Which enzyme catalyses the first CO2 fixation in the Calvin cycle and why is its activity significant?

Which enzyme catalyses the first CO2 fixation in the Calvin cycle and why is its activity significant?

• A. RuBisCO; fixes CO2 to RuBP but can also fix O2, leading to photorespiration and reduced efficiency
• B. PEP carboxylase; fixes CO2 to PEP
• C. Rubisco activase; enhances RuBisCO efficiency
• D. ATP synthase; uses Pi to produce energy

Answer: (A)

RuBisCO is the enzyme that carries out the first carbon fixation step in the Calvin cycle by catalyzing the reaction of CO2 with the five‑carbon RuBP to form two molecules of 3‑phosphoglycerate. This initial fixation is crucial because it introduces inorganic carbon into the organic pathway that will ultimately become sugars. The significance of its activity lies in RuBisCO’s dual nature: it fixes CO2 efficiently under many conditions, but it can also fix O2, leading to a competing reaction known as photorespiration. Photorespiration wastes energy and carbon, reducing photosynthetic efficiency, especially when CO2 is scarce, O2 is plentiful, or temperatures are high. This makes RuBisCO a key limiting factor in photosynthesis and a major focus for efforts to improve crop yields. The other enzymes listed don’t perform the first CO2 fixation in the Calvin cycle: PEP carboxylase fixes CO2 in C4 and CAM pathways (not the Calvin cycle proper); rubisco activase helps activate RuBisCO but doesn’t fix carbon itself; ATP synthase generates ATP, not fixed carbon.

RuBisCO is the enzyme that carries out the first carbon fixation step in the Calvin cycle by catalyzing the reaction of CO2 with the five‑carbon RuBP to form two molecules of 3‑phosphoglycerate. This initial fixation is crucial because it introduces inorganic carbon into the organic pathway that will ultimately become sugars. The significance of its activity lies in RuBisCO’s dual nature: it fixes CO2 efficiently under many conditions, but it can also fix O2, leading to a competing reaction known as photorespiration. Photorespiration wastes energy and carbon, reducing photosynthetic efficiency, especially when CO2 is scarce, O2 is plentiful, or temperatures are high. This makes RuBisCO a key limiting factor in photosynthesis and a major focus for efforts to improve crop yields. The other enzymes listed don’t perform the first CO2 fixation in the Calvin cycle: PEP carboxylase fixes CO2 in C4 and CAM pathways (not the Calvin cycle proper); rubisco activase helps activate RuBisCO but doesn’t fix carbon itself; ATP synthase generates ATP, not fixed carbon.