“
“The membrane-bound alcohol dehydrogenase of Gluconacetobacter diazotrophicus contains one pyrroloquinoline quinone moiety (PQQ), one [2Fe-2S] cluster, and four c-type cytochromes. Here, we describe a novel and inactive enzyme. ADHi, similarly to ADHa, is a heterodimer of 72- and 44-kDa subunits

and contains the expected prosthetic groups. However, ADHa showed a threefold molecular mass as compared to ADHi. Noteworthy, the PQQ, the [2Fe-2S] and most of the cytochromes in purified ADHi is in the oxidized form, contrasting with Wortmannin nmr ADHa where the PQQ-semiquinone is detected and the [2Fe-2S] cluster as well as the cytochromes c remained fully reduced after purification. Reduction kinetics of the ferricyanide-oxidized enzymes showed that while ADHa was brought back by ethanol to its full reduction state, in ADHi, only one-quarter of the total heme c was reduced. The dithionite-reduced ADHi was largely oxidized by ubiquinone-2, thus indicating that intramolecular electron transfer is not impaired in ADHi. The acidic pH of the medium might be deleterious for the membrane-bound ADH by causing conformational changes leading to changes in the relative orientation of heme groups and shift of corresponding redox potential to higher values. This would hamper electron transfer resulting in the low activity observed in ADHi. In Gluconacetobacter diazotrophicus,

the PQQ-dependent enzymes – ethanol dehydrogenase (ADH) Natural Product high throughput screening and aldehyde dehydrogenase (ALDH) – are located in the cytoplasmic membrane and oriented toward the periplasmic space (Matsushita et al., 1992). ADH and ALDH catalyze the two sequential oxidation reactions that convert ethanol to acetic acid; both enzymes transfer electrons to membrane ubiquinone. The ethanol-oxidizing ability in acetic acid bacteria can be easily changed and sometimes lost during cultivation, especially in prolonged shaking Sodium butyrate cultures

of Acetobacter aceti (Muraoka et al., 1982; Ohmori et al., 1982) and Acetobacter pasteurianus (Takemura et al., 1991). Under these conditions, spontaneous mutants unable to oxidize ethanol emerge with high frequency. In Gluconobacter suboxydans, genetic instability has not been detected (Matsushita et al., 1995); instead, a dramatic decay in ADH activity has been observed under particular cultivation conditions, such as low pH and/or with high aeration. The presence of an ADH with a very low enzyme activity level (named as inactive ADH) has been reported (Matsushita et al., 1995). Gómez-Manzo et al. (2008, 2010) have already purified and characterized a highly active ADH (ADHa) from N2-grown Ga. diazotrophicus, using forced aeration and physiological acidification caused by growth. In the present work, we purified and characterized an ADH with very low enzyme activity (ADHi). A comparative study of the molecular and catalytic properties of the active and inactive forms of ADH from Ga.