Novel Pathway for Alcoholic Fermentation of δ-Gluconolactone in the Yeast Saccharomyces bulderi

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Abstract
Under anaerobic conditions, the yeast Saccharomyces bulderi rapidly ferments δ-gluconolactone to ethanol and carbon dioxide. We propose that a novel pathway for δ-gluconolactone fermentation operates in this yeast. In this pathway, δ-gluconolactone is first reduced to glucose via an NADPH-dependent glucose dehydrogenase (EC 1.1.1.47). After phosphorylation, half of the glucose is metabolized via the pentose phosphate pathway, yielding the NADPH required for the glucose-dehydrogenase reaction. The remaining half of the glucose is dissimilated via glycolysis. Involvement of this novel pathway in δ-gluconolactone fermentation in S. bulderi is supported by several experimental observations. (i) Fermentation of δ-gluconolactone and gluconate occurred only at low pH values, at which a substantial fraction of the substrate is present as δ-gluconolactone. Unlike gluconate, the latter compound is a substrate for glucose dehydrogenase. (ii) High activities of an NADP + -dependent glucose dehydrogenase were detected in cell extracts of anaerobic, δ-gluconolactone-grown cultures, but activity of this enzyme was not detected in glucose-grown cells. Gluconate kinase activity in cell extracts was negligible. (iii) During anaerobic growth on δ-gluconolactone, CO 2 production exceeded ethanol production by 35%, indicating that pyruvate decarboxylation was not the sole source of CO 2 . (iv) Levels of the pentose phosphate pathway enzymes were 10-fold higher in δ-gluconolactone-grown anaerobic cultures than in glucose-grown cultures, consistent with the proposed involvement of this pathway as a primary dissimilatory route in δ-gluconolactone metabolism.