Rubisco 

RuBisCO full name of the enzyme often abbreviated as " RuBisCO " is "Ribulose-1,5-bisphosphate carboxylase/oxygenase", is a crucial enzyme in photosynthesis, the process that plants use to make food. It helps grab carbon dioxide during the Calvin cycle, a vital part of photosynthesis shared by all plants, whether they are C3, C4, or CAM plants.

In C3 plants, Rubisco enzyme is found in mesophyll cells, while in C4 plants, it's in bundle sheath cells. Now, the key player here is Ribulose bisphosphate (RuBP), a 5-carbon compound. It's like the hands that catch carbon dioxide in the Calvin cycle. Rubisco works on RuBP, converting it into two 3-PGA (3-phosphoglyceric acid) molecules.

Rubisco is a bit of a multitasker; it can both grab carbon dioxide (carboxylase activity) and work with oxygen (oxygenase activity). But, when it deals with oxygen, it leads to a side process called photorespiration, converting RuBP into one phosphoglycerate and one phosphoglycolate molecule. 

1.0Structure of RuBisCO 

Rubisco, or Ribulose-1,5-bisphosphate carboxylase/oxygenase, is a pivotal enzyme primarily situated in the chloroplasts of plant cells, where the essential process of photosynthesis takes place. Notably, it is also found in cyanobacteria and certain other photosynthetic bacteria, showcasing its fundamental role in carbon fixation.

Widely recognized as the most abundant protein in the biosphere, Rubisco is integral to the conversion of carbon dioxide into organic compounds. In C3 plants, this enzyme is housed in the mesophyll cells of the leaves, while in C4 plants, it takes residence in specialized bundle sheath cells, demonstrating adaptation for different photosynthetic strategies.

The composition of Rubisco typically involves eight large and eight small subunits, totaling 16 subunits. These subunits are encoded by genes located in two distinct genomes: the chloroplast and the nucleus. This dual genetic encoding highlights the intricate coordination required for Rubisco synthesis and function. 

The arrangement of these subunits results in a hexadecameric structure, forming a complex with 16 subunits that are organized in chains. This sophisticated structure underlines the enzyme's significance in orchestrating the crucial biochemical reactions that sustain life through photosynthesis.

2.0Biosynthesis of RuBisCO 

3.0Functions of RuBisCO 

Its primary function is to catalyze the fixation of atmospheric carbon dioxide, the key step in converting light energy into organic compounds that fuel the growth and development of plants. During the Calvin cycle, Rubisco facilitates the binding of carbon dioxide to a five-carbon sugar molecule, ribulose-1,5-bisphosphate (RuBP), resulting in the formation of two molecules of 3-phosphoglycerate. This critical process sets in motion the synthesis of sugars and other organic molecules, providing the building blocks for plant biomass. 

4.0Role 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)

It is a reaction where RuBP is enolised and then carboxylated to give rise to an intermediate 3-keto-2′-carboxyarabinitol-1,5-bisphosphate. Then it is hydrated and further breaks the bond between two carbons to produce 2 molecules of 3-phosphoglycerate (3-PGA). The 3-PGA so produced is further used in glucose and other carbohydrate formation in subsequent steps.

In C3 plants, the process occurs within the mesophyll cells. In the C4 pathway, the Clavin cycle occurs within the bundle sheath cells, which are replete with RuBisCO. This has been an adaptation to reduce photorespiration within C4 plants.

5.0Photorespiration

RuBisCO also has an affinity for oxygen and it oxygenates RuBP in the presence of oxygen. Photorespiration utilizes ATP, hence, results in 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.