Water is widely assumed to be essential for life, although the exact molecular basis of this requirement is unclear. Water facilitates protein motions, and although enzyme activity has been demonstrated at low hydrations in organic solvents, such nonaqueous solvents may allow the necessary motions for catalysis. To examine enzyme function in the absence of solvation and bypass diffusional constraints we have tested the ability of an enzyme, pig liver esterase, to catalyze alcoholysis as an anhydrous powder, in a reaction system of defined water content and where the substrates and products are gaseous. At hydrations of 3 molecules of water per molecule of enzyme, activity is several orders-of-magnitude greater than nonenzymatic catalysis. Neutron spectroscopy indicates that the fast (
this enzyme, implying that one of the biological requirements of water may lie with its role as a diffusion medium rather than any of its more specific properties.