Characteristics of beta, beta-difluoroalanine and beta, beta, beta -trifluoroalanine as suicide substrates for Escherichia coli B alanine racemase.

The alanine racemase from Escherichia coli B has been shown to process DL isomers of beta -fluoroalanine as suicide substrates with an identical partitioning ratio for each enantiomer of 820 catalytic eliminations of HF per enzymatic inactivation event [Wang, E., & Walsh, C. T. (1978) Biochemistry 17, 1313], suggesting the aminoacrylate--PLP complex as a common, symmetrical partitioning species. In an attempt to vary the partition ratio, an index of killing efficiency, systematically the beta, beta-difluoroalanine and beta, beta, beta-trifluoroalanine isomers have now been evaluated for substrate processing, suicidal inactivation kinetics and partitioning ratio, and stability of inactive, derivatized enzyme forms. Both difluoroalanine isomers show high Km values (116 mM for D, 102 mM for L) in catalytic HF loss to form fluoropyruvate. The Vmax for the D isomer is about 14-fold higher than that for the L isomer. Limiting inactivation rate constants, calculated from kcat and observed partition ratios of 5000 and 2600, respectively, are 2.2 min-1 for D-difluoroalanine and 0.33 min-1 for L-difluoroalanine. For comparison, DL-trifluoroalanine turns over less than 10 times per enzyme molecule inactivated and so is a very efficient suicide substrate. The estimated inactivation rate constant is less than or equal to 1.0 min-1. These data are analyzed in terms of partitioning behavior of the monofluoro- and difluoroaminoacrylate--PLP complexes as partitioning intermediates for turnover or for racemase inactivation. While mono- and trifluoroalanines yield stable inactive species, the difluoroalanine isomers produce labile enzyme derivatives, and regain of catalytic activity is analyzed in terms of the anticipated oxidation state at the beta carbon of the substrate fragment adducted to the enzyme.