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 lam inin 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 m etal-binding site (site III) were found to bind Zn2+. Site II is located cl ose 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 g roup and alters the conformations of His(83) and His(136), as well as a dis ordered region over the steroid-binding site. Site III. is formed by the si de chains of His(101) and the carboxylate group of Asp(117) and the distanc e 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 chemical ly-related metal ions (Cd2+ and Hg2+) have similar but less pronounced effe cts. This is not observed when Zn2+ coordination at site II is modified by substituting Gln for His(136). An alteration in the steroid-binding specifi city of human SHBG by Zn2+ occupancy of site II may be relevant in male rep roductive tissues where zinc concentrations are very high.