Glycolytic breakdown of sulfoquinovose in bacteria: A missing link in the sulfur cycle

Sulfoquinovose (6-deoxy-6-sulfo-D-glucopyranose), formed by the hydrolysis of the plant sulfolipid, is a major component of the biological sulfur cycle. However, pathways for its catabolism are poorly delineated. We examined the hypothesis that mineralization of sulfoquinovose to inorganic sulfate is initiated by reactions of the glycolytic and/or Entner-Doudoroff pathways in bacteria. Metabolites of [U-C-13] sulfoquinovose were identified by C-13-nuclear magnetic resonance (NMR) in strains of Klebsiella and Agrobacterium previously isolated for their ability to utilize sulfoquinovose as a sole source of carbon and energy for growth, and cell extracts were analyzed for enzymes diagnostic for the respective pathways. Klebsiella sp. strain ABR11 grew rapidly on sulfoquinovose, with major accumulations of sulfopropandiol (2,3-dihydroxypropanesulfonate) but no detectable release of sulfate. Later, when sulfoquinovose was exhausted and growth was very slow, sulfopropandiol disappeared and inorganic sulfate and small amounts of sulfolactate (2-hydroxy-3-sulfopropionate) were formed. In Agrobacterium sp. strain ABR2, growth and sulfoquinovose disappearance were again coincident, though slower than that in Klebsiella sp. Release of sulfate was still late but was faster than that in Klebsiella sp., and no metabolites were detected by C-13-NMR. Extracts of both strains grown on sulfoquinovose contained phosphofructokinase activities that remained unchanged when fructose 6-phosphate was replaced in the assay mixture with either glucose 6-phosphate or sulfoquinovose. The results were consistent with the operation of the Embden-Meyerhoff-Parnas (glycolysis) pathway for catabolism of sulfoquinovose. Extracts of Klebsiella but not Agrobacterium also contained an NAD(+)-dependent sulfoquinovose dehydrogenase activity, indicating that the Entner-Doudoroff pathway might also contribute to catabolism of sulfoquinovose.