FORMATION OF FORMATE AND HYDROGEN, AND FLUX OF REDUCING EQUIVALENTS AND CARBON IN RUMINOCOCCUS-FLAVEFACIENS FD-1

A pathway for conversion of the metabolic intermediate phosphoenolpyru vate (PEP) and the formation of acetate, succinate, formate, and H-2 i n the anaerobic cellulolytic bacterium Ruminococcus flavefaciens FD-1 was constructed on the basis of enzyme activities detected in extracts of cells grown in cellulose-or cellobiose-limited continuous culture. PEP was converted to acetate and CO2 (via pyruvate kinase, pyruvate d ehydrogenase, and acetate kinase) or carboxylated to form succinate (v ia PEP carboxykinase, malate dehydrogenase, fumarase, and fumarate red uctase). Lactate was not formed even during rapid growth (batch cultur e, mu = 0.35/h). H-2 was formed by a hydrogenase rather than by cleava ge of formate, and C-13-NMR and C-14-exchange reaction data indicated that formate was produced by CO2 reduction, not by a cleavage of pyruv ate. The distribution of PEP into the acetate and succinate pathways w as not affected by changing extracellular pH and growth rates within t he normal growth range. However, increasing growth rate from 0.017/h t o 0.244/h resulted in a shift toward formate production, presumably at the expense of H-2. This shift suggested that reducing equivalents co uld be balanced through formate or H-2 production without affecting th e yields of the major carbon-containing fermentation endproducts.