Binding of inhibitory metals to yeast enolase

Certain divalent cations can inhibit yeast enolase by binding at sites that are distinct from those metal binding sites normally associated with catalytic activity, i.e., the conformational and catalytic binding sites. By using a buffer that does not compete with metal ions (tetrapropylammonium borate) Zn, Co, Mn, Cu, Cd, and Ni are found to exhibit similar inhibitory characteristics. Inhibition by those metals is alleviated by the addition of imidazole or tris buffer and, for zinc, by a metal chelating agent (Calcein). Inhibition by zinc was examined in detail through binding studies and enzymatic activity measurement. In tetrapropylammonium buffers at pH 8.0, enolase binds up to four moles of zinc per mole of enzyme (two moles per subunit). An imidazole concentration of 0.05 M reduces the binding: in the absence of substrate, just two moles of zinc per enzyme are bound. The enzyme will bind two additional moles of zinc upon the addition of substrate in either buffer, but the enzyme in tetrapropylammonium buffer is nearly inactive. Inhibition is, therefore, correlated with the binding of two moles of zinc per mole of enzyme. Some additional metal ions, Ca, Tb, Hg, and Ag also caused inhibition of yeast enolase but not by binding to the inhibitory site described.