Molecular mimicry of substrate oxygen atoms by water molecules in the beta-amylase active site

Soybean beta -amylase (EC 3.2.1.2) has been crystallized both free and comp lexed with a variety of ligands. Four water molecules in the free-enzyme ca talytic cleft form a multihydrogen-bond network with sight strategic residu es involved in enzyme-ligand hydrogen bonds. We show here that the position s of these four water molecules are coincident with the positions of four p otential oxygen atoms of the ligands within the complex. Some of these wate rs ale displaced from the active site when the ligands bind to the enzyme. How many are displaced depends on the shape of the ligand. This means that when one of the four positions is not occupied by a ligand oxygen atom, the corresponding water remains. We studied the functional/structural role of these four waters and conclude that their presence means that the conformat ion of the eight side chains is fixed in all situations (free or complexed enzyme) and preserved from unwanted or forbidden conformational changes tha t could hamper the catalytic mechanism, The water structure at the active p ocket of beta -amylase is therefore essential for providing the ligand reco gnition process with plasticity. It does not affect the protein active-site geometry and preserves the overall hydrogen-bonding network, irrespective of which ligand is bound to the enzyme. We also investigated whether other enzymes showed a similar role for water. Finally, we discuss the potential use of these results for predicting whether water molecules can mimic ligan d atoms in the active center.