Although it has been known that many of the enzymes may be instable in harsh conditions of supercritical CO2, recent experimental studies argued that the enzymes are active and stable in near-critical propane with same pressure and temperature condition. But there are no clear reasons at the molecular level for the activity and stability of the enzymes in such condition. Moreover, it is difficult to experimentally monitor the microstructure and dynamics of the enzyme in a supercritical fluid in situ. In this study the enzyme microenvironment in nearcritical propane is investigated using molecular dynamic simulation. For comparison with other solvent models the enzyme was also simulated in water, hexane and supercritical CO2. We examined the overall structural properties of Candida antarctica Lipase B in these four solvents and found that in supercritical CO2 there are high structural deviations from native form while in near-critical propane deviations are low and very close to those of in the mild aqueous solution and even are lower than those of in hexane. α-helix and β-sheet contents of the enzyme remain intact in near-critical propane. These theoretical results are in agreement with experimental evidences of the stability and activity of the enzyme in near-critical propane. Moreover, it was found that the activity of the enzyme in near-critical propane can be related to the water partitioning on the surface of the enzyme. The results of this study not only confirm the reported experimental findings about enzyme stability and activity in nearcritical propane, but also shed light on the structural situation of the enzyme in this condition. To our knowledge, this is the first molecular dynamic simulation of a protein in a non-CO2- based compressed gas.

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