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Internal Proton Transfer in the External Pyridoxal 5′-Phosphate Schiff Base in Dopa Decarboxylase
journal contribution
posted on 2010-01-12, 00:00 authored by Yen-lin Lin, Jiali GaoCombined quantum mechanical and molecular mechanical (QM/MM) simulations of dopa decarboxylase have been carried out to elucidate the factors that contribute to the tautomeric equilibrium of the intramolecular proton transfer in the external PLP−l-dopa Schiff base. The presence of a carboxylate anion on the α-carbon of the Schiff base stabilizes the zwitterions and shifts the equilibrium in favor of the oxoenamine tautomer (protonated Schiff base). Moreover, protonation of the PLP pyridine nitrogen further drives the equilibrium toward the oxoenamine direction. On the other hand, solvent effects favor the hydroxyimine configuration, although the equilibrium favors the oxoenamine isomer with a methyl group as the substituent on the imino nitrogen. In dopa decarboxylase, the hydroxyimine form of the PLP(H+)−l-dopa Schiff base is predicted to be the major isomer with a relative free energy of −1.3 kcal/mol over that of the oxoenamine isomer. Both Asp271 and Lys303 stabilize the hydroxyimine configuration through hydrogen-bonding interactions with the pyridine nitrogen of the PLP and the imino nitrogen of the Schiff base, respectively. Interestingly, Thr246 plays a double role in the intramolecular proton transfer process, in which it initially donates a hydrogen bond to the phenolate oxygen in the oxoenamine configuration and then switches to a hydrogen bond acceptor from the phenolic hydroxyl group in the hydroxyimine tautomer.
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intramolecular proton transferintramolecular proton transfer processDopa DecarboxylaseCombined quantumequilibriumSchiff basephenolic hydroxyl grouphydroxyimine configurationdopa decarboxylaseoxoenamine isomerprotonated Schiff baseInternal Proton Transferhydrogen bond acceptorPLP pyridine nitrogenimino nitrogenQM
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