The bovine lung 20S proteasome binding to reversible inhibitors: modulation by sodium ion

The effect of sodium ion on the inhibition exerted by Cbz-Leu-Leu-Leu-CHO on the chymotrypsin-like activity of the 20S proteasome isolated from bovine lung was investigated. The experimental data were analyzed using a standard linkage formalism. The calculated equilibrium affinity constants for the sodium ion binding to the free-enzyme and the inhibitor-bound enzyme are compatible to other well-characterized ion-involving heterotropic systems. The functional interdependence between the binding events played by the inhibitor and the sodium ion conforms to a heterotropic modulatory mechanism. (C) 2003 Federation of European Biochemical Societies. Published by Elsevier Science B.V. All rights reserved.