Nonribosomal Peptide Biosynthesis

During the biosynthesis of nonribosomal peptides, there are two ways to incorporate the nonprotein amino acids. They can be incorporated either as a single unit or as an L-a-amino acid, which then undergoes structural modifications, while attached to the carrier protein. In the case of coronamic acid, L-rr//o-isoleucine is loaded onto the carrier protein and a unique biosynthetic pathway produces a cyclopropyl group containing a nonprotein amino acid. Specific examples of the biosynthesis of nonprotein amino acids will be discussed in the following sections. 

Wilkinson B, Micklefield J (2009). Biosynthesis of Nonribosomal Peptide Precursors. Meth Enzymol 458 353... 

It is likely that the biosynthesis of 113 is directed by a hybrid polyketide syn-thase/nonribosomal peptide synthetase enzyme system, as indicated in Figure 11.19. 

Staunton, J. and Wilkinson, B. (2001) Combinatorial biosynthesis of polyketides and nonribosomal peptides. Current Opinion in Chemical Biology, 5, 159. 

Kopp, F. and Marahiel, M.A. (2007) Macrocyclization strategies in polyketide and nonribosomal peptide biosynthesis. Natural Product Reports, 24, 735. 

In this chapter, we will introduce an exciting class of natural product biosynthetic enzymes, the modular synthases, as well as their associated enzyme partners. We will discuss the use of metabolic engineering as a tool for small-molecule discovery and development, both through directed fermentation and combinatorial biosynthesis. In addition, we will review six classes of partner enzymes involved in the modification of polyketide (PK) and nonribosomal peptide (NRP) natural products. We believe that these enzymatic transformations hold great opportunities for synthetic chemists and will serve as the foundation for a new trend in both discovery and process chemistry. 

Duerfahrt, T., Eppelmann, K., Muller, R. andMarahiel, M.A. (2004) Rational design ofabimodular model system for the investigation of heterocyclization in nonribosomal peptide biosynthesis. Chemistry Biology, 11, 261-271. 

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. 

There is only a small selection of nonprotein amino acids that contain carbonyl groups in the form of ketone, aldehyde, and carboxylic acid moieties, as part of the side chain. The examples given in Table 6 are components of nonribosomal peptides isolated from bacteria or fungi and siderophores from bacteria. The biosynthesis of these amino acids is not clear however, some of the amino acids with carboxylic acid side chains may be traced back to the L-a-amino acids aspartic acid and glutamic acid. 

Phosphopantetheine tethering is a posttranslational modification that takes place on the active site serine of carrier proteins - acyl carrier proteins (ACPs) and peptidyl carrier proteins (PCPs), also termed thiolation (T) domains - during the biosynthesis of fatty acids (FAs) (use ACPs) (Scheme 23), polyketides (PKs) (use ACPs) (Scheme 24), and nonribosomal peptides (NRPs) (use T domain) (Scheme 25). It is only after the covalent attachment of the 20-A Ppant arm, required for facile transfer of the various building block constituents of the molecules to be formed, that the carrier proteins can interact with the other components of the different multi-modular assembly lines (fatty acid synthases (FASs), polyketide synthases (PKSs), and nonribosomal peptide synthetases (NRPSs)) on which the compounds of interest are assembled. The structural organizations of FASs, PKSs, and NRPSs are analogous and can be divided into three broad classes the types I, II, and III systems. Even though the role of the carrier proteins is the same in all systems, their mode of action differs from one system to another. In the type I systems the carrier proteins usually only interact in cis with domains to which they are physically attached, with the exception of the PPTases and external type II thioesterase (TEII) domains that act in trans. In the type II systems the carrier proteins selectively interact... 

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Carreras CW, Pieper R, Khosla C (1997) The Chemistry and Biology of Fatty Acid, Polyketide, and Nonribosomal Peptide Biosynthesis. 188 85-126 Ceulemans A (1994) The Doublet States in Chromium (III) Complexes. A Shell-Theoretic View. 171 27-68... 

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