Anaerobic pathway for conversion of the methyl group of aromatic methyl ethers to acetic acid by Clostridium thermoaceticum

Abstract

Clostridium thermoaceticum and other anaerobic acetogenic bacteria can utilize the methyl group of aromatic methyl ethers as a carbon and energy source. It has been unclear what pathway is used to metabolize this methyl group. In the work reported here, the pathway was established by identifying and quantitating the substrates, stable intermediates, and products of O-demethylation of syringic acid. By measuring the dependence of the O-demethylation reaction on purified enzymes of the acetyl-CoA pathway, it was established that CO dehydrogenase, the corrinoid/iron-sulfur protein, and methyltransferase all were required for acetyl-CoA formation. By 13C-NMR spectroscopy it was shown that the O-demethylase from C. thermoaceticum converts the methyl group of syringate to methyltetrahydrofolate (CH3-H4folate). When the reaction was conducted in the presence of CO, H2, or titanium(III), or in the absence of any electron donor, the rate of demethylation of syringic acid at pH 7.2 was approximately 15 nmol min-1 mg-1. In the absence of CO, CH3-H4folate accumulated as a stable product. When CO was added, 13CH3-H4folate was converted to [2-13C]acetyl-CoA, [2-13C]acetyl phosphate, and [2-13C]acetate. Therefore, the acetogenic O-demethylase uses H4folate as acceptor of the methyl group of phenyl methyl ethers and catalyzes the formation of CH3-H4folate. The pathway of conversion of CH3-H4folate, CO, and CoA to acetyl-CoA has been studied previously. Methyltransferase catalyzes the reaction of CH3-H4folate with the corrinoid/iron-sulfur protein to form a methylcobalt species. The nickel/iron-sulfur enzyme CO dehydrogenase then catalyzes the final steps in the formation of acetyl-CoA.(ABSTRACT TRUNCATED AT 250 WORDS)