Sulfolipid Head Group Biosynthesis in Photosynthetic Organisms

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9807943Benning The abundant non-phosphorous sulfolipid sulfoquinovosyl diacylglycerol is specifically associated with photosynthetic membranes of bacteria and plants. The unique head group consists of a 6-sulfonate derivative of glucose called sulfoquinovose. Three major pathways contribute to the biosynthesis of sulfolipid: carbohydrate metabolism, sulfur assimilation, and fatty acid biosynthesis. There is good evidence for the final assembly of the sulfolipid by the transfer of the sulfoquinovosyl moiety from UDP-sulfoquinovose to the sn-3 position of diacylglycerol. However, very little is still known about the biosynthesis of the precursor UDP-sulfoquinovose and the mechanism for the introduction of the sulfonate group. It is the objective of this project, to elucidate the biosynthesis of UDP-sulfoquinovose, thereby determining the pathway of sulfolipid biosynthesis and possibly discovering a novel biochemical reaction crucial to the synthesis of sulfonates in biological systems. One approach is to characterize the proteins encoded by the sqdB sulfolipid genes of photosynthetic bacteria or by the orthologous SQD1 plant gene, and to elucidate the metabolic reaction catalyzed. These proteins are proposed to be involved in the biosynthesis of UDP-sulfoquinovose from UDP-glucose, because their amino acid sequence is similar to those of sugar nucleotide modifying enzymes. Recombinant SQDB or SQD1 proteins convert UDP-glucose to a new compound in vitro. The structural elucidation of this compound, presumably the sulfur acceptor for the formation of UDP-sulfoquinovose, is expected to provide clues to the reaction catalyzed by the SQDB or SQD1 proteins in vivo. A second approach is aimed at the identification of the possible sulfur donor for UDP-sulfoquinovose biosynthesis and, thus, the branch point between general sulfur metabolism and sulfolipid biosynthesis. For this purpose, four mutants of the cyanobacterium Synechocystis sp. PCC6803 deficient in different steps of sulfate assimilation and reduction will be constructed by targeted gene disruption and will be tested for their capability to synthesize sulfolipid. Sulfolipid is a constituent of photosynthetic membranes of bacteria and plants. The head group of the sulfolipid is unique, consisting of a sulfonic acid derivative of glucose called sulfoquinovose. It is the objective of this project to elucidate the crucial biochemical reaction(s) leading to the biosynthesis of this naturally occurring sulfonic acid. The genes encoding the enzyme thought to catalyze the biosynthesis of the head group are available from bacteria and plants. These genes are used to produce recombinant proteins in E. coli that can be studied in detail to elucidate the reaction mechanism. To address the question for the origin of the sulfur in sulfolipid, genes known to encode proteins involved in different reactions of the general sulfur metabolism in cyanobacteria will be inactivated. The resulting mutants will be tested for their competence for sulfolipid biosynthesis. The newly acquired knowledge about the biosynthesis of the sulfolipid head group will not only solve the pathway of sulfolipid biosynthesis, but may also provide clues towards the production of sulfonic acids by genetic engineering. It has been demonstrated in different laboratory tests that sulfolipid has anti-viral and anti-tumor activities. Thus, it may become desirable to produce large quantities of this compound in an inexpensive way in the near future.