The paper explains the 3‑D shape of the human antimicrobial peptide LL‑37 and shows how its structure lets it kill bacteria, break down biofilms, and block some viruses. It highlights which parts of the molecule are most important for sticking to bacterial membranes and DNA.

Host defense antimicrobial peptides are key components of human innate immunity that plays an indispensible role in human health. While there are multiple copies of cathelicidin genes in horses, cattle, pigs, and sheep, only one cathelicidin gene is found in humans. Interestingly, this single cathelicidin gene can be processed into different forms of antimicrobial peptides. LL-37, the most commonly studied form, is not only antimicrobial but also possesses other functional roles such as chemotaxis, apoptosis, wound healing, immune modulation, and cancer metastasis. This article reviews recent advances made in structural and biophysical studies of human LL-37 and its fragments, which serve as a basis to understand their antibacterial, anti-biofilm and antiviral activities. High-quality structures were made possible by using improved 2D NMR methods for peptide fragments and 3D NMR spectroscopy for intact LL-37. The two hydrophobic domains in the long amphipathic helix (residues 2-31) of LL-37 separated by a hydrophilic residue serine 9 explain its cooperative binding to bacterial lipopolysaccharides (LPS). Both aromatic rings (F5, F6, F17, and F27) and interfacial basic amino acids of LL-37 directly interact with anionic phosphatidylglycerols (PG). Although the peptide sequences reported in the literature vary slightly, there is a consensus that the central helix of LL-37 is essential for disrupting superbugs (e.g., MRSA), bacterial biofilms, and viruses such as human immunodeficiency virus 1 (HIV-1) and respiratory syncytial virus (RSV). In the central helix, the central arginine R23 is of particular importance in binding to bacterial membranes or DNA. Mapping the functional roles of the cationic amino acids of the major antimicrobial region of LL-37 provides a basis for designing antimicrobial peptides with desired properties. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova.