RuBisCO

Table of Contents:

Ribulose bisphosphate Carboxylase-Oxygenase or RuBisCO is the most abundant protein in the biosphere. It catalyses the first step of carbon fixation in the Calvin cycle during photosynthesis. It is the common pathway of carbon fixation in all plants, i.e. C₃, C₄ and CAM plants.

RuBisCO catalyses the carboxylation of ribulose bisphosphate (RuBP), which is the primary acceptor of CO₂ in the C₃ pathway or the Calvin cycle. It is the first step of the Calvin cycle.

RuBisCO is present in the mesophyll cells of C₃ plants and bundle sheath cells of C₄ plants. Ribulose bisphosphate (RuBP) is a 5-carbon compound. It acts as a primary carbon dioxide acceptor in the Calvin cycle, and is converted into 2 molecules of 3-PGA (3-phosphoglyceric acid) by the action of RuBisCO.

RuBisCO, as the name suggests, has both carboxylase and oxygenase activity. When molecular oxygen is the substrate, it converts RuBP to one molecule of phosphoglycerate and phosphoglycolate in the process called photorespiration.

Let us now learn more about the structure and enzyme activity of RuBisCO.

RuBisCO Structure and Characteristics

• RuBisCO is present in algae, photosynthetic protists, plants and certain autotrophic bacteria, such as cyanobacteria, proteobacteria, etc.
• RuBisCO is the most abundant protein found in the biosphere. It accounts for about 50% and 30% of total soluble leaf protein in C₃ and C₄ plants, respectively.
• In C₃ plants, RuBisCO is present in mesophyll cells whereas, in C₄ plants, it is present in bundle sheath cells.
• It is a large complex protein made up of large and small chains. The molecular wt. is about 540,000 Da.
• There are generally 8 large chains that form 4 dimers and 8 small chains. The active site for the substrate is present in the large chain. Some bacteria and dinoflagellates only contain large subunits.
• The large chain is coded by chloroplast DNA and the small chains are encoded by nuclear DNA. Small chains are imported to the stroma of chloroplast from the cytoplasm.
• Mg²⁺ is required for the activity of the enzyme.
• RuBisCO is inactive in the dark and gets activated in the light. RuBisCO is activated by binding of CO₂ and Mg²⁺ ions to the lysine residue near the active site, which brings about conformational changes and stability to the active form of the enzyme.

RuBisCO Function

• The main function of RuBisCO is in photosynthesis and photorespiration.
• It catalyses the first step of carbon fixation in the C3 pathway or Calvin cycle, i.e. carboxylation of RuBP. It results in the formation of 2 molecules of 3-PGA.
• RuBisCO also has an affinity for oxygen so it binds to some amount of O₂ in the process known as photorespiration. It leads to the conversion of RuBP to one molecule each of phosphoglycerate and phosphoglycolate.
• Since the affinity of RuBisCO is much higher for CO₂ than for O₂, photosynthesis is preferred over photorespiration.

Role of RuBisCO in Photosynthesis

RuBisCO catalyses the first step of carbon fixation in the Calvin cycle. Calvin cycle occurs in all plants, i.e. C₃, C₄ and CAM.

The first step of the Calvin cycle is carboxylation. Here, CO₂ is fixed into a stable organic intermediate. RuBP is a 5-C compound. It is carboxylated by utilising CO₂ and then C-C bond cleavage results in the formation of 2 molecules of 3-PGA.

The reaction catalysed by RuBisCO is as follows:

RuBP (5C) + CO₂ + H₂O → 2 3-PGA (3C)

The reaction involves enolisation of RuBP followed by carboxylation, which leads to the formation of an intermediate 3-keto-2′-carboxyarabinitol-1,5-bisphosphate. It is followed by hydration, and then subsequent cleavage of the bond between two carbons to give rise to 2 molecules of 3-phosphoglycerate (3-PGA). The 3-PGA thus formed is utilised in the formation of glucose and other carbohydrates in the subsequent steps.

In C₃ plants, this process occurs in the mesophyll cells. In the C₄ pathway, the Clavin cycle occurs in the bundle sheath cells. The bundle sheath cells are rich in RuBisCO. This is an adaptation to reduce photorespiration in C₄ plants.

Photorespiration

RuBisCO also has an affinity for oxygen and it oxygenates RuBP in the presence of oxygen. Photorespiration utilises ATP, hence, leads to the wasting of some energy produced in photosynthesis.

When RuBisCO binds to O₂ it converts RuBP to one molecule of phosphoglycerate (3C) and phosphoglycolate (2 Carbon) each. It is a waste process, it neither generates ATP nor sugar.

This was all about RuBisCO. Explore notes on other important concepts related to NEET, only at BYJU’S.

Further reading:

• Chromoplast
• Carbon Fixation
• Cell – The Unit of Life
• Short Notes Of Biology For NEET CAM Plants
• Difference Between C3 C4 And CAM Pathway

Recommended Video:

Photosynthesis in Higher Plants Class 11 (Ep 15) | NEET Important Questions | NEET 2022