Effect of pressure on the catalytic activity of subtilisin Carlsberg suspended in compressed gases

We studied the effect of pressure up to 300 bar on the catalytic efficiency of subtilisin Carlsberg suspended in compressed propane, near-critical ethane, near-critical carbon dioxide and tert-amyl alcohol, at constant temperature and fixed enzyme hydration. Increasing pressure lowered the catalytic efficiency of the enzyme in all the solvents, resulting in positive activation volumes, Delta V-#. The Delta V-# values in compressed propane and in tert-amyl alcohol were similar and larger in magnitude than the value reported in the literature for the same reaction in an aqueous buffer, although within the range of typical Delta V-# values in aqueous media. In the near-critical fluids, the Delta V-# were much larger, e.g., an increase in pressure of only 200 bar causing a sixfold decrease in the catalytic efficiency of subtilisin in carbon dioxide. These data should reflect the proximity of ethane and carbon dioxide to the critical point, and the resulting condensation of solvent molecules about the solutes, yielding negative solute partial molar volumes. (C) 1998 Elsevier Science B.V.