Ethanol Production by Thermophilic Bacteria: Relationship Between Fermentation Product Yields of and Catabolic Enzyme Activities in <i>Clostridium thermocellum</i> and <i>Thermoanaerobium brockii</i>

<jats:p> Significant quantitative differences in end-product yields by two strains of <jats:italic>Clostridium thermocellum</jats:italic> and one strain of <jats:italic>Thermoanaerobium brockii</jats:italic> were observed during cellobiose fermentation. Most notably, the ethanol/H <jats:sub>2</jats:sub> and lactate/acetate ratios were drastically higher for <jats:italic>T. brockii</jats:italic> as compared with <jats:italic>C. thermocellum</jats:italic> strains LQRI and AS39. Exogenous H <jats:sub>2</jats:sub> addition (0.4 to 1.0 atm) during culture growth increased the ethanol/acetate ratio of both <jats:italic>T. brockii</jats:italic> and AS39 but had no effect on LQRI. All strains had an operative Embden-Meyerhof glycolytic pathway and displayed catabolic activities of fructose-1,6-diphosphate–activated lactate dehydrogenase, coenzyme A acetylating pyruvate and acetaldehyde dehydrogenase, hydrogenase, ethanol dehydrogenase, and acetate kinase. Enzyme kinetic properties (apparent <jats:italic> K <jats:sub>m</jats:sub> </jats:italic> , <jats:italic>V</jats:italic> <jats:sub>max</jats:sub> , and Q <jats:sub>10</jats:sub> values) and the specificity of electron donors/acceptors for different oxidoreductases involved in pyruvate conversion to fermentation products were compared in the three strains. Both species contained ferredoxin-linked pyruvate dehydrogenase and pyridine nucleotide oxidoreductases. Ferredoxin-nicotinamide adenine dinucleotide (NAD) reductase activity was significantly higher in <jats:italic>T. brockii</jats:italic> than in AS39 and was not detectable in LQRI. H <jats:sub>2</jats:sub> production and hydrogenase activity were inversely related to ferredoxin-NAD reductase activity in the three strains. Ferredoxin-NAD phosphate reductase activity was present in cell extracts of both species. Alcohol dehydrogenase activity in <jats:italic>C. thermocellum</jats:italic> was NAD dependent, unidirectional, and inhibited by low concentrations of NAD and ethanol. Ethanol dehydrogenase activity of <jats:italic>T. brockii</jats:italic> was both NAD and NADP linked, reversible, and not inhibited by low levels of reaction products. The high lactate yield of <jats:italic>T. brockii</jats:italic> correlated with increased fructose-1,6-diphosphate. The relation of catabolic enzyme activity and quantitative differences in intracellular electron flow and fermentation product yields of these thermophilic bacteria is discussed. </jats:p>