1 5 Linking amino acids
1.5.1 Linking two amino acids
As we have seen earlier, atoms can combine to form molecules and now we will see how molecules can combine together to make bigger molecules. If two glycine molecules are placed side by side, as in Figure 4a, we can see how it is possible to remove one hydrogen and one oxygen atom from the left-hand one, and one hydrogen atom from the right-hand one, to make a water molecule, leaving a spare bond (‘arm’) to link the carbon (C) atom from the amino acid on the left with the N atom from the one on the right, as in Figure 4b. This bond is called a peptide bond (or sometimes an amide bond) and the new molecule is called a dipeptide (the prefix di- means ‘two’). The word ‘peptide’ comes from the Greek peptos meaning ‘digested’. You might like to check that after this joining process, all the atoms involved still retain the correct number of bonds – 4 for carbon, 3 for nitrogen, 2 for oxygen and 1 for hydrogen.