Composition, Structure and Purpose of Polypeptide Chains

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Primary Structure

If we look at different amino acids in polypeptide chains, and the sequence in which they are linked together, we find that, for a particular polypeptide chain, that they are always the same. This sequence is known as the Primary Structure.

Amino Acids of which there are approximately twenty within a polypeptide chain can be linked together in any format, which means there are nearly an infinite number of possible polypeptide chains. A change in just one amino in the chain will change the protein, which may behave in a different way.

Within a haemoglobin molecule there are two types of polypeptide chains, a α chain and a β chain. Each of these chains has slightly different primary structures and length. The total length of an α chain is 141 amino acids while the β chain is comprised of 146 amino acids.

Each of the twenty different amino acids has its own name - valine, leucine, cysteine...and so on. Often their names are abbreviated to three letters - val, leu and cys.

Secondary Structure

Polypeptide chains do not usually lie straight; they are arranged in regular patterns formed by bonds between different amino acids. These arrangements make up the Secondary Structure.

The shape of an α-helix or β-fold is maintained by hydrogen bonds. A hydrogen bond is an attraction between an atom with a very small negative charge and an atom with a similarly small positive charge. In an α-helix in a polypeptide, the bond forms between the -CO group of one amino acids and the -NH2 group of an amino acid four places up in the chain.

Hydrogen bonds are nowhere near as strong as covalent bonds because of the absence of a shared electron, however the sheer number of hydrogen bonds within


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