NRPS, Nonribosomal peptide synthesis

CPs are integral components of various primary and secondary metabolic pathways, including fatty acid synthesis (FAS), nonribosomal peptide synthesis (NRPS), polyketide synthesis (PKS), and lysine biosynthesis. All CPs harbor... 

Figure 7. General scheme of nonribosomal peptide synthesis (NRPS). Each NRPS module incorporates one amino acid into the growing peptide chain. The modules are composed of several domains Adenylation domain (red) is responsible for substrate selectivity, peptidyl carrier protein domain (orange) and condensation domain (green) work synergistically to form the peptide bond, and thioester domain (blue) which terminates the reaction, resulting in either a linear or cyclic polypeptide.

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-... 

Three categories of synthetases are distinguished, based on their substrate specificity and mode of product synthesis. The two known types of polyketide synthetases (PKSs) (Type I and II) utilize acyl-coenzyme A (CoA) monomers while nonribosomal peptide synthetases (NRPSs) use amino acids and their analogs as substrates. Type I PKS and NRPS oligomerize these building blocks by a modular assembly-line arrangement while type II PKS iteratively assembles monomeric units. 

In the case of linear gramicidin, the N-terminus of the nonribosomal peptide carries a formyl group (10). Just like in the bacterial ribosomal synthesis, only a formylated first building block is processed additionally by the corresponding enzymatic machinery. Thus, one can find a distinct formylation (F) domain at the very N-terminus of the synthetase. Another formylated NRPS product is coelichelin whose N-terminal ornithine residue is believed to be Nj-formylated in trans by a formyltransferase genetically associated with the NRPS (17). Formyl-tetrahydrofolate is used as source of the formyl group by these enzymes. 

The isolated TE domain from the tyrocidine (tyc) NRPS has recently been shown to catalyze the macrocyclization of unnatural substrates to generate a variety of cyclic peptides. In conjunction with standard solid-phase peptide synthesis, Walsh and coworkers demonstrated a broad substrate tolerance for peptidyl-N-acetylcysteamine thioesters by the tyrocidine TE [41,42], Cyclization of peptide analogs, where individual amino acids were replaced with ethylene glycol units, was observed with high efficiency. In addition, hydroxyacid starter units were readily cyclized by the isolated TE domain to form nonribosomal peptide-derived macrolactones. More recently, Walsh and coworkers have demonstrated effective cyclization of PEGA resin-bound peptide/polyketide hybrids by the tyrocidine TE domain [43], Utilization of a pantetheine mimic for covalent attachment of small molecules to the resin, serves as an appropriate recognition domain for the enzyme. As peptide macrocyclizations remain challenging in the absence of enzymatic assistance, this approach promises facile construction of previously unattainable structures. 

Many enzymes are involved in the synthesis of secondary metabolites. The modular biosynthetic enzymes polyketide synthase (PKS) and nonribosomal peptide synthetase (NRPS) are responsible for the generation of a multitude of structurally diverse and biologically important small-molecule natural products. A complex carbon structure is assembled sequentially from simple carbon building blocks (acyl-CoA and amino acids). The elongation of each carbon unit is catalyzed by... 

One of the many unique features found in NRPs is that they are not limited to the 20 standard common amino acids. Rather, NRPs often contain unusual structural elements including D-amino acids, peptide-derived heterocycles, and A-methyl groups." Access to these moieties is the consequence of the way in which NRPs are biosynthesized by nonribosomal peptide synthetases (NRPSs). Perhaps not surprisingly, the molecular logic underpinning NRP synthesis is very similar to that in the PKS systems." ... 

---