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My question is this: Which of the following is NOT correct for rubisco?

a. It is the most abundant enzyme
b. It catalyzes two different reactions
c. It is involved in photorespiration
d. It is not used by CAM plants
e. It is not efficient in fixing CO2

My issues with picking one are because:

a. It is the most abundant enzyme due to its inefficiency
b. It catalyzes both oxygenase and carboxylase
c. It binds O2 meaning its responsible for photorespiration
d. Malate eventually provides a molecule of CO2 to the calvin cycle where it is bound to rubisco, meaning it is used by CAM plants.
e. It only fixes 3 molecules of CO2 per second, making it inefficient. The only reason why this doesn't matter is because of the abundance of rubisco.

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  • $\begingroup$ @RozennKeribin, the problem he has is that all five choices seem to be correct (for the reasons he provides in the second list), and thus none of the five choices seens to be NOT correct, and thus the multiple choice question has no solution (as far as he can see). $\endgroup$
    – mgkrebbs
    Mar 15 '18 at 21:15
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    $\begingroup$ d) is "it is NOT used by CAM plants". In your following text you say it IS used by CAM plants. Seems incorrect to me. $\endgroup$
    – Oosaka
    Mar 15 '18 at 21:34
  • $\begingroup$ Your justifications for your answer need rethinking. (a) It is either abundant or not. You are not asked to give a reason, and your reason is contentious. (e) Efficiency is not the same as speed. Again, focus on what efficiency actually means, not on why you think it doesn't matter. You need to state clearly whether you think each alternative is true and then say why. $\endgroup$
    – David
    Mar 15 '18 at 22:50
  • $\begingroup$ David, I didn’t have to give reasons, just needed to choose an answer. I was just showing my thought process on why I was stuck. $\endgroup$
    – Rayy
    Mar 16 '18 at 0:19
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    $\begingroup$ Rozenn, somehow that literally went right over my head. I was reading it wrong and didn’t even realize I answered the question already. Doh. Thanks. $\endgroup$
    – Rayy
    Mar 16 '18 at 0:20
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Short answer
D

Background
A. Is correct, as it is the most abundant enzyme on land, and probably also in marine environments (Raven, 2013).

B. Is correct, as Rubisco catalyzes the carboxylation, or oxygenation reaction depending upon the molecular concentration of CO2 or O2 (Sudhakar et al., 2016).

C. Is correct, as it reveals the inefficiency of Rubisco, as CO2 and O2 compete for the active site. Fixation of O2 during photosynthesis oxidizes ribulose‐1,5‐bisphosphate into 3‐phosphoglycerate and phosphoglycolate - which is basically a waste of energy.

D. Is incorrect and hence the answer to your question: Rubsico is present in CAM plants.

E. Because of the photorespiration described under B & C, Rubisco is indeed regarded to be inefficient.

References
- Raven, New Phytologist (2013); 198(1): 1–3
- Sudhakar et al., Phenotyping Crop Plants for Physiological and Biochemical Traits, 2016

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