Pathway of propionate formation from ethanol in Pelobacter propionicus

Abstract

Whole cells of Pelobacter propionicus fermented (1-13C) ethanol and CO2 to nearly equal amounts of (2-13C) and (3-13C) propionate and to (1-13C) acetate indicating a randomizing pathway of propionate formation. Enzymes involved in the fermentation were assayed in cell-free extracts and cetyltrimethylammonium bromide-permeabilized cells grown with ethanol as sole substrate. Alcohol dehydrogenase, aldehyde dehydrogenase (benzylviologen-reducing), phosphate acetyl transferase, acetate kinase, pyruvate synthase, methylmalonyl CoA: pyruvate transcarboxylase, propionyl CoA: succinate CoA transferase, and the enzymes of the succinate-methylmalonyl CoA pathway all were detected at activities sufficient to be involved in ethanol fermentation. Very low amounts of a b-type cytochrome were detected in ethanol-grown cells (46 nmol x g rotein- 1). Low cell yields obtained with ethanol as substrate indicate that P. propionicus does not conserve energy by electron transport-linked fumarate reduction. Despite the presence of a hydrogenase and a shift in the fermentation of lactate towards the formation of more propionate in the presence of hydrogen, P. propionicus was unable to catalyze the reduction of acetate and CO2 to propionate, unlike Desulfobulbus propionicus.