Dynamics in the active site of β-secretase: a network analysis of atomistic simulations

The aspartic protease β-secretase (BACE) catalyzes the hydrolysis of the amyloid precursor protein (APP) which leads to amyloid-β aggregation and, ultimately, the perilous Alzheimer's disease. The conformational dynamics and free energy surfaces of BACE at three steps of the catalytic cycle are studied here by explicit solvent molecular dynamics simulations (multiple runs for a total of 2.2 μs). The overall plasticity of BACE is essentially identical for the three states of the substrate: the octapeptide reactant, gem-diol intermediate, and cleavage products. In contrast, the network of hydrogen bonds in the active site is more stable in the complex of BACE with the gem-diol intermediate than the other two states of the substrate. The spontaneous release of the C-terminal (P1'-P4') fragment of the product follows a single-exponential time dependence with a time constant of 50 ns and does not require the opening of the flap. The fast dissociation of the C-terminal fragment is consistent with the transmembrane location and orientation of APP and its further processing by γ-secretase. On the other hand, the N-terminal (P4-P1) fragment of the product does not exit the BACE active site within the simulation time scale of 80 ns. A unified network analysis of the complexes of BACE with the three states of the substrate provides an estimation of the activation free energy associated with the structural rearrangements that involve only noncovalent interactions. The estimated rearrangement barriers are not negligible (up to 3 kcal/mol) but are significantly smaller than the barrier of the peptide bond hydrolysis reaction.