Enzyme identity in carbon fixation: Which enzyme catalyzes the carboxylation step that fixes CO2 in the Calvin cycle?

Introduction / Context:
At the heart of photosynthetic carbon assimilation lies a single, rate-limiting carboxylation. The enzyme performing this reaction is globally abundant and central to plant productivity and the carbon cycle. Recognizing Rubisco as this catalyst is foundational for plant biochemistry.

Given Data / Assumptions:

• CO2 fixation happens in the chloroplast stroma.
• Rubisco catalyzes addition of CO2 to ribulose 1,5-bisphosphate.
• Other enzymes listed participate in electron transport or unrelated metabolic steps.

Concept / Approach:
Match each enzyme to its known role. NADP reductase transfers electrons to NADP+ in light reactions; cytochrome reductase participates in electron transport; glycerol kinase phosphorylates glycerol; phosphoribulokinase regenerates RuBP from ribulose 5-phosphate but does not fix CO2. Only Rubisco performs carboxylation of RuBP.

Step-by-Step Solution:

Verification / Alternative check:
Purified Rubisco exhibits both carboxylase and oxygenase activities, explaining the competing process of photorespiration at high O2/low CO2 ratios, which is unique to this enzyme among the choices.

Why Other Options Are Wrong:

• NADP reductase and cytochrome reductase: light-reaction electron carriers, not CO2-fixing enzymes.
• Glycerol kinase: unrelated to photosynthetic carbon fixation.
• Phosphoribulokinase: regenerates RuBP but does not add CO2.

Common Pitfalls:
Confusing regeneration (phosphoribulokinase) with fixation (Rubisco). The names are similar, but the functions are distinct.

Final Answer:
Ribulose bisphosphate carboxylase (Rubisco)