76. Following are some enzyme-substrate combinations involved in the conversion of photosynthetically formed triose-phosphate to sucrose.
A. Fructose-1,6-bisphosphate aldolase : Dihydroxyacetonephosphate.
B. UDP-glucose pyrophosporylase : UDP-Glucose.
C. Sucrose 6-phosphate synthase : Fructose-6-phosphate.
D. Phosphofructo kinase : Fructose 6 phosphate
Which one of the following combinations of above enzyme-substrate combinations is correct?
(1) A, B and D        (2) A, C and D
(3) B. C and D        (4) A, B and C

Introduction

The conversion of triose-phosphate intermediates, produced during photosynthesis, into sucrose involves several key enzymes. Understanding the correct substrate pairs for these enzymes is essential for comprehending the metabolic pathway leading to sucrose synthesis in plants.

Explanation of Each Option

A. Fructose-1,6-bisphosphate aldolase : Dihydroxyacetone phosphate

• Correct. Aldolase catalyzes the condensation of dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate, forming fructose-1,6-bisphosphate in the cytosolic pathway.

B. UDP-glucose pyrophosporylase : UDP-Glucose

• Correct. UDP-glucose pyrophosphorylase converts glucose-1-phosphate and UTP to UDP-glucose, which is critical for sucrose formation.

C. Sucrose 6-phosphate synthase : Fructose-6-phosphate

• Correct. Sucrose 6-phosphate synthase catalyzes the reaction between UDP-glucose and fructose-6-phosphate, yielding sucrose-6-phosphate, a direct precursor to sucrose.

D. Phosphofructokinase : Fructose 6 phosphate

• Incorrect in this pathway context. Phosphofructokinase acts on fructose-6-phosphate but primarily in glycolysis, converting it to fructose-1,6-bisphosphate. This step is not central in the direct sucrose biosynthesis pathway.

Why Option (4) A, B and C Is Correct

• A, B, and C represent enzymes and substrates directly involved in the sucrose biosynthesis pathway from triose-phosphates: aldolase initiates formation, UDP-glucose pyrophosphorylase provides a key sugar-nucleotide, and sucrose-6-phosphate synthase forms the immediate sucrose precursor.

• D, though important in glycolysis, is not a direct enzyme of the cytosolic triose-phosphate-to-sucrose pathway.

This biochemical pathway knowledge is core for plant physiology and biochemistry exams.