Enzyme of the Month March edition - RuBisCo

 

I hope you enjoyed reading about DNA polymerase last month! This month we are discussing an essential enzyme also known as Rubisco. We have to respect this mighty enzyme which happens to be the most abundant enzyme on this planet. Without this plant enzyme, planet Earth would have no life. Hence, dedicating a blog to this ultra-important enzyme makes sense, right? 

RuBisCo or Ribulose biphosphate carboxylase/oxygenase is found in the plants and is a major enzyme in the process of photosynthesis. We will first briefly go over the process of photosynthesis. Ideally, photosynthesis is the reverse pathway to cellular respiration (refer to the January edition of the blog to understand cellular respiration). Unlike cellular respiration which is a catabolic (breaking down the energy source) pathway, photosynthesis is an anabolic pathway (synthesizing an energy source) that synthesizes glucose from water and carbon dioxide. The reaction formula of photosynthesis is as follows:

6 CO₂ + 6 H₂O → C₆ + H₁₂

This chemical formula makes it appear that photosynthesis is a reverse reaction of cellular respiration reaction which is as follows: C₆ + H₁₂ → 6 CO₂ + 6 H₂O. But, this is biology; nothing that appears simple is simple. It is complicated. Nevertheless, just for our basic understanding, we can assume that photosynthesis makes the sugar to store energy, and cellular respiration breaks down the sugar to release the energy. Photosynthesis occurs in 2 stages:

1.      The light-dependent reaction that features light and chlorophyll¹

2.      The light-independent reaction that occurs with the help of the products formed in the light-dependent reaction

            In this section of the blog, we will talk about the Light-dependent reaction. The chlorophyll pigment absorbs a certain wavelength of light (aka photon). This absorption of photon excites electrons in the reaction center that functions to convert light energy into chemical energy (please refer to January blog to read more about chemical energy in the form of ATP). The main purpose of this reaction is to produce ATP and NADPH². 

Image Citation link

            Now we move to the Light-independent reactions, also regarded as the Calvin Cycle, which is broken down into three stages: Carbon Fixation, Reduction, and Regeneration. The products of the Light-dependent reactions are used in this process. The carbon atoms which are taken in from the atmosphere by the stomata of the leaves are fixed into three-carbon compounds called 3-phosphoglyceric acid (3-PGA). In the reduction phase, 3-PGA is reduced to a sugar molecule, glyceraldehyde-3 phosphate (G3P). This phase utilizes the NADPH and ATP produced in the Light-dependent reactions as the NADPH donates hydrogen atoms to reduce 3-GPA. After the glucose molecule in the form of G3P is produced, some molecules are transported out of the photosynthesis site to produce glucose while other glucose molecules are regenerated to make a 5-C RuBP compound which is required to carry out the Calvin Cycle again. Hence, this stage is referred to as the Regeneration step. 

Where does the enzyme RuBisCo fall into this complex process? The role of RuBisCo is evident in the Carbon Fixation step of Calvin Cycle (Please refer to the image below). RuBisCo fixes the atmospheric CO₂  molecule with a carbon acceptor molecule Ribulose-1, 5-biphosphate (RuBP). After this step, the reactive 6-carbon compound is broken down into PGA. This whole reaction is catalyzed by RuBisCo. Without this reaction, photosynthesis would have been impossible. That is why this enzyme is crucial for the existence of life on Earth. As through this enzyme, plants produce food that powers life on Earth! So I am not lying when I say that respect this enzyme!

Image citation link

 

Footnotes:

1.      A pigment found in the leaves of the leaves that produces a green color and absorbs a certain wavelength of light to initiate photosynthesis.

2.      Similar to NADH, however, NADPH is found in the anabolic pathways that donate Hydrogens to other molecules and therefore acts as a reducing agent, unlike NADH which is an oxidizing agent.

 

Works Cited:

Aparna Vidyasagar. (2018, October 15). What Is Photosynthesis? Livescience.com; Live Science. https://www.livescience.com/51720-photosynthesis.html

 

BD Editors. (2016, November 10). Calvin Cycle. Biology Dictionary. https://biologydictionary.net/calvin-cycle/

BD Editors. (2017, February 14). NADPH. Biology Dictionary. https://biologydictionary.net/nadph/

 

Cellular respiration | Process & Products | Britannica. (2021). In Encyclopædia Britannica. https://www.britannica.com/science/cellular-respiration

RuBisCO - an overview | ScienceDirect Topics. (2017). Sciencedirect.com. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/rubisco

 

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