Biochemical and Functional Evaluation of the Intramolecular Disulfide Bonds in the Zinc-Chelating Antimicrobial Protein Human S100A7 (Psoriasin)

Abstract

Human S100A7 (psoriasin) is a metal-chelating protein expressed by epithelial cells. It is a 22-kDa homodimer with two EF-hand domains per subunit and two transition-metal-binding His₃Asp sites at the dimer interface. Each subunit contains two cysteine residues that can exist as free thiols (S100A7_red) or as an intramolecular disulfide bond (S100A7_ox). Herein, we examine the disulfide bond redox behavior, the Zn(II) binding properties, and the antibacterial activity of S100A7, as well as the effect of Ca(II) ions on these properties. In agreement with prior work [Hein, K. Z., et al. (2013) Proc. Natl. Acad. Sci. U. S. A. 112, 13039-13044], we show that apo S100A7_ox is a substrate for the mammalian thioredoxin system; however, negligible reduction of the disulfide bond is observed for Ca(II)- and Zn(II)-bound S100A7_ox. Furthermore, metal binding depresses the midpoint potential of the disulfide bond. S100A7_ox and S100A7_red each coordinate 2 equiv of Zn(II) with subnanomolar affinity in the absence and presence of Ca(II) ions, and the cysteine thiolates in S100A7_red do not form a third high-affinity Zn(II) site. These results refute a prior model implicating the Cys thiolates of S100A7_red in high-affinity Zn(II) binding [Hein, K. Z., et al. (2013) Proc. Natl. Acad. Sci. U. S. A. 112, 13039-13044]. S100A7_ox and the disulfide-null variants show comparable Zn(II)-depletion profiles; however, only S100A7_ox exhibits antibacterial activity against select bacterial species. Metal substitution experiments suggest that the disulfide bonds in S100A7 may enhance metal sequestration by the His₃Asp sites and thereby confer growth inhibitory properties to S100A7_ox.