Rubisco catalyzes RuBP reaction with O2 as well as CO2

As its full name indicates, rubisco is a carboxylase (adding CO2 to the acceptor molecule RuBP) as well as an oxygenase (adding O2 to RuBP). These two reactions compete with each other. So when RuBP reacts with O2, it cannot react with CO2. This reaction reduces the overall CO2 that is converted to carbohydrates, and therefore limits plant growth.

When O2 is added to RuBP, one of the products is a two-carbon compound, phosphoglycolate:

RuBP + O2 ^ phosphoglycolate + 3PG

Plants have evolved a metabolic pathway that partially recovers the carbon that has been channeled away from the

CH2O P

l| In the chloroplasts,

RuBP reacts with O2. Glycolate is formed.

| Glycolate diffuses into a peroxisome, where it is converted to glycine.

l| In the chloroplasts,

RuBP reacts with O2. Glycolate is formed.

| Glycolate diffuses into a peroxisome, where it is converted to glycine.

Photorespiration
Glycine is converted to glycerate in the mitochondria and CO2 is released.

8.15 Organelles of Photorespiration The reactions of photorespiration take place in the chloroplasts, peroxisomes, and, finally, in the mitochondria.

Calvin-Benson cycle. The phosphoglycolate forms glycerate, which diffuses into membrane-enclosed organelles called peroxisomes (Figure 8.15). There, a series of reactions converts it to the amino acid glycine:

glycolate ^ ^ glycine

The glycine then diffuses into a mitochondrion, where two glycine molecules are converted to glycerate (a three-carbon molecule), and CO2:

2 glycine ^ glycerate + CO2

This pathway is called photorespiration because it consumes O2 and releases CO2. It uses ATP and NADPH produced in the light reactions, just like the Calvin-Benson cycle. The net effect is to take two two-carbon molecules and make one three-carbon molecule. So one carbon of the four is released as CO2 and three of the carbons (75%) are recovered as fixed carbon. In other words, photorespiration reduces net carbon fixation by 25 percent compared with the Calvin-Benson cycle.

How does rubisco "decide" whether to act as an oxygenase or a carboxylase? First, rubisco has 10 times more affinity for CO2 than O2, and so favors CO2 fixation. Another consideration is the relative concentrations of CO2 and O2 in the leaf. If O2 is relatively abundant, rubisco acts as an oxyge-nase, and photorespiration ensues. If CO2 predominates, ru-bisco fixes it, and the Calvin-Benson cycle occurs. Temperature is also a factor: photorespiration is more likely at high temperatures.

On a hot, dry day, the stomata that allow water to evaporate from the leaf close to prevent water loss (see Figure 8.1). But this also prevents gases from entering and leaving the leaf. The CO2 concentration in the leaf falls because CO2 is being used up by the light-driven photosynthetic reactions, and the O2 concentration rises because of these same reactions. As the ratio of CO2 to O2 in the leaf falls, the reaction of rubisco with O2 is favored, and photorespiration proceeds.

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Responses

  • Dodinas Brockhouse
    How does rubisco “decide” whether to act as an oxygenase or a carboxylase?
    7 years ago
  • scudamor
    When rubp reacts with o2?
    7 years ago
  • amina
    What catalyzes the reaction of o2 and co2?
    7 years ago
  • severo
    How glycine diffuses to the mitochondria?
    7 years ago
  • Jan Kunze
    Why does oxygen react with rubisco?
    7 years ago
  • alfreda
    Does RuBP react with oxygen?
    7 years ago
  • Lasse
    When RuBP reacts with oxygen?
    7 years ago
  • selamawit
    Why does rubisco have 10 times more affinity for O2 than CO2?
    7 years ago
  • Leonardo
    What are the two gases rubisco catalyzes?
    6 years ago
  • theodore
    How rubisco acts as oxygenase?
    6 years ago
  • salvo
    Why does rubisco need to react with rubp?
    5 years ago
  • longo
    Does rubisco catalyze a phosphorylation reaction?
    5 years ago
  • ines
    When would RUBISCO catalyze the reaction of RuBP with CO2?
    5 years ago
  • reilly
    Why does RUBISCO prefer O2 to CO2?
    3 years ago
  • haben filmon
    How RuBisCO react with both CO2 andO2?
    2 years ago
  • amanda
    What is the reaction that rubisco catalyzes?
    2 years ago
  • mylie
    Why does Rubisco have an oxygenase reaction?
    2 years ago
  • jenni
    Does rubp catalyze a phosphorylation reaction?
    2 years ago
  • martha
    How does rubisco work in presence of o2 and co2?
    1 year ago
  • marcel
    When rubisco acts as oxygenase it adds what to rubp?
    7 months ago

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