Mechanism of Anaerobic Ether Cleavage (original) (raw)

2002, Journal of Biological Chemistry

2-Phenoxyethanol is converted into phenol and acetate by a strictly anaerobic Gram-positive bacterium, Acetobacterium strain LuPhet1. Acetate results from oxidation of acetaldehyde that is the early product of the biodegradation process (Frings, J., and Schink, B. (1994) Arch. Microbiol. 162, 199-204). Feeding experiments with resting cell suspensions and 2-phenoxyethanol bearing two deuterium atoms at either carbon of the glycolic moiety as substrate demonstrated that the carbonyl group of the acetate derives from the alcoholic function and the methyl group derives from the adjacent carbon. A concomitant migration of a deuterium atom from C-1 to C-2 was observed. These findings were confirmed by NMR analysis of the acetate obtained by fermentation of 2-phenoxy-[2-13 C,1-2 H 2 ]ethanol, 2-phenoxy-[1-13 C,1-2 H 2 ]ethanol, and 2-phenoxy-[1,2-13 C 2 ,1-2 H 2 ]ethanol. During the course of the biotransformation process, the molecular integrity of the glycolic unit was completely retained, no loss of the migrating deuterium occurred by exchange with the medium, and the 1,2deuterium shift was intramolecular. A diol dehydrataselike mechanism could explain the enzymatic cleavage of the ether bond of 2-phenoxyethanol, provided that an intramolecular H/OC 6 H 5 exchange is assumed, giving rise to the hemiacetal precursor of acetaldehyde. However, an alternative mechanism is proposed that is supported by the well recognized propensity of ␣-hydroxyradical and of its conjugate base (ketyl anion) to eliminate a ␤-positioned leaving group. Ether linkages are comparably stable, and their cleavage requires rather rigorous conditions. Such cleavage reactions represent challenges also to microbes and their enzymes, and this difficulty causes the relative stability of many ether compounds in nature (1). An important group of xenobiotic ether compounds, the linear polyether PEG 1 and its derivatives, is released into the * This work was supported in part by a grant from the Deutsche Forschungsgemeinschaft, Bonn in its priority program "Radicals in enzymatic catalysis." The costs of publication of this article were defrayed in part by the payment of page charges.