These amino acids contain additional amine groups or other groups which contain easily protonated nitrogen atoms in their side chains. Their isoelectric point lies in the "basic area".
The substituents of the side chains are primarily responsible for the formation of hydrogen bonds. In this way, they contribute to the quaternary structure of the proteins.
The imidazole fragments of histidine are one of the most important binding sites of metal ions in proteins.
(Examples: Haemoglobine, Urease, Zinc fingers, Carboanhydrase, Haemocyanin, Azurin)
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