Catalysis and Linear Free Energy Relationships in Aspartic Proteases

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Abstract
Aspartic proteases are receiving considerable attention as potential drug targets in several serious diseases, such as AIDS, malaria, and Alzheimer's disease. These enzymes cleave polypeptide chains, often between specific amino acid residues, but despite the common reaction mechanism, they exhibit large structural differences. Here, the catalytic mechanism of aspartic proteases plasmepsin II, cathepsin D, and HIV-1 protease is examined by computer simulations utilizing the empirical valence bond approach in combination with molecular dynamics and free energy perturbation calculations. Free energy profiles are established for four different substrates, each six amino acids long and containing hydrophobic side chains in the P1 and P1‘ positions. Our simulations reproduce the catalytic effect of these enzymes, which accelerate the reaction rate by a factor of ∼1010 compared to that of the corresponding uncatalyzed reaction in water. The calculations elucidate the origin of the catalytic effect and allow a rationalization of the fact that, despite large structural differences between plasmepsin II/cathepsin D and HIV-1 protease, the magnitude of their rate enhancement is very similar. Amino acid residues surrounding the active site together with structurally conserved water molecules are found to play an important role in catalysis, mainly through dipolar (electrostatic) stabilization. A linear free energy relationship for the reactions in the different enzymes is established that also demonstrates the reduced reorganization energy in the enzymes compared to that in the uncatalyzed water reaction.