Sulfate is activated to adenosine phosphosulfate (APS) by a phosphate bound with consumption of ATP and release of pyrophosphate by ATP sulfury-lase (ATP: sulfate adenylyl transferase, E.C. 18.104.22.168, ATP-S; Fig. 4,i). Since the reaction equilibrium of ATP-S favors the reverse reaction (ATP and SO42- formation) (Saito 2004) and in plants pyrophosphate concentration is around 0.3 mM, in vivo activity of ATP-S depends on the consumption of APS in further reduction or phosphorylation reactions. Arabidopsis ATP-S genes are chloro-plast targeted therefore the metabolic steps that reduce sulfate to sulfide are exclusively chloroplast localized, although ATP-S activity can also be measured in the cytoplasm. One hypothesis is that APS generated in the cytoplasm is used
Vv ATP-S 1; VvAfP-S2 ATP+SO42- lAPS+P-Pi
Fig 4. Flow chart of reactions of sulfate assimilation in Vitis vinifera: VvATP-S1,2 - sulfate-ATP: adenylyl transferase (E.C. 22.214.171.124, APS sulfotransferase); APS - adenosine 5'-phosphosulfate; VvAPSR - GSH:APS sulforeductase (E.C. 2.8.2n, GSH-APS reductase); VvSIR - S2-:ferredoxin oxidoreductase (E.C. 126.96.36.199, sulfite reductase, SIR); VvSAT - serine acetyl transferase (E.C. 188.8.131.52, SAT); OAS - O-acetyl-serine; VvOASTL1-13 - O-acetyl-serine sulfydrilase (E.C. 4. 2.99.8 O-acetyl-L-serine(thiol)-lyase, OASTL).
in sulfation reactions (Rotte and Leustek 2000) which occur, for instance, in glucosinolate biosynthesis.
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