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sulfoquinovosyl diacylglycerol biosynthesis

Photosynthetic membranes, such as plant chloroplast membranes, need a certain amount of anionic lipids for their proper function. These anionic lipids can be either phospholipids such as phosphatidylglycerol or sulfolipids such as |FRAME: SULFOQUINOVOSYLDIACYLGLYCEROL sulfoquinovosyldiacylglycerol| (SQDG). SQDG is also classified as |FRAME: Glycolipids|. However the characteristic structure of SQDG is the unique head group constituent |FRAME: CPD-10247|, which is a derivative of glucose where 6-hydroxy is replaced by sulfonate |CITS: [15012227]|. SQDG is synthesized from |FRAME: UDP-SULFOQUINOVOSE| by the action of |FRAME: EC-2.4.1.M2|. The presence of a possible sulfoquinovose nucleotide in algal extracts was suggested already in 1963 |CITS: [Shibuya63]|, but its existence was confirmed only in 1995, by its accumulation in an |FRAME: G-44027| mutant |CITS: [7592762]|. Insight into this pathway was provided with the discovery of a four genes cluster (sqdA, sqdB, sqdC, and sqdD) encoding sulfolipid biosynthesis in |FRAME: TAX-1063| |CITS: [1400200][1551852]|. It has been shown that |FRAME: G-44027| encodes |FRAME: EC-2.4.1.M2| |CITS: [7592762]|. Out of these four genes, only |FRAME: SYNPCC7942_0578| has homologues in cyanobacteria. However, a gene next to it in the genome of the cyanobacterium |FRAME: TAX-1140|, |FRAME: SYNPCC7942_0579|, was shown to encode the cyanobacterial sulfoquinovosyldiacylglycerol synthase |CITS: [10629209]|. The cyanobacterial |FRAME: SYNPCC7942_0578| gene was used to isolate a cDNA for the orthologous |FRAME: TAX-3702| |FRAME: G-741| gene |CITS: [9465123]|, which was subsequently shown to encode |FRAME: EC-| |CITS: [11073956]|. The synthesis of SQDG is not crucial under normal growth condition. It only becomes important when phosphate is limited |CITS: [9465123]|. Under such conditions, the increase of SQDG levels compensates for the decrease in phospholipids and ensures a constant proportion of anionic lipids in chloroplast membranes |CITS:[14675442]|. While the general scheme of the pathway is understood, there are still some unanswered questions |CITS: [17334828]|. The mechanism of SQDG assembly in bacteria like |FRAME: TAX-1063| remains unclear, as the function of the essential genes sqdA and sqdC is still unknown. Disruption of sqdC led to the accumulation of |FRAME: CPD-17933|, suggesting it may be an intermediate in the pathway in that organism |CITS: [9143325]|. In addition, the SQD1 enzyme of |FRAME: TAX-3702| was found to form a complex with |FRAME: EC- "ferredoxin-dependent glutamate synthase"|, and the significance of this finding is also not understood |CITS: [15752726]|.

from BIOCYC source record: META_PWYQT-4427
Type: pathway
Taxonomic scope
conserved biosystem

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