Short but promising - how nature modulates the antimicrobial activity of proline-rich fragment of salivary MUC-7.

Antimicrobial peptides (AMPs), including mucin-derived sequences, play a vital role in host defense at mucosal surfaces by modulating microbial interactions and supporting innate immunity. However, their susceptibility to proteolytic cleavage limits their protective efficacy. This study investigates the peptide FPNPHQPPKHPDK (L1), derived from human salivary mucin MUC7, and its proteolytic fragments L2 (FPNPHQPPK) and L3 (HPDK), generated by trypsin cleavage. Using a combination of potentiometry, UV-vis spectroscopy, circular dichroism (CD), electron paramagnetic resonance (EPR), electrospray ionization mass spectrometry (ESI-MS), density functional theory (DFT) calculations, and antimicrobial assays, we elucidate the structural and thermodynamic aspects of metal ion coordination with Cu(II) and Zn(II), and assess their impact on antimicrobial efficacy. Our findings reveal that the L3 fragment forms the most thermodynamically stable complexes with both Cu(II) and Zn(II) ions, and exhibits the strongest antimicrobial activity, which is pH-dependent. These results suggest a mechanism involving metal sequestration, consistent with the concept of 'nutritional immunity'. Notably, natural proteolytic processing of the parent peptide enhances its functional properties upon metal coordination. This highlights a potential evolutionary advantage of peptide fragmentation in modulating antimicrobial activity, supporting the development of MUC7-derived peptides as promising templates for metal-based antimicrobial agents.