Bacteria and fungi use large multifunctional enzymes, the so-called nonribosomal peptide synthetases (NRPSs), to produce peptides of broad structural and biological activity. Biochemical studies have contributed substantially to the understanding of the key principles of these modular enzymes that can draw on a much larger number of catalytic tools for the incorporation of unusual features compared with the ribosomal system. Several crystal structures of NRPS-domains have yielded deep insight into the catalytic mechanisms involved and have led to a better prediction of the products assembled and to the construction of hybrid enzymes. In addition to the structure-function relationship of the core- and tailoring-domains of NRPSs, which is the main focus of this review, different biosynthetic strategies and essential enzymes for posttranslational modification and editing are discussed.

The ability to synthesize nonribosomally small bioactive peptides that find application in modern medicine is widely spread among microorganisms. As broad as the spectrum of biological activities is the structural diversity of these peptides, which are mostly cyclic or branched cyclic compounds containing non-proteinogenic amino acids, small heterocyclic rings and other unusual modifications in the peptide backbone. They are synthesized by multimodular enzymes, the so-called nonribosomal peptide synthetases (NRPSs), from simple building blocks. Biochemical and genetic studies have unveiled the key principles of nonribosomal peptide syntheses, as well as the realization of many structural features of these peptides. This review focuses on recent results in NRPS research and highlights how this knowledge can be exploited for biotechnological purposes. In addition, possibilities and limitations for prediction of structural features of uncharacterized NRPSs and approaches for their engineering are discussed.

4 -Phosphopantetheine transferases (PPTases) transfer the 4 -phosphopantetheine moiety of coenzyme A onto a conserved serine residue of acyl carrier proteins (ACPs) of fatty acid and polyketide synthases as well as peptidyl carrier proteins (PCPs) of nonribosomal peptide synthetases. This posttranslational modification converts ACPs and PCPs from their inactive apo into the active holo form. We have investigated the 4 -phosphopantetheinylation reaction in Bacillus subtilis, an organism containing in total 43 ACPs and PCPs but only two PPTases, the acyl carrier protein synthase AcpS of primary metabolism and Sfp, a PPTase of secondary metabolism associated with the nonribosomal peptide synthetase for the peptide antibiotic surfactin. We identified and cloned ydcB encoding AcpS from B. subtilis, which complemented an Escherichia coli acps disruption mutant. B. subtilis AcpS and its substrate ACP were biochemically characterized. AcpS also modified the D-alanyl carrier protein but failed to recognize PCP and an acyl carrier protein of secondary metabolism discovered in this study, designated AcpK, that was not identified by the Bacillus genome project. On the other hand, Sfp was able to modify in vitro all acyl carrier proteins tested. We thereby extend the reported broad specificity of this enzyme to the homologous ACP. This in vitro cross-interaction between primary and secondary metabolism was confirmed under physiological in vivo conditions by the construction of a ydcB deletion in a B. subtilis sfp ؉ strain. The genes coding for Sfp and its homolog Gsp from Bacillus brevis could also complement the E. coli acps disruption. These results call into question the essential role of AcpS in strains that contain a Sfp-like PPTase and consequently the suitability of AcpS as a microbial target in such strains.4Ј-Phosphopantetheine (Ppant) 1 is an essential prosthetic group of several acyl carrier proteins involved in pathways of primary and secondary metabolism. These include acyl carrier proteins (ACPs) of fatty acid synthases (FASs), ACPs of polyketide synthases (PKSs), and peptidyl carrier proteins (PCPs) and aryl carrier proteins of nonribosomal peptide synthetases (NRPSs) (1, 2). The free thiol moiety of Ppant serves to covalently bind the acyl reaction intermediates as thioesters during the multistep assembly of the monomeric precursors, typically acetyl, malonyl, and aminoacyl groups. Ppant fulfills two demands in these biosynthetic pathways. First, the intermediates remain covalently tethered to the multifunctional enzyme templates in an energy-rich linkage. Second, the flexibility and length of Ppant (about 20 Å) facilitates the transport of the intermediates to the spatially distinct reaction centers. The Ppant moiety is derived from coenzyme A (CoA) and posttranslationally transferred onto an invariant serine side chain. This Mg 2ϩ -dependent conversion of the apoproteins to the holoproteins is catalyzed by the 4Ј-phosphopantetheine transferases (PPTases) (1, 2) (see Fig. 1).Most organisms that employ more t...

