Aspartic acid peptides naturally occurring

From the ring opening of p-lactams by amines and a-amino acids, p-amino amides and fi-amino acid derived peptides are generated. These structures are of interest because of their presence in several naturally occurring macrocyclic compounds [82, 83]. One example of the latter is the p-hydroxy aspartic acid derived tripeptide 45 found in the macrocyclic peptide lactone antibiotic lysobactin 40 [84], Fig. 4. 

Example Reaction of the naturally occurring amino acids, aspartic acid (Asp) and phenylalanine (Phe), produces the artificial sweetener aspartame (Asp-Phe), a dipeptide which contains two residues. Notice the peptide link, -CO-NH-, shown in the box. 

The third mechanism is represented by nonmodular one-step peptide synthesis. Enzymes belonging to this group catalyze the biosynthesis of poly(amino acids). Naturally occurring poly(amino acids) comprise cyanophy-cin [multi-L-arginyl-poly-(L-aspartic acid) cyanophycin granule polypeptide, (CGP)], (poly-(e-lysine) (PL), and poly-(y-glutamate) (PGA). As a consequence of non-ribosomal biosynthesis these peptides reveal a polydisperse mass distribution. 

In 1981, aspartame became the first new sweetener to be approved by the U.S. Food and Drug Administration (FDA) in nearly 25 years. It is about 160 times sweeter than sucrose. Structurally, aspartame is the methyl ester of a dipeptide of two amino acids that occur naturally in proteins—aspartic acid and phenylalanine—and is sold under the trade name NutraSweet . (Amino acids and peptides will be discussed in the next chapter.)... 

Figure 39.1a shows the generic structure of microcystins (MC), with the two most variable positions marked by X and Z. The numbering of the individual positions was done before the elucidation of the biosynthetical pathway and is arbitrary (see below). MCs are cyclic heptapeptides with several structural peculiarities. The moiety characteristic of microcystins and nodularins is the Adda, abbreviated for 3-amino-9-methoxy-2,6,8,-trimethyl-10-phenyl-4,6-decadienoic acid, a P-amino acid that is not known from other natural products. Further peculiarities of the molecule are o-amino acids. These stereoisomers of commonly occurring L-amino acids are not found in ribosomally synthesized proteins and peptides and are therefore called nonproteinogenic amino acids. A-methyl-dehydroalanine (Mdha) in position 7 also is a nonproteinogenic amino acid as it is the case with o-erythro-P-methyl-iso-aspartate (o-MeAsp). 

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