Rubisco
Table of Contents:
RuBisCO, also known as Ribulose Bisphosphate Carboxylase-Oxygenase, is the most abundant protein in the biosphere. It is responsible for catalyzing the first step of the Calvin cycle in photosynthesis and is the common pathway of carbon fixation in all plants, including C3, C4 and CAM plants.
RuBisCO catalyzes the carboxylation of ribulose bisphosphate (RuBP), which is the primary acceptor of CO2 in the C3 pathway or the Calvin cycle. This is the first step of the Calvin cycle.
RuBisCO is found in the mesophyll cells of C3 plants and the bundle sheath cells of C4 plants. Its substrate, Ribulose bisphosphate (RuBP), is a 5-carbon compound. When RuBisCO acts on RuBP, it produces two molecules of 3-PGA (3-phosphoglyceric acid) as part of the Calvin cycle.
RuBisCO has both carboxylase and oxygenase activity. When molecular oxygen is the substrate, RuBisCO converts RuBP to one molecule of phosphoglycerate and phosphoglycolate in the process called photorespiration.
Let’s Dive Deeper into the Structure and Enzyme Activity of RuBisCO
RuBisCO Structure and Characteristics
RuBisCO is found in algae, photosynthetic protists, plants and certain autotrophic bacteria, including cyanobacteria and proteobacteria.
RuBisCO is the most abundant protein found in the biosphere, comprising approximately 50% of total soluble leaf protein in C3 plants and 30% of total soluble leaf protein in C4 plants.
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 composed of both large and small chains, with a molecular weight of approximately 540,000 Da.
The active site for the substrate is present in the large chain of the 8 large chains that form 4 dimers and 8 small chains. Some bacteria and dinoflagellates only contain the large subunits.
The large chain is encoded by chloroplast DNA, while the small chains are encoded by nuclear DNA. The small chains are imported to the stroma of the chloroplast from the cytoplasm.
Mg2+ is required for the activity of the enzyme.
RuBisCO becomes inactive in the dark and gets activated in the light. Activation occurs when CO2 and Mg2+ ions bind to the lysine residue near the active site, resulting in conformational changes and stability of the active form of the enzyme.
RuBisCO Function
The main function of RuBisCO is to facilitate both photosynthesis and photorespiration.
The enzyme RuBisCO catalyzes the first step of carbon fixation in the C3 pathway or Calvin cycle, which is the carboxylation of RuBP. This results in the formation of two molecules of 3-PGA.
RuBisCO has an affinity for oxygen, resulting in it binding to some O2 in the process known as photorespiration. This leads to the conversion of RuBP into one molecule each of phosphoglycerate and phosphoglycolate.
Since RuBisCO has a much higher affinity for CO2 than for O2, photosynthesis is favored over photorespiration.
The Role of RuBisCO in Photosynthesis
RuBisCO catalyses the first step of carbon fixation in the Calvin cycle, which occurs in all plants, including C3, C4 and CAM plants.
RuBP is carboxylated by utilising CO2, forming a stable organic intermediate. This is the first step of the Calvin cycle. C-C bond cleavage then results in the formation of two molecules of 3-PGA, a 5-C compound.
The reaction catalyzed by RuBisCO is:
2 3-PGA (3C) ← RuBP (5C) + CO2 + H2O
The reaction begins with enolisation of RuBP, followed by carboxylation, resulting in the formation of an intermediate 3-keto-2′-carboxyarabinitol-1,5-bisphosphate. This is then followed by hydration and subsequent cleavage of the bond between two carbons, yielding two molecules of 3-phosphoglycerate (3-PGA). The 3-PGA thus formed is then utilised in the formation of glucose and other carbohydrates in the subsequent steps.
In C3 plants, the Calvin Cycle occurs in the mesophyll cells. Whereas, in the C4 pathway, the Calvin Cycle occurs in the bundle sheath cells which are rich in RuBisCO. This is an adaptation to reduce photorespiration in C4 plants.
Photorespiration
RuBisCO has an affinity for oxygen and it oxygenates RuBP when oxygen is present. This process of photorespiration uses ATP, resulting in the wasting of some of the energy produced in photosynthesis.
RuBisCO binds to O2, resulting in the conversion of RuBP into one molecule of phosphoglycerate (3C) and one molecule of phosphoglycolate (2 Carbon); however, this is a wasteful process, as it does not generate ATP or sugar.
Frequently Asked Questions
What is the Role of RuBisCO?
RuBisCO is an enzyme that plays an important role in the Calvin Cycle of photosynthesis. It catalyzes the reaction that combines carbon dioxide with ribulose-1,5-bisphosphate to form two molecules of 3-phosphoglycerate.
RuBisCO is essential for carbon fixation in C3, C4 and CAM plants. It is present in the mesophyll and bundle sheath cells. It catalyses the carbon fixation reaction in the Calvin cycle, i.e. carboxylation of RuBP.
What is RuBisCO in Photosynthesis?
RuBisCO stands for Ribulose 1,5-bisphosphate carboxylase-oxygenase. It is responsible for carbon fixation during photosynthesis.
What is RuBisCO and when is it used?
RuBisCO is an enzyme found in plants that catalyzes the first step of the Calvin cycle, which is the process by which plants convert carbon dioxide into organic compounds. It is used in photosynthesis to convert carbon dioxide and water into glucose and oxygen.
RuBisCO is an enzyme present in the chloroplast and it is utilized in the dark reaction of photosynthesis.
Yes, RuBisCO is present in C4 plants.
Yes, RuBisCO is present in C4 plants. It is located in the bundle sheath cells and plays an important role in the Calvin cycle.
Where is RuBisCO found?
In C3 plants, RuBisCO is active in mesophyll cells whereas in C4 plants it is present in the bundle sheath cells.
The full form of RuBisCO is Ribulose-1,5-bisphosphate carboxylase/oxygenase.
The full form of RuBisCO is Ribulose bisphosphate Carboxylase-Oxygenase.
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