Summary: The role of the pyruvate dehydrogenase complex (PDC) in the formation of different fermentation products by was studied. This organism was grown on a semi-defined medium under various conditions in the presence or absence of lipoic acid, an essential cofactor of the enzyme complex. When grown on a medium without added lipoic acid, a very low activity, both and , of the PDC was observed. When pyruvate served as the energy source, lipoic acid was found to be essential for growth under anaerobic conditions at low culture pH values. The presence of lipoic acid in the culture medium had a marked effect on the production of acetoin: in the presence of lipoic acid, acetoin was produced only when the intracellular pyruvate concentration was relatively high, whereas in the absence of lipoic acid, acetoin was a common product. Under potassium-limited conditions, lactate was the main product and culture pH significantly affected the bacterial dry weight. After instantaneous addition of lipoic acid to a glucose + pyruvate-limited chemostat culture, an immediate activation of the PDC took place as deduced from the change in fermentation pattern. Reconstitution of the PDC by the addition of lipoic acid was also possible in cell-free extracts, although pre-incubation with ATP and lipoic acid for 90 min was necessary for maximal activation. The effects of an active PDC on product formation and the physiological role of the complex under anaerobic growth conditions are discussed.


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