Reduction of human defensin 5 affords a high-affinity zinc-chelating peptide

Human defensin 5 (HD5) is a 32-residue cysteine-rich host-defense peptide that exhibits three disulfide bonds in the oxidized form (HD5_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 _red, is a metal-ion chelator. HD5 has a midpoint potential of -257 mV at pH 7.0. HD5_red utilizes its cysteine residues to coordinate one equivalent of Zn(II) with an apparent K_d1 value in the midpicomolar range. Zn(II) or Cd(II) binding perturbs the oxidative folding pathway of HD5_red to HD5_ox. Whereas HD5_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.