Enzymatic catalysis and dynamics in low-water environments.

Enzymes suspended in organic solvents represent a versatile system for studying the involvement of water in enzyme structure and function. Addition of less than 1% (vol/vol) water to tetrahydrofuran containing 1 M 1-propanol leads to a substantial increase in the transesterification activity of subtilisin Carlsberg (from Bacillus licheniformis) that correlates with a sharp increase in the active-site polarity and a 90% decrease in the rotational correlation time (i.e., increase in mobility) of a nitroxide spin label within the active site. Water in excess of 1% has little additional effect on active-site polarity and coincides with a further increase in spin-label mobility, yet the transesterification activity decreases dramatically. Thus, transesterification activity increases and then decreases with increasing enzyme hydration and flexibility (which are presumably coupled through dielectric screening), suggesting that the conformation of partially hydrated subtilisin is different from that of the nearly dry enzyme--i.e., enzyme containing less than 9% (wt/wt) water.