Question

In the native Calvin-Benson cycle regeneration steps, what is the benefit of including SBPase in the regeneration steps other than providing the driving force?

Answer 1

Ans). SBPase (Sedoheptulose-bisphosphatase) is an enzyme that is involved in the regeneration of 5-carbon sugars during the Calvin-Benson cycle. It plays a important role in controlling the flux of carbon through the Calvin cycle.

• Other than providing driving force in Calvin Benson cycle SBPase activity is also involved in catalyzing dephosphorylation of sedoheptulose-1, 7-bisphosphate to sedoheptulose-7-phosphate in photosynthetic plants.
• Mg2+ concentration has a significant impact on the activity of SBPase and the rate of the reactions it catalyzes. SBPase is inhibited by acidic conditions (low pH). This results in the inhibition of carbon fixation when the pH rate is low inside stroma of the chloroplast.
• reductions in SBPase activity lead to significant decreases in photosynthesis, providing direct evidence that SBPase is tightly linked with carbon flux in the Calvin cycle.
• Like other Calvin cycle enzymes, SBPase is activated in the presence of light through a ferredoxin/thioredoxin system. In the light reactions of photosynthesis, light energy powers the transport of electrons to eventually reduce ferredoxin.
• Our findings are that (1) increased levels of SBPase activity promote photosynthetic activity; (2) different levels of SBPase affect growth and development in tomato plants by altering photosynthetic efficiency and carbohydrate status; (3) higher levels of SBPase activity confer improved chilling tolerance by accumulating more carbohydrate and maintaining relatively higher RuBP regeneration rates and quantum efficiency of photosystem II in tomato plants.
• The enzyme ferredoxin-thioredoxin reductase use ferredoxin to reduce thioredoxin. Finally, the reduced thioredoxin is used to reduced a cysteine-cysteine disulfide bond in SBPase to a dithiol, which converts the SBPase into its active form.

Explaination:

• SBPase catalyses the dephosphorylation of sedoheptulose-1,7-bisphosphate to sedohep-tulose-7-phosphate. This enzyme is unique to the Calvin cycle and unlike many of the other enzymes of this cycle has no cytosolic counterpart.
• SBPase is one of the two bisphosphatase enzymes in the Calvin cycle, the other being fructose-1,6-bisphosphatase (FBPase).
• SBPase is found only in photosynthetic organisms, previously it was thought that a single enzyme catalysed both chloroplast bisphosphatase reactions, as no distinct SBPase enzyme had been isolated.
• Indeed, in photosynthetic bacteria, Rhodobacter and Alcaligenes ‘FBP’ enzymes with dual function are known to catalyse the dephosphorylation of both sed-1,7-b and fructose-1,6-bisphosphate.

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