1.5.2 Linking more amino acids
Consider what would be possible if you placed another glycine molecule to the right of the dipeptide shown in Figure 4b. Another peptide bond could be formed between them to give a chain of three glycine molecules, a tripeptide (tri-means ‘three’ as in a word like tricycle). Adding another one would give a chain of four glycines, and so on. In fact, you could make a molecule of as many glycine molecules as you wanted. You might find it easier to think of them as railway trucks, with a hook on each end, being joined together to form a long train. Of course, all the amino acids have ‘hooks’ on each end of the molecule, so an almost infinite variety of proteins are possible, using the whole range of amino acids. In the train analogy, you could join all sorts of different trucks and carriages together to make a train, as long as they all had the same sort of hook on the end of them. In chemical terms, a chain of many amino acids joined together is called a polypeptide (poly- means ‘many’). A long polypeptide chain, with various amino acids attached together in the correct order is called a protein, though some proteins are made up of more than one polypeptide chain. Biologists often use the words ‘polypeptide’ and ‘protein’ interchangeably.
Can you recall from Figure 4, how many amino acids are linked together to form a typical protein molecule?
Several hundred amino acids is typical. For example, the protein part of the haemoglobin molecule, which transports oxygen around the body consists of four polypeptide chains, two made up from 141 amino acids and two from 146 amino acids.