81. Which one of the following correctly states the action of sucrose phosphate synthase enzyme?
(1) UDP-glucose and Fructose-6-phosphate are used as substrates.
(2) UDP-glucose and Fructose-6-phosphate are the products
(3) Sucrose is formed as product
(4)Sucrose-6-phosphate and UDP-glucose are the products

Introduction

Sucrose phosphate synthase (SPS) plays a central role in the cytosolic pathway of sucrose biosynthesis in plants. SPS catalyzes the reaction joining UDP-glucose and fructose-6-phosphate, forming sucrose-6-phosphate and releasing UDP—a critical step prior to sucrose formation.

Explanation of Each Option

(1) UDP-glucose and Fructose-6-phosphate are used as substrates.

• Partially correct. These are indeed substrates, but this option does not specify the products of the reaction, which is essential for fully stating the enzyme’s action.

(2) UDP-glucose and Fructose-6-phosphate are the products.

• Incorrect. These are the initial substrates, not products, of the SPS-catalyzed reaction.

(3) Sucrose is formed as product.

• Incorrect. SPS forms sucrose-6-phosphate initially; sucrose is generated after sucrose-6-phosphate is dephosphorylated by sucrose phosphatase.

(4) Sucrose-6-phosphate and UDP-glucose are the products.

• Correct. The enzyme mediates the biosynthetic step: UDP-glucose + Fructose-6-phosphate → Sucrose-6-phosphate + UDP. This is the precise stoichiometry and product set of the SPS reaction.​

Why Option (4) is Correct

Option (4) is correct because it describes the genuine products of the sucrose phosphate synthase reaction, a step vital for regulated sucrose production in the plant cytosol and export to food sinks.