Nonribosomally synthesized peptides

Fig. 10. Structures of nonribosomally synthesized peptides of bacterial origin (1-6) and fungal origin (7-9). Me, N-methylated peptide boni Orn, ornithine 4-MHA, 4-methyl-3-hydroxyanthranilic acid Aad, aminoadipic acid Aeo, 2-amino-9,10-epoxy-8-oxodecanoic acid D-Hiv, D-hydroxyisovaleric acid Bmt, (4i )-4-[( )-2-butenyl]-4-methyl-L-threonine Abu, a-aminoisobutyric acid Sar, sarcosine. The boxes signify gene products for peptide synthetases composed of modules which activate and process the indicated amino acids...

The existing literature on toxins of entomopathogenic fungi, although substantial, deals with relatively few species 6, 12). Virtually nothing is known of the secondary chemistry of many fiingai pathogens of invertebrates. Of those toxins that have been described, the three principal classes of chemistries are polyketides (PK), nonribosomally synthesized peptides (NRSP), and alkaloids. 

Some nonribosomally synthesized peptides recognize and incorporate carboxy acids other than amino acids into nonribosomally synthesized peptides (Marahiel et al., 1997 von Dohren et al., 1997). Examples of non-amino acid substrates include 2-hydroxyvaleric acid in AM-toxin and enniatin, 2,3-dihydroxybenzoic acid in enterobactin, and lysergic acid in the ergopeptines. The incorporation of non-amino acids into peptides appears to be catalyzed by typical peptide synthetase modules (Haese et al., 1993 Gehring et al., 1998 Johnson et al., 2000). 

A glycopeptide consists of a carbohydrate linked to a peptide composed of l- and/or D-amino acids. This would also include a biologically important class of glycopeptides that are nonribosomally synthesized, for example, vancomycin and bleomycin. However, this class of natural products will not be covered in this chapter as excellent reviews on this topic can be found elsewhere. ... 

Earlier in this chapter, it was mentioned that many of the nonprotein amino acids are components of nonribosomal peptides. During such a biosynthesis, the peptide is attached to a carrier protein through a thioester bond, until chain termination occurs and the final product is released. The carrier protein is posttranslationally modified by the attachment of a phosphopantetheinyl group from coenzyme A. This step gives rise to the active carrier protein with a phosphopantetheine arm upon which amino acids are added to during NRPS. As an example, loading of isoleucine onto the carrier protein is depicted below (Scheme 5). Further details about nonribosomal peptide syntheses and enzymatic reactions can be found in Chapter 5.19. 

Based on our current understanding of ribosomal protein synthesis, several strategies have been developed to incorporate amino acids other than the 20 standard proteinogenic amino acids into a peptide using the ribosomal machinery . This allows for the design of peptides with novel properties. On the one hand, such a system can be used to synthesize nonstandard peptides that are important pharmaceuticals. In nature, such peptides are produced by nonribosomal peptide synthetases, which operate in complex pathways. On the other hand, non-natural residues are a useful tool in biochemistry and biophysics to study proteins. For example, incorporation of non-natural residues by the ribosome allows for site-specific labeling of proteins with spin labels for electron paramagnetic resonance spectroscopy, with... 

Additionally, a vast array of natural lipopeptides with remarkable structural diversity is produced by microorganisms living in different habitats, from aquatic to terrestrial environments. These products are not gene encoded, but are synthesized nonribosomally by large multifunctional enzymes, the peptide synthetases. 6"8 ... 

As beta-lactam antibiotics continue to be a major contributor to human health preservation, research on the biosynthesis of penicillin, an almost ancient drug, continues to open up roads to new technologies and perspectives. The provision of precursor peptides to be transformed enzymatically with chemically unachieved efficiency into mono- or bicyclic antibiotics has been termed by Jack Baldwin and colleagues the irreversible commitment of metabolic carbon to the secondary metabolism [1]. The synthesis of such peptides is indeed performed by a remarkable class of synthetases which, in contrast to the protein-synthesizing machinery, have been termed a nonribosomal system or nonribosomal peptide synthetases (NRPS) [2]. These peptide synthetases have been shown to catalyze the irreversible synthesis of peptides differing both in sequence and stmctural variability, thus extending the scope of directly gene-encoded poly-... 

A similar modular type arrangement of activities is present in the nonribosomal peptide synthetases (NRPS). Several examples have been described that use a hybrid PKS-NRPS system to synthesize epothiolones,370 rifamycin,371 and C-1027372 with reviews on these hybrid systems being described.373 374... 

Nonribosomal peptide synthetases have a modular structure and carry out the syntheses of peptides about two to fifteen amino acids in length... 

The mammalian multifunctional fatty acid synthase is a member of a large family of complex enzymes termed megasynthases that participate in step-by-step synthetic pathways. Two important classes of compounds that are synthesized by such enzymes are the polyketides and the nonribosomal peptides. The antibiotic erythromycin is an example of a polyketide, whereas penicillin (Section 8.5.5) is a nonribosomal peptide. 

Many natural peptides are synthesized by a sequence of enzyme-controlled processes carried out by a multifunctional enzyme of modular arrangement, similar to some polyketide synthases. These nonribosomal peptide synthetases (NRPSs) typically consist of an adenylation domain, a peptidyl carrier protein domain, and a condensation or elongation domain in order to carry out amide bond formation and some derivations of amino acid residues. 

PN is synthesized by three enzymes encoded by a cluster of three genes (Fig. 10.IB). The arrangement of PN genes in a cluster was first demonstrated in A. nidulans The first enzyme is a nonribosomal peptide synthetase, ACV synthetase (ACVS), catalyzing the condensation of L-a-aminoadipic acid (L-a-AAA), L-cysteine, and L-valine into a tripeptide, 8-(L-a-aminoadipyl)-L-cysteinyl-D-valine (ACV) (Fig. 10.4). In A. nidulans, ACV synthetase is encoded by the intronless. 

In bacterial cells and in yeast, which are, for obvious reasons, much more commonly studied than other animals and plants, amino acids, synthesized de novo or by degradation of proteins of various kinds and sizes, are found in the cytosol. The amino acids from these proteins are used for the synthesis of peptides. Much of the synthesis occurs either on/in the ribosomes of the endoplasmic reticulum (ER) or through nonribosomal pathways using acyl carrier protein-like condensation reactions. 

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