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| a-amino acids
Proteins, whether large or small, are remarkably intricate pieces of molecular engineering. To understand what they are like and how they work, we need to be able to imagine what their molecules might look like. In this chapter you will see how scientists picture protein molecules and you will be introduced to some of the technical terms they use to describe them. Peptides are molecules made by joining a-amino acid units. Peptides with more than about 50 a-amino acid units are called polypeptides. Proteins are made from a single polypeptide molecule or from several linked together. The links can be covalent bonds, using disulphide bridges (-S-S-), or non-covalent, intermolecular forces (see Figure 1). |
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Peptide and protein structure |
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| 3.1 Primary structure All polypeptide molecules (chains) share a common structure called the peptide backbone. The backbone differs in length from one polypeptide to another. In nature all polypeptide chains are linear, not branched. |
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 Figure 2 A small section of a polypeptide chain |
In addition there are side chain groups (residues) that belong to each a-amino acid unit (Figures 2, 3 on this page, and Figure 3 Chapter 2). The number of each type of residue and their order along the backbone give each polypeptide its unique properties.
The a-amino acid composition of a polypeptide tells us the numbers of each a-amino acid unit in the molecule. The primary structure of a polypeptide tells us the sequence in which the different units are linked. Protein chemists write amino acid sequences with the free -NH2 (or -NH3+) end of the molecule on the left and the free -COOH (or -COO-) end on the right. In Chapter 4 you will see how genes determine the primary structures of proteins. |
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Primary sequence of a small peptide |
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Unilever Education Advanced Series: Proteins | ||||||||||
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