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Theoretical Studies on the Catalytic Cycle of Histidine Acid Phosphatases Revealing an Acid Proof Mechanism.

After reporting the mechanisms of purple acid phosphatases against acid environments and alkaline phosphatases against alkaline environments, in the present work, we continued investigating the relationship between catalytic structures of histidine acid phosphatases (HAPs) and acid environments. On the basis of the comparison of the crystal structures of several HAP members, a series of models were constructed and calculated using density functional theory. Our calculations describe a complete catalytic cycle of HAPs, including a free stage and a catalytic reaction stage. This cycle reveals a definite mechanism for HAPs to survive in acidic environments, which can be used to nicely interpret acidic pH optima of HAPs. It also suggests that a free water molecule from a solvent should be the nucleophile for hydrolyzing the phosphohistidine intermediate. Our studies are focused on the biological significance of enzymatic mechanisms and raise two concrete criteria: the logic-complete catalytic cycle and the evolutional relation with family members and molecular environments.