Figure 2
The conventional way to show the 3D shape of a molecule - in this case an amino acid. |
Chirality in an a-amino acid is due to the a-carbon atom in the molecule being joined to four different atoms or groups. The a-carbon is called the chiral centre. We can draw the configurations of two enantiomers using a convention to show three dimensions (see Figure 2). Nearly all a-amino acids in nature share the same configuration, called the L configuration. Its mirror image is called the D configuration. |
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Questions:
(Refer to Figure 3 - Structures of the a-amino acids - to help you answer these questions.)
1. Glycine is the only a-amino acid that is not chiral . Explain why.
2. Which two a-amino acids have a chiral centre in their side group? Draw out the structures and label these centres.
3. Using 3D diagrams like those in Figure 2, draw out the configurations of L-alanine, D-alanine, L-cysteine and L-proline. |
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 |
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