The term rare amino acids is misleading in some respects. It refers to all those amino acids that are not incorporated into proteins. We already got to know some of them like ornithine or citrulline that are rather common intermediates of the basic metabolism. Roughly 220 different structures are known, most of which occur in plant cells in a free state though glutamate-, oxalate- or acetyl- derivatives can also sometimes be found.
In some fungi, rare amino acids are polymerized to small, sometimes also cyclic polypeptides like in phalloidine or the amanitins, the toxins of the amanita.
Derivatives of nearly all well-known 20 amino acids have been isolated and described. A certain rare amino acid occurs often only in one or a few plant species.
Of the 220 different types of molecules, only the syntheses of two derivatives of arginine, octopine and nopaline will be shown here. They became known when it was found out that a plasmid of Agrobacterium tumefaciens, the pathogen of crown galls, is the carrier of the genetic information necessary for their syntheses.
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