Non-ribosomal peptide synthetases are important modular enzymes involved in the synthesis of peptide containing molecules in the cell. They are prevalent in bacterial and fungi cells but can also be found in animal and plant cells to synthesize specialised secondary metabolites. Each NRPS is specific to the synthesis of a single peptide but interestingly can use a larger range of substrates including D-amino acids (which are non-proteinogenic) and modified amino acids. Furthermore, the backbone of substrates can often be further modified than what you would see in ribosomal derived peptides- including cyclization against the backbone or oxidation, reduction and dehydration. ## Structure NRPSs are composed of repeating units called modules, each typically containing three core domains; **A domain** the A domain is the adenolation domain, it recognises a specific amino or hydroxy acid and then activates via the hydrolysis of ATP. **PCP domain** After activation, the substrate is loaded into the Peptidyl carrier protein domain. **C domain** The condensation domain then catalyses the linkages between substrates in neighbouring PCP domains, of differing modules. These modules can be part of a technically different NRPSs HOWEVER they must be linearly attached. this type of peptide elongation (where the PCP domains providing the substrates are from different NRPSs) is called non-colinear NRPS assembly and follows typical colinear biosynthetic logic (where for example PCP1 of module 1 and PCP2 of module 2 are attached together). This is a little confusing so I will restate this again: PCP domains that are involved in a condensation reaction can be from **different NRPSs** but they must be **in the correct linear assembly order**, ie, they act as if they are part of a single continuous chain of modules. This means: - upstream NRPS or module provides the donor PCP (with the growing peptide) - The downstream NRPS or module has the acceptor PCP and a C domain that condenses the next amino acid onto the peptide. **In the Initiation module there is no C domain.** **Termination module** The termination module releases the final product by cleaving the thioester bond between the peptide and the PCP domain. the termination domain is often a thioesterase (TE) or reductase (R) domain. TE types often perform hydrolysis or cyclization to release the peptide and the reductase reduces the thioester bond, releasing the peptide. ![[NRPS.JPG|400]] (Huang et al., 2024) **References** - [Huang, Z., Peng, Z., Zhang, M., Li, X. and Qiu, X. (2024). Structure, Function and Engineering of the Nonribosomal Peptide Synthetase Condensation Domain. _International Journal of Molecular Sciences_, [online] 25(21), p.11774. doi:https://doi.org/10.3390/ijms252111774.](https://www.mdpi.com/1422-0067/25/21/11774) - [Thierry Izoré, Ho, C., Kaczmarski, J.A., Athina Gavriilidou, Chow, K.-H., Steer, D., Robert, Schittenhelm, R.B., Julien Tailhades, Tosin, M., Challis, G.L., Krenske, E.H., Ziemert, N., Jackson, C.J. and Cryle, M.J. (2021). Structures of a non-ribosomal peptide synthetase condensation domain suggest the basis of substrate selectivity. _Nature Communications_, 12(1). doi:https://doi.org/10.1038/s41467-021-22623-0.](https://www.nature.com/articles/s41467-021-22623-0)