Enzyme catalysis in organic solvents with low water content at high temperatures. The adenosinetriphosphatase of submitochondrial particles

Abstract

A system composed of toluene, phospholipids, and Triton X-100 in which the ATPase activity of bovine heart submitochondrial particles can be studied at low water concentrations and high temperatures is described. In this system, ATPase activity starts to appear at 0.5% (v/v) water and increases as the concentration of water is increased. At 3.8% water, the enzyme exhibits saturation kinetics with respect to Mg-ATP with a Km similar to that observed in an all-water system (approximately 300 microM), but the Vmax is about 100 times lower (6 nmol min-1 mg-1) than that in water. At concentrations of water between 0.5% and 2%, the enzyme catalyzes ATP hydrolysis at temperatures of up to 91 degrees C. The conditions for achieving catalysis at high temperatures are described. Even though at low water concentrations the enzyme catalyzes ATP hydrolysis at temperatures significantly higher than in totally aqueous media, the optimal temperature for hydrolysis (approximately 58 degrees C) is independent of the water content. The half-life of the enzyme at high temperatures is significantly higher at low water concentrations than in aqueous media. In the system described, the enzyme is located in a compartment distinct from that of the substrate and products of the reaction. Apparently, the enhancement of catalytic rates by water is due to a higher conformational mobility of the protein; the same factor causes a decrease in the thermostability of the enzyme.