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Dear colleagues: We have a 1.4 Angstrom structure of an enzyme showing a water molecule enclosed in a triangular pocket formed by the hydroxyl oxygens of 2 serine residues and a sulfhydryl group of an essential cysteine. The water molecule is forming a 2.8 Angstrom hydrogen bond with each of the hydroxyl groups of the 2 serines and a 2.9 Angstrom hydrogen bond with the sulfhydryl group of the cysteine. Is it possible for such a water molecule to lower the pKa of the cysteine and activate the thiol group? I would appreciate any comments or suggestions or information on any literature that I need to look up :> Sincerely, Deepak