ABSTRACT
Zinc binds in solution to SEA with a Kv of 0.3 pM [100]. The zinc binding site seen in the crystals of SEA could indicate this protein can form dimers in the presence of zinc. However, gel permeation chromatography in the presence of Zn2+ concentra tions as high as 100 pM showed no evidence of dimer formation [100]. In contrast, the same experiment performed with Staphylococcus aureus enterotoxin type D (SED) yielded dimers at a 1 pM zinc concentration. SED crystallizes as a Zn2+-dependent dimer with two high-affinity zinc sites located between the C-terminal (3 sheets of the two monomers [102]. Each zinc is tetrahedrally coordinated by H isll8 from one SED molecule and Aspl82, His220, and Asp222 from the other. A similar zinc binding site is observed in the superantigen SEC [103] (Table 4).
