the antimicrobial peptides

Antimicrobial Peptides

Antimicrobial peptides are small-molecular-weight proteins that exhibit antimicrobial activity by interacting with negatively charged microbial membranes. They usually contain 7-100 amino acids. Antimicrobial peptides are usually found in the innate immune system. Antimicrobial peptides, such as RiPPs, begin as linear precursors composed of a leader and a core peptide region (Baumann, Nickling, Bartholomae, Buivydas, Kuipers, & Budisa, 2017). Posttranslational alteration is then used to make chemical modifications to the inactive nucleus. Finally, the leader is severed from the nucleus, resulting in the release of a mature and functioning antimicrobial peptide.

The Study by Baumann et al. (2017)

The study by Baumann et al. (2017) aims to increase the chemical diversity of the antimicrobial peptides by the use of post- translational modification enzymes and the incorporation of non- canonical amino acids (nCAA).

Challenges in Modification of Antimicrobial Peptides

The modification of antimicrobial peptides is challenging in many aspects, especially in large-scale synthesis. The yield during large scale production is one of the biggest challenges. An example is the synthesis of lactocin S which includes a massive 71 reaction steps with a final yield of just 10 percent (Baumann et al., 2017). Strict control over peptide precursor and PTM enzyme expression is necessary for SPI. There is also a challenge with quality control during tRNA charging. The proofreading mechanism which discriminates hydroxylated variants of phenylalanine impedes the production process leading to the drastic reduction of yield. For the SCS method, there is also need for well- balanced setups due to the genetic complexity of tRNA, AMP precursor peptide, aaRS, and PTM enzyme expression (Baumann et al., 2017).

Figures 1 and 2: Chemical Modification of Antimicrobial Polypeptides

Figures 1 and 2 in the article are illustrations of the chemical modification of antimicrobial polypeptides. Figure 1 reviews modification through the SPI method, the SCS method, and post- translational modification. In the SPI method, isostructural variants of the canonical amino acids, such as Met*, in the illustration are charged onto the tRNA by E. coli methionyl- tRNA synthetase (Baumann et al., 2017). In the SCS method, the amber stop codon is recognized by a suppressor tRNA thus leading to consequent charging with the target non- canonical amino acid. In the PTM illustration, dehydratase NisB is responsible for dehydration of residues in preparation for eventual cyclization by NisC. Figure 2, on the other hand, shows the efficacy of antimicrobial nisin which has been incorporated with non- canonical amino acids (Baumann et al., 2017).

Sequencing of Antimicrobial Peptides using SPI

Antimicrobial peptides are usually sequenced by SPI using proline- auxotrophic E. coli which is based on SPI using L. lactis. The expression and secretion of NisP by L. lactis activates recombinant production of nisin variants by E. coli (Baumann et al., 2017). The addition of non- canonical amino acids leads to the increase of the bacterial cell densities. This method has proved feasible in the production of bioactive nisin.

References

Baumann, T., Nickling, J.H., Bartholomae, M., Buivydas, A., Kuipers, O., Budisa, N. (2017). Prospects of in vivo incorporation of non-canonical amino acids for the chemical diversification of antimicrobial peptides. Frontiers in Microbiology, 8, 124.