Modeling hydration mechanisms of enzymes in nonpolar and polar organic solvents

A comprehensive study of the hydration mechanism of an enzyme in nonaqueous media was done using molecular dynamics simulations in five organic solvents with different polarities, namely, hexane, 3-pentanone, diisopropyl ether, ethanol, and acetonitrile. In these solvents, the serine protease cutinase from Fusarium solani pisi was increasingly hydrated with 12 different hydration levels ranging from 5% to 100% (w/w) (weight of water/weight of protein). The ability of organic solvents to 'strip off' water from the enzyme surface was clearly dependent on the nature of the organic solvent. The rmsd of the enzyme from the crystal structure was shown to be lower at specific hydration levels, depending on the organic solvent used. It was also shown that organic solvents determine the structure and dynamics of water at the enzyme surface. Nonpolar solvents enhance the formation of large clusters of water that are tightly bound to the enzyme, whereas water in polar organic solvents is fragmented in small clusters loosely bound to the enzyme surface. Ions seem to play an important role in the stabilization of exposed charged residues, mainly at low hydration levels. A common feature is found for the preferential localization of water molecules at particular regions of the enzyme surface in all organic solvents: water seems to be localized at equivalent regions of the enzyme surface independently of the organic solvent employed.