Recent Developments on the Biochemistry of Methanogenesis from Acetate

Abstract

Athough acetate is the major precursor for methanogenesis in nature, biochemical studies on this pathway have lagged behind studies on the pathway of carbon dioxide reduction to methane. In 1955, it was demonstrated that the methyl group of acetate is metabolized intact to methane by mixed cultures [17]. Research was stimulated 23 years later when Mah et al. [12] showed that Methanosarcina barkeri utilizes acetate as a sole growth substrate and produces methane at rates comparable to that observed in nature. Methods for large-scale culture of these slow growing organisms have been improved by use of the pH auxostat which delivers the large amounts of acetic acid required for growth while maintaining a constant pH [21]. Methanogenesis from acetate in cell exracts [1, 9] is an important development. Coenzyme M was shown to be a methyl carrier in the pathway by demonstrating deuterated methyl coenzyme M in Methanosarcina thermophila cells utilizing deuterated methyl acetate for methanogenesis [11]. Recently, the involvement of coenzyme M was also demonstrated in Methanosarcina barkeri using ¹⁴C-methyl labelled acetate [6]. The terminal step in methanogenesis from acetate is the reductive demethylation of methyl coenzyme M [6, 14]; thus, a common mechanism exists for the terminal step in methanogenesis from all known substrates. However, the mechanism of acetate cleavage and the steps leading to methyl coenzyme M in acetotrophic methanogens are unknown.