Heterocyclic Amino Acids and Related Substances

There are four amino acids, amongst the 20 that make up proteins, that have an aromatic side-chain, and of these, two have a heteroaromatic side-chain - histidine, with an imidazole, and tryptophan, with an indole. Both of these are amongst the essential amino acids , i.e. they need to be part of the diet since they cannot be biosynthesised by human beings. Decarboxylation of histidine produces the hormone histamine. Proline is the only heterocyclic DNA-coded a-amino acid - it is based on pyrrolidine hydroxyproline is an essential component of collagen, the hbrous structural protein that supports tissues and is the main component of cartilage. 

The ability of an imidazole (24.1.1.1 and 24.4.1) to act as both an acid (A-hydrogen) and a base (the imine nitrogen) is put to good use in the active sites of several enzymes in which the imidazole rings of appropriately placed histidines effectively shuffle protons from one place to another. One example is the 

Heterocyclic Chemistry 5th Edition John Joule and Keitii Mills 2010 Blackwell F iblishing Ltd 

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