Original video: https://youtu.be/pkqkajc-2Tw
Delivered on: 27 NOVEMBER 2023
Photorespiration is a complex process that occurs in plants when the enzyme rubisco, responsible for carbon fixation in photosynthesis, binds with oxygen instead of carbon dioxide. This creates a toxic byproduct that the plant must recycle through a series of reactions involving three organelles: the chloroplast, peroxisome, and mitochondrion. This process, while metabolically expensive, is essential for plants to recover carbon, maintain metabolic stability, and protect themselves from damage.
Video Transcript
It goes out and get into the peroxisome. So now you are in the peroxisome. Peroxisome, okay. So, what happens to glycolate in the peroxisome? So, glycolate in the peroxisome will be oxidized, okay? So, when it is oxidized, meaning that oxygen is added. The enzyme responsible for this should be glycolate oxidase. You get a new compound called glyoxylate plus peroxide.
This glyoxylate will proceed with another reaction. However, just a side reaction, this H2O2 is very, very damaging, right? It is so because you can use it to bleach. So, plants will break down H2O2 on the sideline using the enzyme catalase. Catalase breaks it down. Now you get H2O plus oxygen. Two rounds of breaking down, you will get a molecule of oxygen, O2, okay?
Glycolate is converted to glyoxylate. Glyoxylate proceeds with the next step. However, H2O22 peroxide is very damaging. So, this hydrogen peroxide on the sideline will be, you can call it neutralised, will be neutralized by the action of catalase. Catalase means it breaks down as water and oxygen. Two times break down, you got a molecule O2 of oxygen, okay?
Now, coming back to glyoxylate. So, what happens to glyoxylate now? So, this glyoxylate, it will now react, remember you are still in the peroxisome, okay? Glyoxylate now reacts with an amino acid called glutamate. Okay, and this is a special process because it uses one amino acid to make another amino acid. So, in this case here, glyoxylate here reacts with glutamate in order to produce glycine plus 2-oxoglutarate, okay?
And the enzyme responsible for this conversion to get glycine and 2-oxoglutarate, this is called glutamate glyoxylate aminotransferase. You see, this enzyme aminotransferase, meaning that this process here in the book you will say is called transamination. It uses glutamic acid and then it reacts with glyoxylate to get a new amino acid. Glutamic acid is an amino acid, okay? Glyoxylate is not an amino acid, but this enzyme does the reshuffling and rearranging to get glycine.
What about this 2-oxoglutarate? This actually, maybe in, when you learn your biochemistry, you learn, do you learn the Krebs cycle? The Krebs cycle has a molecule in that called alpha-ketoglutarate. So, this is the synonym as alpha-ketoglutarate. It is an intermediate in the Krebs cycle or citric acid cycle, right? What's the function of this thing? This thing actually, you will see in the cycle involve over and over again, whenever there is an involvement to create an amino acid from another amino acid, okay? Meaning that this newly synthesised amino acid is not from any precursor, it's actually from another amino acid gets converted. When conversion happens through the process of transamination, this is needed, okay?
To prove you to that point, you can see that 2-oxoglutarate is being used again in reaction number eight, when you want to convert amino acid serine to hydroxypyruvate, you use that again, right? And number 12, you use that again to convert glutamine back into glutamine. They are changing from one form to another, okay? Right, so 2-oxoglutarate, we are not talking about it because that is the reactant in many other things. You see, 2-oxoglutarate can go here, can jump here, can go, can go to chloroplast. Now, we want to proceed here.
Keywords: Photorespiration, C2 cycle, Calvin cycle, Plant metabolism, Carbon fixation, Oxygenation, Peroxisome
Watch full video: https://youtu.be/X3c81GRbN2M
Watch the Introduction and the Brief Operation Mechanism photorespiration: https://youtu.be/UkPosfJ8IwA
Watch the Chloroplast Reactions - Rubisco's Oxygenase Activity & Phosphoglycolate Production: https://youtu.be/ISE3Ln_5aHY
Watch the reactions in Mitochondrion - Converting Glycine to Serine & Releasing CO2: https://youtu.be/n8siyvawuiE
Watch the reactions in Peroxisome & Chloroplast - The Final Path from Serine to Phosphoglycerate: https://youtu.be/43m8xoweG48
Reference book: Plant Physiology and Development 7th Edition
by Lincoln Taiz, Ian Max Møller, Angus Murphy, Eduardo Zeiger
Attribution 4.0 International — CC BY 4.0 - Creative Commons
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