Physicochemical Principles Governing the Intrinsically Disordered Salivary Peptide Histatin 5: Ensemble Structure, Electrostatics, and Environmental Responsiveness.

Histatin 5 (Hst5) is a histidine-rich intrinsically disordered protein (IDP) whose biological function arises from a highly heterogeneous conformational ensemble and environmental sensitivity. Unlike structured antimicrobial peptides that rely on persistent secondary motifs, Hst5 remains disordered across a wide range of conditions, enabling continuous adaptation to changes in pH, ionic strength, metal-ion concentration, macromolecular crowding, and interfaces such as membranes and mineral surfaces. Recent advances in small-angle X-ray scattering (SAXS), neutron-based methods, and surface-sensitive techniques, combined with computer simulations, have allowed quantitative characterization of Hst5's ensemble structure, thermodynamics, and interactions. This work synthesizes current understanding of how intrinsic disorder, charge regulation, histidine chemistry, and multiscale interactions govern Hst5 behavior. Beyond its biological relevance, Hst5 has emerged as a benchmark system for elucidating general physicochemical principles of IDPs and for testing integrative and machine-learning approaches that map sequence features to ensemble architecture.