Sucrose-phosphate synthase (SPS) is a key enzyme in the biosynthesis of sucrose in plants. Following are certain statements regarding SPS:

A. Uridine-diphosphate glucose and fructose-6-phosphate are the substrates for SPS.
B. SPS directly converts its substrate into sucrose.
C. Phosphorylation activates while dephosphorylation inactivates SPS.
D. SPS converts its substrate into sucrose-6-phosphate which is then converted to sucrose by the action of sucrose-phosphate phosphatase.
E. Glucose-6-phosphate activates while Pi inactivates SPS.

Which one of the following options represents the combination of all correct statements?

Correct option is D

Sucrose-phosphate synthase (SPS) is an essential enzyme in the sucrose biosynthesis pathway in plants. It catalyzes the reaction that forms sucrose-6-phosphate from its substrates, which is then converted into sucrose by sucrose-phosphate phosphatase. Let’s analyze each statement carefully:

• Statement A (Correct): SPS utilizes uridine-diphosphate glucose (UDP-glucose) and fructose-6-phosphate as its substrates. These two molecules participate in the formation of sucrose-6-phosphate.
• Statement B (Incorrect): SPS does not directly convert its substrates into sucrose. Instead, it forms sucrose-6-phosphate, which is then dephosphorylated by sucrose-phosphate phosphatase to form sucrose.
• Statement C (Incorrect): Phosphorylation inactivates SPS, while dephosphorylation activates it. The statement given in the question is the opposite of the actual mechanism, making it incorrect.
• Statement D (Correct): SPS converts its substrates into sucrose-6-phosphate, which is subsequently converted to sucrose by sucrose-phosphate phosphatase. This is the correct enzymatic pathway.
• Statement E (Correct): Glucose-6-phosphate (G6P) activates SPS, while inorganic phosphate (Pi) inhibits it. This regulatory mechanism helps in controlling sucrose synthesis based on cellular energy requirements.

Thus, the correct combination of correct statements is A, D, and E.

Information Booster:

1. Sucrose-Phosphate Synthase (SPS) Function: SPS catalyzes the reaction between UDP-glucose and fructose-6-phosphate to form sucrose-6-phosphate.
2. Key Regulatory Mechanisms: SPS activity is regulated by phosphorylation (inactivation) and dephosphorylation (activation).
3. Role of Glucose-6-Phosphate: Acts as an allosteric activator of SPS, enhancing its activity when energy is available.
4. Inorganic Phosphate (Pi) Regulation: Acts as an inhibitor, ensuring sucrose synthesis is reduced when energy levels are low.
5. Final Step in Sucrose Formation:Sucrose-6-phosphate phosphatase removes the phosphate group from sucrose-6-phosphate to yield free sucrose.
6. Importance in Plants: SPS activity is crucial for sucrose production, which is essential for carbon transport and storage in plants.
7. Environmental Regulation: Light, temperature, and nutrient availability influence SPS activity, impacting overall sucrose synthesis.