Phloroglucinol pathway in the strictly anaerobic Pelobacter acidigallici : fermentation of trihydroxybenzenes to acetate via triacetic acid

Abstract

The strictly anaerobic, fermenting bacterium Pelobacter acidigallici degrades several trihydroxybenzene derivatives to stoichiometric amounts of acetate. We now report on the enzymatic activities in cell extracts which are responsible for the fermentative degradation of these aromatic compounds, and postulate a novel phloroglucinol pathway involving triacetic acid as an unusual metabolic intermediate. Gallate is decarboxylated to pyrogallol by a specific, Mg 2+-dependent, soluble enzyme activity, followed by conversion of pyrogallol to phloroglucinol, involving an unusual intermolecular transhydroxylation described previously. Phloroglucinol is then reduced to dihydrophloroglucinol (5-hydroxy-1,3-cyclohexanedione) by an NADPH-dependent phloroglucinol reductase. Dihydrophloroglucinol is cleaved hydrolytically to 3-hydroxy-5-oxohexanoic acid, which is then oxidized to triacetic acid (3,5-dioxohexanoic acid) by a unique, NADP § dehydrogenase. Triacetic acid is activated by CoA transfer from acetyl-CoA, and then converted to 3 acetyl-CoA by two subsequent fl-ketothiolase reactions. ATP is generated via phosphotransacetylase and acetate kinase.