Is Rubisco An Enzyme? 9 Facts You Should Know

In this article, we get know about 9 Important Facts regarding ‘Is Rubisco An Enzyme?’, along with their origin, characteristics, functions and examples.

Ribulose Bisphosphate Carboxylase Oxygenase is known as RuBisCO. It is created from the five-carbon ketose sugar ribulose bisphosphate (RuBP). The enzyme that is most prevalent on earth is RuBisCO. Its ability to bind both CO2 and O2 at its active site gives it its characteristic nomenclature.

Let us discuss some facts and try to understand “Is Rubisco An Enzyme?”

  • Why is rubisco an enzyme?
  • How is rubisco an enzyme?
  • What group of enzyme is rubisco?
  • Where is rubisco enzyme found?
  • Functions of rubisco enzyme
  • Structure of rubisco enzyme
  • Can rubisco be a substrate?
  • Is rubisco enzyme a protein?

Key features:

  • It binds CO2 and O2 with a strong affinity, and the binding is competitive. Which of the two will attach to the enzyme depends on the relative levels of CO2 and O2.
  • Due to a mechanism present in C4 plants that raises the concentration of CO2 at the enzyme site, RuBisCO performs greater carboxylation in these plants.
  • When the bundle sheath cells break down the C4 acid first from mesophyll cells during the C4 route, it releases CO2, which raises the content of CO2 inside the cells.
  • Therefore, RuBisCO acts as a carboxylase, binds to plants, and increases carboxylation.

Why is rubisco an enzyme?

Ribulose-1,5-bisphosphate carboxylase/oxygenase catalyses the rate-limiting phase of the Calvin-Benson cycle, which transforms atmospheric carbon into a physiologically useful carbon source (Rubisco). Rubisco must be produced in large quantities due to its low substrate specificity and slower catalytic rate.

is rubisco an enzyme
Overview of carbon fixation and the Calvin cycle from Wikipedia

Ex:

  • Because RubisCO is abundant and has a high molecular weight in the plant leaf chloroplasts of C3 type plants, the fractionation of extract of leaves using ammonium sulphate produced “Fraction 1,” a homogenous protein fraction that accounts for a significant amount of leaf proteins (550 kDa).
  • For simplicity, this fraction is referred to as “Fraction 1 protein,” whose composition has been thoroughly documented by electrophoresis, analytical centrifugation, crystallisation, etc. and has been used in a variety of applications.
  • However, it was still unclear how it worked as an enzyme. It was found in a different way. Investigators set out to identify the enzyme that catalyses the dismutation of 3-PGA into two molecules and the binding of the CO2 molecule to Ribulose 1,5-diphosphate after comprehending the Benson-Bassham-Calvin Cycle.
  • Called carboxydismutase, this enzyme was found in 1956. Ribulose bisphosphate (or RuBP) Carboxylase was quickly shown to be the same protein as the “Fraction 1 protein” and the enzyme that had been previously identified.

How is rubisco an enzyme?

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) is the key enzyme in CO2 assimilation. It catalyses the reaction between gaseous carbon dioxide and ribulose-1,5-bisphosphate (RuBP) to generate two molecules of 3-phosphoglyceric acid (3-PGA).

is rubisco an enzyme
Galdieria sulphuraria‘s RuBisCO activity site with CO2 from Wikipedia
  • The term RubisCO comes from the fact that this enzyme is bifunctional and also acts as an oxygenase in addition to its carboxylase function (Ribulose biphosphate Carboxylase Oxygenase).
  • For more than three billion years, an enzyme particular to photosynthesis called RubisCO has served as the entry point for carbon into the widely held of the organic compounds on the planet.
  • Photorespiration, a metabolic process that emerged considerably later with the rise in atmospheric oxygen level, also has its roots in this process.
  • It is the biosphere’s enzyme with the highest quantitative importance. Furthermore, it serves as the primary source of organic nitrogen in the leaves.

What group of enzyme is rubisco?

