Fluorine-Induced Elimination Drives Mechanism-Based Inactivation of Isocitrate Lyase

Abstract

Fluorinated succinate analogues were evaluated as mechanistic probes of Mycobacterium tuberculosis isocitrate lyase (MtICL). However, 2,2-difluorosuccinate (1; Ki = 6.1 mM) and 2,2,3-trifluorosuccinate (2; Ki = 23.5 μM) act as reversible noncompetitive inhibitors and meso-2,3-difluorosuccinate (3) displayed slow-onset reversible inhibition (Ki = 30 μM), the 2-fluorosuccinate enantiomers ((R)-4 and (S)-4) produced time-dependent irreversible inactivation. Inactivation by 4 was observable under turnover conditions in the presence of glyoxylate and succinate, consistent with a two-step kinetic mechanism. The S enantiomer inactivated more efficiently than (R)-4, consistent with stereoelectronic alignment required for elimination of HF following abstraction of the pro-S proton. 1H NMR analysis detected maleate formation from (S)-4, and mass spectrometry revealed a +132 Da adduct consistent with covalent modification of Cys191. Notably, kinact/KI values for 4 exceeded that measured for maleate, indicating that covalent capture occurs from an enzyme-bound intermediate prior to product release. These results support a mechanism in which fluorine substitution redirects the enolate-generating half-reaction of MtICL toward elimination and covalent modification. (S)-2-Fluorosuccinate therefore represents a succinate-analogue mechanism-based inactivator that exploits a catalytic step distinct from previously described isocitrate-analogue inhibitors.