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

Abstract

Significant quantitative differences in end-product yields by 2 strains of C. thermocellum and 1 strain of T. brockii were observed during cellobiose fermentation. Most notably, the ethanol/H2 and lactae/acetate ratios were drastically higher for T. brockii as compared with C. thermocellum strains LQRI and AS39. Exogenous H2 addition (0.4-1.0 atm) during culture growth increased the ethanol/acetate ratio of both T. brockii 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, CoA acetylating pyruvate and acetaldehyde dehydrogenase, hydrogenase, ethanol dehydrogenase and acetate kinase. Enzyme kinetic properties (apparent Km, Vmax and Q10 values) and the specificity of electron donors/acceptors for different oxidoreductases involved in pyruvate conversion to fermentation products were compared in the 3 strains. Both species contained ferredoxin-linked pyruvate dehydrogenase and pyridine nucleotide oxidoreductases. Ferredoxin-NAD reductase activity was significantly higher in T. brockii than in AS39 and was not detectable in LQRI. H2 production and hydrogenase activity were inversely related to ferredoxin-NAD reductase activity in the 3 strains. Ferredoxin-NADP reductase activity was present in cell extracts of both species. Alcohol dehydrogenase activity in C. thermocellum was NAD dependent, unidirectional and inhibited by low levels of reaction products. The high lactate yield of T. brockii 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.