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Download fileA New Class of Conformationally Rigid Analogues of 4-Amino-5-halopentanoic Acids, Potent Inactivators of γ-Aminobutyric Acid Aminotransferase
journal contribution
posted on 2000-02-04, 00:00 authored by Jian Qiu, Richard B. SilvermanRecently, we found (Qiu, J.; Pingsterhaus, J. M.; Silverman, R. B. J. Med. Chem. 1999, 42,
4725−4728) that conformationally rigid analogues of the GABA aminotransferase (GABA-AT)
inactivator vigabatrin were not inactivators of GABA-AT. To determine if this is a general
phenomenon of GABA-AT inactivators, several mono- and di-halogen-substituted conformationally rigid analogues (7−15) of other GABA-AT inactivators, 4-amino-5-halopentanoic
acids, were synthesized as potential inactivators of GABA-AT. Four of them, (+)-7, (−)-9,
(+)-10, and (+)-15, were inactivators, although not as potent as the corresponding open-chain analogues. The maximal inactivation rate constants, kinact, for the fluoro- and bromo-substituted analogues were comparable, indicating that cleavage of the C−X bond is not
rate determining. Consistent with that observation is the finding that [3-2H]-10 exhibits a
deuterium isotope effect on inactivation of 3.3, suggesting that C−H bond cleavage is the
rate-determining step. The rate of inactivation of GABA-AT by the fluorinated analogue 7
is 1/15 that of inactivation by the corresponding open-chain analogue, 4-amino-5-fluoropentanoic acid (3a). Whereas inactivation by 3a releases only one fluoride ion, inactivation
by 7 releases 148 fluoride ions, accounting for the less efficient inactivation rate. Inactivation leads to covalent attachment of 2 equiv of inactivator after gel filtration; upon urea
denaturation, 1 equiv of radioactivity remains bound to the enzyme. This suggests that, unlike the open-chain anlogue, the conformationally rigid analogue becomes, at least partially,
attached to an active-site residue. It appears that the conformational constraint has a larger
effect on inactivators that inactivate by a Michael addition mechanism than by an enamine
mechanism.
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inactivation rate constantsk inactcovalent attachmenturea denaturationdeuterium isotope effect7 releases 148 fluoride ionsNew Classgel filtrationinactivation rateanalogue 72 equivenamine mechanismconformationallyfluoride ioninactivator vigabatrinMichael addition mechanismPotent InactivatorsGABA aminotransferase1 equiv