82. Sucrose-phosphate synthase (SPS) and sucrose-phosphate phosphatase (SPP) are two key enzymes involved in the biosynthesis of sucrose. Following are certain statements regarding these two enzymes:
A. Fructose-6-phosphate is one of the substrates of SPS enzyme.
B. Fructose-6-phosphate and UDP-glucose are the substrates of SPP enzyme.
C. Sucrose is the final product of SPP enzyme.
D. UDP is one of the products of SPS enzyme while Pi is one of the products of SPP enzyme.
Which one of the following options represents the combination of all correct statements?
(1) A, B and C                  (2) A, B and D
(3) A, C and D                  (4) B, C and D

Introduction

Sucrose biosynthesis in plants involves a two-step enzymatic pathway centered on sucrose-phosphate synthase (SPS) and sucrose-phosphate phosphatase (SPP). Each enzyme uses distinct substrates and produces specific products critical for efficient sucrose production and plant metabolism.

Explanation of Each Statement

A. Fructose-6-phosphate is one of the substrates of SPS enzyme.

• Correct. SPS catalyzes the reaction between UDP-glucose and fructose-6-phosphate to form sucrose-6-phosphate and UDP.

B. Fructose-6-phosphate and UDP-glucose are the substrates of SPP enzyme.

• Incorrect. These are substrates for SPS, not for SPP. SPP acts on sucrose-6-phosphate.

C. Sucrose is the final product of SPP enzyme.

• Correct. SPP catalyzes the dephosphorylation of sucrose-6-phosphate to yield sucrose, the transportable carbohydrate.

D. UDP is one of the products of SPS enzyme while Pi is one of the products of SPP enzyme.

• Correct. SPS reaction releases UDP as a product. SPP reaction produces inorganic phosphate (Pi) after cleaving the phosphate group from sucrose-6-phosphate.

Why Option (3) A, C and D Is Correct

• A, C, and D correctly match the enzymatic steps of sucrose biosynthesis: SPS uses fructose-6-phosphate and UDP-glucose, producing sucrose-6-phosphate and UDP; SPP converts sucrose-6-phosphate into sucrose and releases Pi.

• Statement B incorrectly assigns SPS substrates to SPP.