Electrochemical Insights into Histatin 5 Antimicrobial Activity: Cu(II) Redox Chemistry and Model Lipid Layer Interactions

Histatin 5 (Hst5) is one of the most potent antifungal members of the salivary histatin family, exhibiting activity against the opportunistic fungus Candida albicans, with reported intracellular localisation to mitochondria and effects on mitochondrial function. This localization, together with Hst5's ability to bind redox-active Cu(II) ions, suggested that Cu(II)-Hst5 complexes could catalyze ROS production via Cu(II)/Cu(I) cycling. The high stability of the primary Cu(II)-Hst5 complex engaging the ATCUN motif and its high resistance to the reduction call into question the feasibility of such a mechanism. Employing electrochemical methods and spectroscopic measurements, we investigated how copper coordination influences redox reactivity in Hst5 and its N-terminal fragments, examining two possible sources of redox activity in Cu(II)/Hst5 mixtures. The first involves either incomplete Cu(II) coordination within the ATCUN motif or Cu(II) ions not bound to Hst5. The second is the binding of excess Cu(II) to additional Cu(II)-binding sites in the Hst5 sequence. We also utilized electrochemical techniques to investigate the interactions of Hst5 and its Cu(II) complexes with DMPC-based lipid layers, revealing stronger perturbation by full-length Hst5 than by its N-terminal fragments.