Functional optimisation and structure-activity relationship studies of helix-loop peptides from the skin secretions of the frogs, Odorrana schmackeri and Amolops wuyiensis

It was found that addition of a leucine residue at position 2, enhanced both antibacterial and anti-cancer activity. Moreover, the incorporation of a D-leucine residue significantly decreased haemolytic activity. These data indicated that N-terminal hydrophobicity and the C-terminal cyclic rana box domain, are critical for the bioactivity of brevinin-1 peptides and propose B1OS-D-L as an attractive therapeutic candidate for clinical development.<br><br>In Chapter 4, a novel ranatuerin-2 peptide was isolated and identified from the skin secretion of Amolops wuyiensis and named ranatuerin-2-AW (R2AW).

Five analogues were progressively designed to evaluate the role of the rana box and optimise the dual antibacterial and anti-cancer activities of R2AW. The results illustrated that the absence of a disulphide bridge and rana box domain did not affect the antibacterial activity of R2AW. Additionally, a cationicity- and hydrophobicity-enhanced variant, R2AW-LK, which displayed significantly improved antibacterial and anti-cancer activity, was successfully designed.

Thus, this study demonstrated that rational design and modification are ideal for optimising the dual activities of ranatuerin-2 peptides and propose R2AW-LK as a promising agent for clinical application.<br>In Chapter 5, a novel brevinin-2 peptide, brevinin-2-OS (B2OS), was isolated and identified from the skin secretion of Odorrana schmackeri. The modification ideas from Chapters 2 and 4 were merged to design a C-terminal truncated and D-leucine incorporated analogue, t-D-B2OS-NH2, with enhanced antibacterial activity and optimised haemolytic activity.

The results suggested that the artificial deletion of the rana box resulted in better antibacterial activity, faster killing efficacy and lower toxicity. Overall, this study indicated that the rana box domain is dispensable in brevinin-2 and ranatuerin-2 helix-loop peptides and proposed the design strategy to optimise activity by incorporating a D-leucine residue on the hydrophobic face.<br><br><i>Thesis embargoed until 31 July 2027</i>