Slow Conformational Changes of the Inorganic Pyrophosphatase from Bakers' Yeast Induced by Divalent Metal Ions

A study of the role of divalent metal ions, in particular of Mg 2+ and Mn 2+ in the reaction of the inorganic pyrophosphatase of bakers' yeast has been carried out. It is shown that the free metal ions compete with each other for the same metal binding site. Binding to this site causes reversible conformational changes of the enzyme as indicated by the kinetics. This assumption is in agreement with significant alterations in the circular dichroism spectrum. In the presence of inorganic pyrophosphate the changes of the enzyme are slow enough to be followed kinetically over a period of minutes. Other possibilities for a slow transition of the enzyme velocity which might exist according to the hysteretic enzyme concept of Frieden could be rejected. To explain the experimental data a kinetic scheme is proposed. The results show that, the metal ions affect, the hydrolysis at the substrate site through a conformational change of the protein.