Crystal structure of glucooligosaccharide oxidase from Acremonium strictum

Introduction Sugar oxidases and dehydrogenases that catalyze carbohydrate oxidation into the corresponding lactones are of considerable commercial importance. Glucooligosaccharide oxidase (GOOX) from Acremonium strictum was screened with the aim of identifying enzymes with potential applications such as an oligosaccharide acid production and various alternative carbohydrate assays. Screening of more than 50 carbohydrates and derivatives showed that D-glucose, maltose, lactose, and cellobiose are good substrates. In addition, this enzyme can react with maltoand cellooligosaccharides, and hence the name of this novel oxidase. The broad substrate specificity of GOOX, particularly towards oligosaccharides, suggests that it may have great potentialapplicability. To facilitate further characterization and the potential industrial use of A. strictum GOOX, we have cloned and expressed the encoding gene. Interestingly, even though GOOX shares some substrate specificity with glucose oxidase and cellobiose dehydrogenase, it has no sequence similarity to them. In contrast, GOOX displays significant sequence homology to the FADbinding domain of plant berberine bridge enzyme-like proteins, particularly the characteristic flavinylation site (RSGGH). To gain structural insights into the FAD incorporation, substrate specificity and catalytic mechanism, we have determined the GOOX structure at 1.55-Å resolution.