Steroid-binding specificity of human sex hormone-binding globulin is influenced by occupancy of a zinc-binding site

One calcium-binding site (site I) and a second poorly defined metal-binding site (site II) have been observed previously within the amino-terminal laminin G-like domain (G domain) of human sex hormone-binding globulin (SHBG). By soaking crystals of this structure in 2.5 mM ZnCl2, site II and a new metal-binding site (site III) were found to bind Zn2+. Site II is located close to the steroid-binding site, and Zn2+ is coordinated by the side chains of His(83) and His(136) and the carboxylate group of Asp(65). In this site, Zn2+ prevents Asp(65) from interacting with the steroid 17 beta-hydroxy group and alters the conformations of His(83) and His(136), as well as a disordered region over the steroid-binding site. Site III. is formed by the side chains of His(101) and the carboxylate group of Asp(117) and the distance between them (2.7 Angstrom) is increased to 3.7 Angstrom in the presence of Zn2+. The affinity of SHBG for estradiol is reduced in the presence of 0.1-1 mM Zn2+, whereas its affinity for androgens is unchanged, and chemically-related metal ions (Cd2+ and Hg2+) have similar but less pronounced effects. This is not observed when Zn2+ coordination at site II is modified by substituting Gln for His(136). An alteration in the steroid-binding specificity of human SHBG by Zn2+ occupancy of site II may be relevant in male reproductive tissues where zinc concentrations are very high.