Rubisco is an important enzyme in the Calvin cycle because it acts as a carboxylase to catalyze addition of carbon dioxide and water to ribulose-1,5-bisphosphate, which then forms two molecules of 3-phosphoglycerate

Question

Rubisco also acts as an oxygenase to catalyze addition of molecular oxygen rather than carbon dioxide to ribulose-1,5-bisphosphate.
a) What is (are) the product(s) of the rubisco oxygenase reaction activity and why is this a problem for plants?
b) Explain why rubisco's much lower affinity for oxygen than for carbon dioxide does not mitigate the rubisco oxygenase problem for plants.
c) Identify the pathway plants use to metabolize the problematic product of rubisco oxygenase and the final fate of its carbons.
d) Explain why the oxygenase activity of rubisco is particularly problematic for plants in arid environments.
e) What strategies do plants in arid environments use to deal with this problem?
What will be an ideal response?

Answer 1

Answer:
a) Rather than the carboxylase activity yielding two molecules of 3-phosphoglycerate to feed into the Calvin cycle, rubisco oxygenase activity yields one three-carbon product (3-phosphoglycerate) and one two-carbon product (phosphoglycolate) that cannot be used directly in the next step of the Calvin cycle. Producing the two-carbon product appears to be a wasteful diversion of material from carbon assimilation.
b) The Earth's atmosphere contains a much higher concentration of O2 than CO2, so oxygenase activity could have a significant negative impact on carbon fixation.
c) The glycolate pathway transforms ~75 percent of the phosphoglycolate carbon into 3-phosphoglycerate and the remaining 25 percent is released as CO2.
d) Higher temperatures decrease carbon dioxide solubility relative to oxygen solubility. In addition, these plants close their stomata to reduce water loss, which further reduces the CO2 concentration in the leaf.
e) C4 plants confine rubisco to cells containing high carbon dioxide levels. CAM plants only open their stomata at night.