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

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) <i>Proc. Natl. Acad. Sci. U. S. A.</i> <i>112</i>, 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) <i>Proc. Natl. Acad. Sci. U. S. A.</i> <i>112</i>, 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<sub>ox