Structural basis for antibiotic action of the B1 antivitamin 2'-methoxy-thiamine

The natural antivitamin 2'-methoxy-thiamine (MTh) is implicated in the suppression of microbial growth. However, its mode of action and enzyme-selective inhibition mechanism have remained elusive. Intriguingly, MTh inhibits some thiamine diphosphate (ThDP) enzymes, while being coenzymatically active in others. Here we report the strong inhibition of Escherichia coli transketolase activity by MTh and unravel its mode of action and the structural basis thereof. The unique 2'-methoxy group of MTh diphosphate (MThDP) clashes with a canonical glutamate required for cofactor activation in ThDP-dependent enzymes. This glutamate is forced into a stable, anticatalytic low-barrier hydrogen bond with a neighboring glutamate, disrupting cofactor activation. Molecular dynamics simulations of transketolases and other ThDP enzymes identify active-site flexibility and the topology of the cofactor-binding locale as key determinants for enzyme-selective inhibition. Human enzymes either retain enzymatic activity with MThDP or preferentially bind authentic ThDP over MThDP, while core bacterial metabolic enzymes are inhibited, demonstrating therapeutic potential.

Carbon Kinetic Isotope Effects and the Mechanisms of Acid-Catalyzed Decarboxylation of 2,4-Dimethoxybenzoic Acid and CO2 Incorporation into 1,3-Dimethoxybenzene

The rate of decarboxylation of 2,4-dimethoxybenzoic acid (1) is accelerated in parallel to the extent that the carboxyl group acquires a second proton (1H⁺). However, the conjugate acid would resist C-C bond breaking as that would lead to formation of doubly protonated CO₂. An alternative via formation of a higher-energy protonated phenyl tautomer (2H⁺) prior to C-C bond breaking would produce protonated CO₂, an energetically inaccessible species that can be avoided by transfer of the carboxyl proton to water in the same step. Headspace sampling of CO₂ that evolves in the acid-catalyzed process and analysis by GC-IRMS gives a smaller than expected value of 1.022 for the carbon kinetic isotope (CKIE), k¹²/k¹³. While this value establishes that C-C cleavage is part of the rate-determining process, intrinsic CKIEs for decarboxylation reactions are typically greater than 1.03. Computational analysis of the C-C bond cleavage from 2H⁺ gives an intrinsic CKIE of 1.051 and suggests two partially rate-determining steps in the decarboxylation of 1: transfer of the second carboxyl proton to the adjacent phenyl carbon and C-C cleavage in which the carboxyl proton is also transferred to water. Applying the principle of microscopic reversibility to fixation of CO₂ in acidic solutions reveals the importance of proton transfers to both carbon and oxygen in the overall fixation process.