Abstract
Non-ribosomal peptide synthetases (NRPSs) are modular biosynthetic megaenzymes producing many important natural products and refer to a specific set of peptides in bacteria’s and fungi’s secondary metabolism. With the actual purpose of providing advantages within their respective ecological niche, the bioactivity of the structurally highly diverse products ranges from, e.g., antibiotic (e.g., vancomycin) to immunosuppressive (e.g., cyclosporin A) to cytostatic (e.g., echinomycin or thiocoralin) activity.
An NRPS module consists of at least three core domains that are essential for the incorporation of specific substrates with the 'multiple carrier thiotemplate mechanism' into a growing peptide chain: an adenylation (A) domain selects and activates a cognate amino acid; a thiolation (T) domain shuffles the activated amino acid and the growing peptide chain, which are attached at its post-translationally 4ʹ-phosphopantetheine (4'-PPant) group, between the active sites; a condensation (C) domain links the upstream and downstream substrates. NRPS synthesis is finished with the transfer of the assembled peptide to the C-terminal chain-terminating domain. Accordingly, the intermediate is either released by hydrolysis as a linear peptide chain or by an intramolecular nucleophilic attack as a cyclic peptide.
The NRPS’s modular character seems to imply straightforward engineering to take advantage of their features but appears to be more challenging. Since the pioneering NRPS engineering approaches focused on the reprogramming and replacement of A domains, several working groups developed advanced methods to perform a complete replacement of subdomains or single or multiple catalytic domains.
The first part of this work focusses parts of the publication with the title 'De novo design and engineering of non-ribosomal peptide synthetases', which follows up assembly line engineering with the development of a new guideline. Thereby, the pseudodimeric V-shaped structure of the C domain is exploited to separate the N-terminal (CDSub) and C-terminal (CASub) subdomains alongside a four-AA-long linker. This results in the creation of self-contained, catalytically active CASub-A-T-CDSub (XUC) building blocks. As an advantage over the previous XU concept, the characteristics (substrate- and stereoselectivity) assigned to the C domain subunits are likewise exchanged, and thus, no longer represent a barrier. Furthermore, with the XUC concept, no important interdomain interfaces are disrupted during the catalytic cycle of NRPS, allow to expect much higher production titers. Moreover, the XUC concept shows a more flexible application within its genus origin of building blocks to create peptide libraries. Additionally, with this concept only 80 different XUC building blocks are needed to cover the entire proteinogenic amino acid spectrum.
The second part of this work addresses the influence of the C domain on activity and specificity of A domains. In a comprehensive analysis, a clear influence of different C domains on the in vitro activation rate and the in vivo substrate spectrum could be observed. Further in situ and in silico characterizations indicate that these influences are neither the result of the respective A domains promiscuity nor the C domain’s proofreading, but due to an 'extended gatekeeping' function of the C domain. This novel term of an 'extended gatekeeping' function describes the very nature of interfaces that C domains can form with an A domain of interest. Therefore, the C-A interface is assumed to have a more significant contribution to a selectivity filter function.
The third part of this work combines the NRPS engineering with phylogenetic/evolutionary perspectives. At first, the C-A interface could be precisely defined and further identified to encode equivalent information corresponding to the complete C-A didomain. Moreover, the comparison of NRPSs topology reveals hints for a co-evolutionary relatedness of the C-A didomain and could be shown to reassemble even after separation. In this regard, based on a designed CAopt.py algorithm, the reassembling-compatibility of hybrid interfaces could be determined by scoring of the co-expressed NRPS hybrids. This algorithm also enables the randomization of the interface sequences, thus, leading to the identification of more functional interface variant, which cause significantly higher peptide production and could even be applied to other native and hybrid interfaces.
