Micellar and Metal Ion Catalysis in the Pyridoxal-Promoted α,β-Elimination of S-Phenylcysteine

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Abstract
The effects of metal ions and detergents on the pyridoxal-catalyzed α,β-elimination reaction of S-phenylcysteine have been investigated in 0.01 M borate buffer at 50 °C (the copper ion catalysis at 40 °C in the presence of CTAB micelles). Both copper(II) and nickel(II) ions markedly accelerated the elimination reaction of S-phenylcysteine. Since the catalytic efficiency of nickel(II) seems to be obtained due to the poor coordination ability of the leaving group in the present reaction, the previous working hypothesis which was provided for the elucidation of catalytic roles of metal ions in the α,β-elimination of O-phosphothreonine has now been approved. Among cationic, anionic, and non-ionic detergents employed in this work as apoenzyme models, only CTAB of cationic nature showed a considerable rate acceleration even in the absence of metal ions. The micellar binding constant was evaluated from the saturation-type correlation between reaction rate and CTAB concentration. The surfactant micelles demonstrated some enzyme-like features of catalysis in the pyridoxal-promoted α,β-elimination reaction, where the reaction is controlled by the entropy term. The CTAB catalysis was greatly inhibited by organic and inorganic anions due to the electrostatic nature of the micellar surface. The catalytic mechanisms for the micellar reactions and their significance as an enzyme model system are discussed.