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Background  The alcohol dehydrogenase (ADH) system plays a critical role in sugar metabolism involving in not only ethanol formation and consumption but also the general “cofactor balance” mechanism.  Candida maltosa  is able to ferment glucose as well as xylose to produce a significant amount of ethanol. Here we report the ADH system in  C. maltosa  composed of three microbial group I ADH genes ( CmADH1 ,  CmADH2A  and  CmADH2B ), mainly focusing on its metabolic regulation and physiological function.    Methodology/Principal Findings  Genetic analysis indicated that  CmADH2A  and  CmADH2B  tandemly located on the chromosome could be derived from tandem gene duplication.  In vitro  characterization of enzymatic properties revealed that all the three CmADHs had broad substrate specificities. Homo- and heterotetramers of CmADH1 and CmADH2A were demonstrated by zymogram analysis, and their expression profiles and physiological functions were different with respect to carbon sources and growth phases. Fermentation studies of ADH2A-deficient mutant showed that  CmADH2A  was directly related to NAD regeneration during xylose metabolism since CmADH2A deficiency resulted in a significant accumulation of glycerol.    Conclusions/Significance  Our results revealed that  CmADH1  was responsible for ethanol formation during glucose metabolism, whereas  CmADH2A  was glucose-repressed and functioned to convert the accumulated ethanol to acetaldehyde. To our knowledge, this is the first demonstration of function separation and glucose repression of ADH genes in xylose-fermenting yeasts. On the other hand,  CmADH1  and  CmADH2A  were both involved in ethanol formation with NAD regeneration to maintain NADH/NAD ratio in favor of producing xylitol from xylose. In contrast,  CmADH2B  was expressed at a much lower level than the other two CmADH genes, and its function is to be further confirmed.

The Alcohol Dehydrogenase System in the Xylose-Fermenting Yeast Candida maltosa