INVESTIGATION OF THE FUMARATE METABOLISM OF THE SYNTROPHIC PROPIONATE-OXIDIZING BACTERIUM STRAIN MPOB

The growth of the syntrophic propionate-oxidizing bacterium strain MPO B in pure culture by fumarate disproportionation into carbon dioxide a nd succinate and by fumarate reduction with propionate, formate or hyd rogen as electron donor was studied. The highest growth yield, 12.2 g dry cells/mol fumarate, was observed for growth by fumarate disproport ionation. In the presence of hydrogen, formate or propionate, the grow th yield was more than twice as low: 4.8, 4.6, and 5.2 g dry cells/mol fumarate, respectively. The location of enzymes that are involved in the electron transport chain during fumarate reduction in strain MPOB was analyzed. Fumarate reductase, succinate dehydrogenase, and ATPase were membrane-bound, while formate dehydrogenase and hydrogenase were loosely attached to the periplasmic side of the membrane. The cells co ntained cytochrome c, cytochrome b, menaquinone-6 and menaquinone-7 as possible electron carriers. Fumarate reduction with hydrogen in membr anes of strain MPOB was inhibited by 2-(heptyl)4-hydroxyquinoline-N-ox ide (HOQNO). This inhibition, together with the activity of fumarate r eductase with reduced 2,3-dimethyl-1,4-naphtoquinone (DMNH2) and the o bservation that cytochrome b of strain MPOB was oxidized by fumarate, suggested that menequinone and cytochrome b are involved in the electr on transport during fumarate reduction in strain MPOB. The growth yiel ds of fumarate reduction with hydrogen or formate as electron donor we re similar to the growth yield of Wolinella succinogenes. Therefore, i t can be assumed that strain MPOB gains the same amount of ATP from fu marate reduction as W. succinogenes, i.e. 0.7 mel ATP/mol fumarate. Th is value supports the hypothesis that syntrophic propionate-oxidizing bacteria have to invest two-thirds of an ATP via reversed electron tra nsport in the succinate oxidation step during the oxidation of propion ate. The same electron transport chain that is involved in fumarate re duction may operate in the reversed direction to drive the energetical ly unfavourable oxidation of succinate during syntrophic propionate ox idation since (1) cytochrome b was reduced by succinate and (2) succin ate oxidation was similarly inhibited by HOQNO as fumarate reduction.