Regulation of Sucrose-Phosphate Synthase (SPS) in Plants

Sucrose-phosphate synthase (SPS) plays a central role in sucrose biosynthesis in plants. It catalyzes the formation of sucrose-6-phosphate from UDP-glucose and fructose-6-phosphate, which is later converted into sucrose.

The activity of SPS is tightly regulated through reversible phosphorylation, making it responsive to the plant’s metabolic status and environmental cues.

Enzymes Involved in SPS Regulation

• SPS-kinase: Inactivates SPS by phosphorylating it.

• SPS-phosphatase: Activates SPS by dephosphorylating it.

These regulatory enzymes themselves are modulated by metabolites, including:

• Glucose-6-phosphate (G6P): A signal of carbon abundance.

• Inorganic phosphate (Pi): Often linked to low-energy status.

Key Regulatory Mechanism

• A high ratio of Glucose-6-phosphate to inorganic phosphate favors active SPS.

• This indicates that G6P promotes SPS activity either by directly activating SPS or by inhibiting its inhibitor, SPS-kinase.

Thus, Glucose-6-phosphate inhibits SPS-kinase, preventing SPS from being inactivated and promoting sucrose synthesis when carbon and energy are abundant.

Analysis of Options

1. Glucose-6-phosphate inhibits SPS-phosphatase
   → Incorrect. This would reduce SPS activity, which contradicts the known effect of G6P.

2. Inorganic phosphate activates SPS-kinase
   → Partially correct but not the primary explanation of the regulatory effect of a high G6P:Pi ratio.

3. Glucose-6-phosphate inhibits SPS-kinase
   → Correct. This supports the observation that G6P promotes SPS activation.

4. Inorganic phosphate activates SPS-phosphatase
   → Incorrect. Pi is associated with reduced SPS activity, not activation.

Conclusion

The correct answer is:

3. Glucose-6-phosphate inhibits SPS-kinase

This mechanism ensures that sucrose synthesis is promoted when carbon (in the form of glucose-6-phosphate) is plentiful and energy demand is low.