Reduction of Human Defensin 5 Affords a High-Affinity Zinc-Chelating Peptide

Abstract

Human defensin 5 (HD5) is a 32-residue cysteine-rich host-defense peptide that exhibits three disulfide bonds in the oxidized form (HD5[subscript ox]). It is abundant in small intestinal Paneth cells, which release HD5 into the intestinal lumen and house a labile Zn(II) store of unknown function. Here, we consider the redox properties of HD5 and report that the reduced form, HD5[subscript red], is a metal-ion chelator. HD5 has a midpoint potential of −257 mV at pH 7.0. HD5[subscript red] utilizes its cysteine residues to coordinate one equivalent of Zn(II) with an apparent K[subscript d1] value in the midpicomolar range. Zn(II) or Cd(II) binding perturbs the oxidative folding pathway of HD5[subscript red] to HD5[subscript ox]. Whereas HD5[subscript red] is highly susceptible to proteolytic degradation, the Zn(II)-bound form displays resistance to hydrolytic breakdown by trypsin and other proteases. The ability of a reduced defensin peptide to coordinate Zn(II) provides a putative mechanism for how these peptides persist in vivo.