The acyl carrier proteins (ACPs) of fatty acid synthase and polyketide synthase as well as peptidyl carrier proteins (PCPs) of nonribosomal peptide synthetases are modified by 4-phosphopantetheinyl transferases from inactive apo-enzymes to their active holo forms by transferring the 4-phosphopantetheinyl moiety of coenzyme A to a conserved serine residue of the carrier protein. 4-Phosphopantetheinyl transferases have been classified into two types; the AcpS type accepts ACPs of fatty acid synthase and some ACPs of type II polyketide synthase as substrates, whereas the Sfp type exhibits an extraordinarily broad substrate specificity. Based on the previously published co-crystal structure of Bacillus subtilis AcpS and ACP that provided detailed information about the interacting residues of the two proteins, we designed a novel hybrid PCP by replacing the Bacillus brevis TycC3-PCP helix 2 with the corresponding helix of B. subtilis ACP that contains the interacting residues. This was performed for the PCP domain as a single protein as well as for the TycA-PCP domain within the nonribosomal peptide synthetase module TycA from B. brevis. Both resulting proteins, designated hybrid PCP (hPCP) and hybrid TycA (hTycA), were modified in vivo during heterologous expression in Escherichia coli (hPCP, 51%; hTycA, 75%) and in vitro with AcpS as well as Sfp to 100%. The designated hTycA module contains two other domains: an adenylation domain (activating phenylalanine to Phe-AMP and afterward transferring the Phe to the PCP domain) and an epimerization domain (converting the PCP-bound L-Phe to DPhe). We show here that the modified PCP domain of hTycA communicates with the adenylation domain and that the co-factor of holo-hPCP is loaded with Phe. However, communication between the hybrid PCP and the epimerization domain seems to be disabled. Nevertheless, hTycA is recognized by the next proline-activating elongation module TycB1 in vitro, and the dipeptide is formed and released as diketopiperazine.

Phosphopantetheinyl-dependent carrier proteins are part of fatty-acid synthases (primary metabolism), polyketide synthases, and non-ribosomal peptide synthetases (secondary metabolism). For these proteins to become functionally active, they need to be primed with the 4-phosphopantetheine moiety of coenzyme A by a dedicated phosphopantetheine transferase (PPTase). Most organisms that employ more than one phosphopantetheinyl-dependent pathway also have more than one PPTase. Typically, one of these PPTases is optimized for the modification of carrier proteins of primary metabolism and rejects those of secondary metabolism (AcpS-type PPTases), whereas the other, Sfp-type PPTase, efficiently modifies carrier proteins involved in secondary metabolism. We present here a new type of PPTase, the carrier protein synthase of Pseudomonas aeruginosa, an organism that harbors merely one PPTase, namely PcpS. Gene deletion experiments clearly show that PcpS is essential for growth of P. aeruginosa, and biochemical data indicate its association with both fatty acid synthesis and siderophore metabolism. At first sight, PcpS is a PPTase of the monomeric Sfp-type and was consequently expected to have catalytic properties typical for this type of enzyme. However, in vitro characterization of PcpS with natural protein partners and non-cognate substrates revealed that its catalytic properties differ significantly from those of Sfp. Thus, the situation in P. aeruginosa is not simply the result of the loss of an AcpS-type PPTase. PcpS exhibits high catalytic efficiency with the carrier protein of fatty acid synthesis and shows a reduced although significant conversion rate of the carrier proteins of non-ribosomal peptide synthetases from their apo to holo form. This association with enzymes of primary and secondary metabolism indicates that PcpS belongs to a new sub-class of PPTases. 4Ј-Phosphopantetheine (Ppant)1 -dependent carrier proteins (CP) are the central entity in fatty-acid synthases (FAS), polyketide synthases (PKS), and non-ribosomal peptide synthetases (NRPSs) (1, 2). The superfamily of CP includes the acidic acyl and aryl carrier proteins and the neutral peptidyl carrier proteins (PCPs). They can be part of a larger polypeptide chain or exist as distinct proteins but always fulfill the same job; during the multistep assembly of the product, the reaction intermediates of the growing acyl or polypeptide chain remain covalently tethered to the Ppant cofactor moiety of these proteins. This moiety is about 20 Å in length and enables the bound intermediates to move between the reaction centers of multifunctional proteins. The thioester linkage that is used to bind the intermediates and final product is energy-rich, which facilitates cleavage after the final step of the assembly. After ribosomal synthesis, however, the carrier protein exists only in the inactive apo form. The Ppant moiety is post-translationally transferred from coenzyme A to a conserved serine residue of the CP in a Mg 2ϩ -dependent reaction by a dedicated phosph...