What is the role of RuBisCO in carbon fixation and why is it potentially problematic?

• A. RuBisCO is a transporter protein in the chloroplast.
• B. RuBisCO catalyzes NADP+ reduction in the stroma.
• C. RuBisCO catalyzes CO2 fixation to RuBP; its oxygenase activity leads to photorespiration, reducing efficiency, especially at high O2/low CO2.
• D. RuBisCO stores energy as ATP in the chloroplast.

Answer: (C)

RuBisCO is the enzyme that starts carbon fixation by attaching CO2 to a five-carbon sugar called RuBP in the Calvin cycle, producing two molecules of 3-phosphoglycerate that flow toward carbohydrate synthesis. The problem is that RuBisCO can also react with O2 in the same active site. When oxygen binds, it leads to the formation of one molecule of 3-PGA and one molecule of phosphoglycolate, a portion that has to be recycled through photorespiration. This salvage pathway uses energy and releasing CO2, so it wastes carbon and reducing power that could have gone toward making sugars. The efficiency hit is greatest when oxygen levels are high relative to CO2, which happens under high O2/low CO2 conditions, higher temperatures, or in plants without CO2 concentrating mechanisms. Because RuBisCO is relatively slow and cannot perfectly discriminate between CO2 and O2, this oxygenase activity inherently limits the rate of net carbon fixation in many plants. Some plants mitigate this with carbon concentrating mechanisms (like C4 or CAM pathways) to favor the carboxylase reaction and suppress photorespiration.