The importance of N-methylations for the stability of the $$\beta ^{6.3}$$ -helical conformation of polytheonamide B.

Polytheonamide B (pTB), a highly cytotoxic peptide produced by a symbiotic bacterium of the marine sponge Theonella swinhoei, forms a transmembrane pore consisting of 49 residues. More than half of its residues are posttranslationally modified. Epimerizations result in alternating L- and D-amino acids that allow the peptide to adopt a $$\beta ^{6.3}$$ -helical conformation. Unusually, the wide $$\beta ^{6.3}$$ -helix of pTB is stable in a polar environment, which is in contrast to gramicidin A, an antibiotic with similar function and structure. The role of the other posttranslational modifications (PTMs) such as side chain hydroxylations, C- and N-methylations is not well understood. In this study, the importance of these PTMs for the stability of $$\beta ^{6.3}$$ -helix is investigated using computational tools. By reverting the modified residues to their precursors and monitoring the effect on the dominant structure, we show that the N-methylations are crucial for the stability of the $$\beta ^{6.3}$$ -helix in a polar environment. They are the driving force for the formation of stable side chain hydrogen-bond chains that act as an 'exoskeleton.' Such exoskeletons could present a general design strategy for helical peptides. [ABSTRACT FROM AUTHOR]