Functionalized bilayer membranes as artificial tryptophan synthase. Characterization of catalytic efficiency, substrate specificity, and reaction selectivity

Functionalized bilayer membranes having vitamin B₆ activity effectively catalyzed β-replacement reactions of serine with indoles to afford the corresponding tryptophan derivatives in aqueous media under mild conditions. Catalytic capability of the present artificial enzyme was subjected to change by changing a combination of molecular components constituting the catalyst system. The structural mode of a hydrophobic pyridoxal derivative as the coenzyme model, the catalytic ability of an amino acid residue placed in a peptide lipid which forms single-walled bilayer vesicles as the apoenzyme model, and the coordination property of added metal ions were found to be responsible for the overall catalytic performance. Multifunctional assistance was observed in the β-replacement reaction of serine with indole, and the reaction proceeded in preference to other side reactions, such as β-elimination, dealdolation, and transamination reactions. Substrate selectivity was found to be primarily dependent on the nucleophilicity of indole derivatives.