Effects of metal ions on the rates and enantioselectivities of reactions catalyzed by a series of semisynthetic transaminases created by site directed mutagenesis

Fatty acid binding proteins are a class of small 15 kDa proteins with a simple architecture that forms a large solvent sequestered cavity. In previous work, we demonstrated that reductive amination reactions could be performed in this cavity by covalent attachment of a pyridoxamine cofactor and that the rate, enantioselectivity and substrate specificity of these reactions could be altered by site directed mutagenesis. Herein, we show that the chemistry performed by these conjugates can be extended to include catalytic transamination and describe the effects of added metal ions on reaction rate and enantioselectivity. We conclude that metal ions can be used to increase the rate of reactions catalyzed by semisynthetic transaminases; however, the addition of metal ions can also retard the reaction rate. Furthermore, it appears that the presence of metal ions almost always results in an erosion of reaction enantioselectivity. This limits their utility as a practical means of increasing reaction rate. The results reported here, for four independent systems, should be considered in future designs of artificial transaminases.