Ribulose bisphosphate carboxylase oxygenase is known as RuBisCO. It catalyses the carboxylation and oxygenation reactions during photosynthesis and photorespiration, respectively, and has an affinity for both CO2 and O2.

Ex:

  • One of the numerous enzymes in the Calvin cycle is RuBisCO. Two molecules of glycerate-3-phosphate are created when Rubisco aids in the oxidation of CO2 at the RuBP C2 carbon and following bond cleavage between the C3 and C2 carbons. Enolization, carboxylation, hydration, C-C bond cleavage, and protonation are the processes involved in the conversion.
  • The RuBisCo enzyme, which is more correctly known as RUBP Carboxylase-Oxygenase, has an affinity for both oxygen and carbon dioxide.

Where is rubisco enzyme found?

Most autotrophic organisms, including eukaryotic algae and higher plants, as well as a variety of prokaryotes, including photosynthetic and chemolithoautotrophic bacteria and archaea, contain rubisCO.

Ex: In plants, the Calvin cycle of photosynthesis is aided by the enzyme RuBisCO, which is located in the chloroplast. After reacting with RuBP (ribulose 1,5 bisphosphate) to create enediol, it next combines with CO2 to form PGA (3-phosphoglycerate) after passing through a few intermediates.. 

Origin of the RubisCO

RubisCO-like proteins (RLP) and RubisCO itself are members of the same superfamily and are found in different living creatures. These proteins don’t all function as carboxylases. A three-step evolutionary route is suggested by phylogenetic study of the genes that encode these proteins.

  • A non-CO2-binding enolase-type enzyme is likely the source of the RPL/RubisCO superfamily of proteins. A functional carboxylase would have first appeared in this family;
  • Prior to the emergence of RubisCO-dependent autotrophy in the bacteria with the Benson-Bassham-Calvin cycle, RubisCO would have first appeared in an autotrophic environment in archaea through nucleotide metabolism;
  • To improve the flow of carbon As a result, RubisCO would evolve from an independent enzyme to an enzyme complex, with the last stage of evolution occurring in cyanobacteria with the carboxysomes or pyrenoids of algae.

Functions of rubisco enzyme

  • RuBisCO’s primary roles are in photosynthesis and photorespiration.
  • It catalyses the carboxylation of RuBP, the initial stage of carbon fixation in the C3 pathway or Calvin cycle. Two molecules of 3-PGA are produced as a result of it.
  • RuBisCO also attaches to some oxygen in the photorespiration process because it has an affinity for oxygen. RuBP is transformed into one molecule each of phosphoglycerate and phosphoglycolate as a result.
  • Photosynthesis is preferred over photorespiration because RuBisCO has a significantly higher affinity for CO2 than for O2..

Role of RuBisCO in Photosynthesis

  • The Calvin cycle’s initial phase of carbon fixation is catalysed by RuBisCO. All plants undergo the Calvin cycle, which includes C3, C4, and CAM.
  • Carboxylation is the Calvin cycle’s initial stage. Here, CO2 is converted into an organic intermediate that is stable. A 5-C compound is RuBP. It is carboxylated using CO2, and after the C-C link is broken, two molecules of 3-PGA are produced.

The reaction catalysed by RuBisCO is as follows:

RuBP (5C) + CO2 + H2O → 2 3-PGA (3C)

  • Enolization of RuBP is followed by carboxylation, which creates an intermediary compound called 3-keto-2′-carboxyarabinitol-1,5-bisphosphate. Following hydration, the link between two carbons is subsequently broken to produce two molecules of 3-phosphoglycerate (3-PGA). The resulting 3-PGA is used in the succeeding steps to create glucose and other carbohydrates..
  • C3 plants’ mesophyll cells are where this process occurs. The bundle sheath cells of the C4 pathway are where the Calvin cycle occurs. RuBisCO is abundant in the cells of the bundle sheath. This modification helps C4 plants reduce photorespiration.

