70. Following are certain statements regarding Rubisco, the predominant protein in plant leaves that catalyzes the initial reaction of the Calvin-Benson cycle.
A. During the oxygenase activity of Rubisco, O₂ is used as substrate to produce three carbon molecule, 3- phosphoglycerate and two-carbon molecule, 2- phosphoglycolate
B. In red and brown algae, the large subunit of Rubisco is localized in the chloroplast while small subunit is localized in the nucleus.
C. The bound sugar phosphates in Rubisco are specifically removed by an ATP dependent enzyme, Rubisco activase.
D. The active form of Rubisco catalyzes carboxylation or oxygenation reactions in five steps.
Which one of the following combinations of above statements is correct?
(1) A, B and C              (2) A, B and D
(3) B, C and D              (4) A, C and D

Introduction

Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) is the most abundant protein in plant leaves and orchestrates carbon fixation in the Calvin-Benson cycle. Its dual enzymatic action and regulatory mechanisms are fundamental topics in plant physiology, genetics, and biotechnology.

Explanation of Each Statement

A. During the oxygenase activity of Rubisco, O2 is used as substrate to produce three-carbon molecule 3-phosphoglycerate and two-carbon molecule 2-phosphoglycolate

• Correct. In photorespiration, Rubisco’s oxygenase activity utilizes O₂ and RuBP, generating one molecule each of 3-phosphoglycerate (3-PGA) and 2-phosphoglycolate, a classic hallmark of its dual function.

B. In red and brown algae, the large subunit of Rubisco is localized in the chloroplast while small subunit is localized in the nucleus.

• Correct. As in higher plants, in red and brown algae, the Rubisco large subunit is encoded by the chloroplast genome and the small subunit by the nuclear genome, reflecting their evolutionary origins and dual genomic coordination.

C. The bound sugar phosphates in Rubisco are specifically removed by an ATP-dependent enzyme, Rubisco activase.

• Correct. Rubisco activase uses ATP to remove inhibitory sugar phosphates bound to Rubisco, ensuring the enzyme returns to an active state and can continue catalyzing carboxylation or oxygenation.

D. The active form of Rubisco catalyzes carboxylation or oxygenation reactions in five steps.

• Incorrect. While Rubisco’s reaction mechanism is complex, textbook descriptions typically do not enumerate it as “five steps” in defining catalysis. This statement is either inaccurate or ambiguous in standard biochemical contexts.

Why Option (1) A, B and C is Correct

Statements A, B, and C accurately describe Rubisco’s dual activity, genetic localization in algae, and essential regulation by Rubisco activase. Statement D is ambiguous and lacks specificity according to standard biochemistry sources.

This consolidated knowledge is fundamental for plant physiology, molecular biology exams, and advanced plant science studies.