Xylosevergärung mit dem thermophilen Bakterium Thermoanaerobacter Finnii

This study presents results of the fermentation of xylose with the thermophilic bacterium $\textit{Thermoanaerobacter finnii}$, especially the effects of substrate concentration on end product formation. 1. Comparative studies on differences in morphological and physiological features, GC-content and DNA-homologies of $\textit{AKO-1}$ with other thermophilic bacteria were performed. The newly isolated bacterium $\textit{AKO-1}$ is closely related to the nonsporeformer $\textit{Thermoanaerobacter ethanolicus}$. It was classified as $\textit{Thermoanaerobacter finnii}$. 2. Fermentation of xylose yields ethanol, acetate, L-lactate, H$_{2}$ and CO$_{2}$ in varying amounts depending on the substrate concentration. Batch fermentations with increasing xylose concentrations higher than 66 mM resulted in a decrease of the ethanol yield from 1.25 to 0.85 mol/mol xylose (maximum ethanol yield is 1.66 mol/mol xylose) and in an increase in lactate formation. Fed-batch-fermentations with stationary concentrations of xylose between 20 and 60 mM lead to a very low lactate production (2mM), so that from 200 mM of xylose 230 mM ethanol were produced, corresponding to 70% of the maximal achievable yield. Increasing xylose consumption rates lead to increasing lactate formation rates (from 0.04 to 1.52 mmol/ 1-h), whereas the ethanol formation rate remained largely constant (5.2 and 4.6 mmol/ 1-h respectively). The inhibitory effect of ethanol on growth and product formation was circumvented by continous removal of the produced alcohol from the culture broth by an inert gas stream. That way xylose consumption could be raised from 200 to 400 mM. 3. Enzymatic studies showed that $\textit{T. finnii}$ catabolises xylose to xylulose and xylulose-5-phosphate which is then degraded to pyruvate via the pentosephosphate pathway and glycolysis. Fructose-1,6-bisphosphate has been shown to be an activator of the lactate-dehydrogenase and causes a decrease of the K$_{M}$ values for NADH and pyruvate. The lactate-dehydrogenase has a pH-optimum at pH7.0 and is completely inhibited by 2 mM NADPH. The activity of the ferredoxin-NAD-reductase (1.1 U/mg protein) was 10 times higher than the ferredoxin-NADP-reductase activity. For the first time a transhydrogenase activity (0,9 U/mg protein) was detected in thermophilic ethanol producing bacteria, which transfered electrons from NADH to NADP. The activity of the acetaldehyde-dehydrogenase in $\textit{T. finnii}$ was very low (0.03 U/mg protein) compared to the other catabolic enzyme activities. 4. The intracellular pyridine nucleotide levels were between 1.5 to 4 nmol NADPH/mg dry weight and 0.6 to 2 nmol NADH/mg dry weight, whereas the NAD and NADP levels reached only 0.1 to 1.2 and 0.5 to 1.2 nmol/mg dry weight, respectively. The intracellular fructose-1,6-bisphosphate concentrations ranged between 1.2 and 8.4 nmol/mg dry weight. These fructose-1,6-bisphosphate levels are sufficient for complete activation of the lactate-dehydrogenase, so that presumably the lactateformation depends $\underline{not}$ $\underline{only}$ on the intracellular fructose-1,6-bisphosphate level, but also on the intracellular concentration of pyruvate