Characterization of a unique Caulobacter crescentus aldose-aldose oxidoreductase having dual activities
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We describe here the characterization of a novel enzyme called aldose-aldose oxidoreductase (Cc AAOR; EC 1.1.99) from Caulobacter crescentus. The Cc AAOR exists in solution as a dimer, belongs to the Gfo/Idh/MocA family and shows homology with the glucose-fructose oxidoreductase from Zymomonas mobilis. However, unlike other known members of this protein family, Cc AAOR is specific for aldose sugars and can be in the same catalytic cycle both oxidise and reduce a panel of monosaccharides at the C1 position, producing in each case the corresponding aldonolactone and alditol, respectively. Cc AAOR contains a tightly-bound nicotinamide cofactor, which is regenerated in this oxidation-reduction cycle. The highest oxidation activity was detected on d-glucose but significant activity was also observed on d-xylose, l-arabinose and d-galactose, revealing that both hexose and pentose sugars are accepted as substrates by Cc AAOR. The configuration at the C2 and C3 positions of the saccharides was shown to be especially important for the substrate binding. Interestingly, besides monosaccharides, Cc AAOR can also oxidise a range of 1,4-linked oligosaccharides having aldose unit at the reducing end, such as lactose, malto- and cello-oligosaccharides as well as xylotetraose. 1H NMR used to monitor the oxidation and reduction reaction simultaneously, demonstrated that although d-glucose has the highest affinity and is also oxidised most efficiently by Cc AAOR, the reduction of d-glucose is clearly not as efficient. For the overall reaction catalysed by Cc AAOR, the l-arabinose, d-xylose and d-galactose were the most potent substrates.
KeywordsEnzyme catalysis Glucose-fructose oxidoreductase Nuclear magnetic resonance Tightly-bound cofactor Carbohydrate
This study was financially supported by the Academy of Finland through the Centre of Excellence in White Biotechnology–Green Chemistry (decision number 118573). We thank Outi Liehunen and Jonas Excell for excellent technical assistance. Tobias Greuter (Bruker, Fällanded, Switzerland) is acknowledged for providing the modified firmware for the NMR sample changer.
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Conflict of interest
The authors declare that they have no competing interests.
This article does not contain any studies with human participants or animals performed by any of the authors.
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