Photorespiration

  • Additionally having a preference for oxygen, RuBisCO oxygenates RuBP when oxygen is present. Because photorespiration uses ATP, some of the energy from photosynthesis is lost.
  • RuBP is changed into one molecule of each phosphoglycerate (3C) and phosphoglycolate (2C) when RuBisCO binds to O2. It is a waste process because it does not produce ATP or sugar..

Structure of rubisco enzyme

In photosynthetic organisms, rubisCO is made up of a set of eight large subunits (referred to as L), each measuring 51 to 58 kDa, and eight tiny subunits (referred to as S), each measuring 12 to 18 kDa..

is rubisco an enzyme
Ribulose-1,5-bisphosphate carboxylase oxygenase- Rubisco (3D MODEL) from Wikipedia
  • The chloroplast genome encodes the major components in the chloroplast stroma;
  • The small components are encoded in the nuclear genome of the photosynthetic cells and transported into the stroma of the chloroplast organelle through its outer and inner membranes;
  • Chaperone proteins are required for the folding of the polypeptides matching to the subunits and their assemblage to form the functional RubisCO;
  • The big sub-units contain the catalytic sites at the functional level. Its little sub-units, which are required for operation, have a regulatory role.

RuBisCO Characteristics

  • Algae, plants, photosynthetic protists, and some autotrophic bacteria, such as cyanobacteria and proteobacteria, among others, all contain RuBisCO.
  • The most prevalent protein in the biosphere is called RuBisCO. In C3 and C4 plants, it makes up around 50% and 30% of the total soluble leaf protein, respectively.
  • RuBisCO is found in bundle sheath cells in C4 plants as opposed to mesophyll cells in C3 plants.
  • It is a substantial, complex protein made up of both long and short chains. The molecular weight is approximately 540,000 Da..
  • Typically, there are 8 little chains and 8 giant chains, which combine to produce 4 dimers. The big chain contains the substrate’s active site. Some dinoflagellates and bacteria only have big subunits.
  • Nuclear DNA codes for the tiny chains, while chloroplast DNA codes for the big chains. From the cytoplasm, little chains are transported into the stroma of chloroplasts.
  • Mg2+ is necessary for the enzyme to function.
  • RuBisCO is inactive at night and becomes active during the day. By attaching CO2 and Mg2+ ions to the lysine residue close to the active site, RuBisCO is activated, causing conformational changes and stabilising the enzyme’s active state.

Can rubisco be a substrate?

RuBisCO uses carbon dioxide and ribulose-1,5-bisphosphate as substrates (distinct from the “activating” carbon dioxide). When catalysed by RuBisCO, ribulose-1,5-bisphosphate and molecular oxygen (O2) react instead of carbon dioxide (CO 2).

  • Fixation, reduction, and regeneration are the three fundamental phases that make up the Calvin cycle reactions. The enzyme RuBisCO and the molecule ribulose bisphosphate are both present in the stroma in addition to CO2 to start the Calvin cycle (RuBP). RuBP has a phosphate group on each of its ends and five carbon atoms.
  • A six-carbon molecule is created when RuBP combines with CO2, and RuBisCO subsequently splits that molecule into two three-carbon molecules. This procedure is known as carbon fixation because CO2 is “fixed” from its inorganic state into organic molecules.

Is rubisco enzyme a protein?

The protein RuBisCO can account for up to 50% of the total protein fraction in plants’ green portions. RuBisCO is thus thought to be the most prevalent protein on earth and is the primary photosynthetic enzyme in green plant leaves.

For instance, RuBisCO is an enzyme, an unique class of protein. RuBisCO, like other enzymes, has active sites that bind to the substrates, speeding up processes. RuBisCO is a component of the Calvin Cycle, a process in photosynthesis where carbon dioxide is converted to sugar.

Conclusion

In the above article, we studied about RuBisCo: whether it is an enzyme and some interesting facts to prove such thing. Structure, classification, functions and the origin of RuBisCo has been studied in detail.

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