It has been proposed that BACE1 and aspartic proteases catalyze peptide bond hydrolysis through an acid-base catalysis mechanism mediated by Wat1 and the Asp pair as follows. (i) After substrate binding, the free Asp pair activates Wat1 by forming a hydrogen bond with it. (ii) Next, the activated Wat1, it has been proposed, nucleophilically attacks the scissile-bond carbonyl. The resulting geminal diol intermediate is stabilized by hydrogen bonds with the carboxyl group of Asp.Finally,decomposition of the scissile C-N bond is accompanied by the transfer of a proton from Asp to the leaving amino group (see scheme 1 in reference 32). In addition, kinetics studies suggested that Wat1 competitively inhibits the binding of inhibitor to the activesite(21).On the otherhand,the second conserved water molecule (Wat2) is involved in the hydrogen bond, with a conserved Tyr residue in the flap. Wat2 also participates in a conserved hydrogen-bonding network Wat2-Ser35-Asp32- Wat1-Asp228 and was proposed to assist in the catalytic reaction (2, 8). Thus, BACE1 activity is directly affected by the behaviors of these water molecules.