What enzyme is essential for the first major step of carbon fixation in photosynthesis?

Rubisco, or ribulose bisphosphate carboxylase/oxygenase, is essential for the first major step of carbon fixation in photosynthesis as it catalyzes the reaction between carbon dioxide and ribulose bisphosphate (RuBP) to form 3-phosphoglycerate (3-PGA). This reaction is critical because it initiates the process of converting inorganic carbon (in the form of CO2) into organic compounds that can be used by plants to produce glucose during the Calvin cycle.

Rubisco is considered one of the most abundant enzymes on Earth due to its vital role in the global carbon cycle, and its activity is fundamental for life as it allows for the incorporation of carbon from the atmosphere into biological systems. In the overall context of photosynthesis, this enzyme facilitates a critical step that ultimately leads to the production of sugars, which serve as energy sources for the plant and, indirectly, for other organisms within the ecosystem.

The other choices, while involved in various biochemical processes, either do not play a direct role in carbon fixation or are specific to different metabolic pathways. Phosphoenolpyruvate carboxylase is involved in C4 and CAM plants for fixing CO2 in a different manner, carbon