Catabolism of sulfoquinovose (SQ; 6-deoxy-6-sulfoglucose), the ubiquitous sulfosugar produced by photosynthetic organisms, is an important component of the biogeochemical carbon and sulfur cycles. Here, we describe a pathway for SQ degradation that involves oxidative desulfurization to release sulfite and enable utilization of the entire carbon skeleton of the sugar to support the growth of the plant pathogen Agrobacterium tumefaciens. SQ or its glycoside sulfoquinovosyl glycerol are imported into the cell by an ATP-binding cassette transporter system with an associated SQ binding protein. A sulfoquinovosidase hydrolyzes the SQ glycoside and the liberated SQ is acted on by a flavin mononucleotide-dependent sulfoquinovose monooxygenase, in concert with an NADH-dependent flavin reductase, to release sulfite and 6-oxo-glucose. An NAD(P)H-dependent oxidoreductase reduces the 6-oxo-glucose to glucose, enabling entry into primary metabolic pathways. Structural and biochemical studies provide detailed insights into the recognition of key metabolites by proteins in this pathway. Bioinformatic analyses reveal that the sulfoquinovose monooxygenase pathway is distributed across Alpha- and Betaproteobacteria and is especially prevalent within the Rhizobiales order. This strategy for SQ catabolism is distinct from previously described pathways because it enables the complete utilization of all carbons within SQ by a single organism with concomitant production of inorganic sulfite.
Glycoside hydrolases (GHs) are a diverse group of enzymes that catalyze the hydrolysis of glycosidic bonds. The Carbohydrate-Active enZymes (CAZy) database organizes GHs into families based on sequence data and function, with fewer than 1% of the predicted proteins characterized biochemically. Consideration of genomic context can provide a useful guide to infer possible enzyme activities for proteins of unknown function. We used the MultiGeneBLAST tool to discover a putative gene cluster inMarinovumsp., a member of the marineRoseobacterclade, that encodes homologues of enzymes belonging to the sulfoquinovose monooxygenase pathway. This putative gene cluster lacks a gene encoding a classical family GH31 sulfoquinovosidase (SQase) candidate, but which instead includes an uncharacterized family GH13 protein (MsGH13). We show that recombinantMsGH13 lacks SQase activity but is a broad spectrum α-glucosidase active on a diverse array of α-linked disaccharides, including: maltose (100%), sucrose (7.9%), nigerose (52%), trehalose (20%), isomaltose (6.4%), and kojibiose (3.8%). Using AlphaFold, a 3D model for the MsGH13 enzyme was constructed that predicted its active site shared close similarity with a narrower specificity α-glucosidase fromHalomonassp. H11 of the same GH13 subfamily. This study supports recent findings that bacteria belonging to theRoseobacterclade can metabolize SQ without a classical SQase encoded in their genomes.
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