The 'oxygenase' activity of RUBISCO initiates :

Question

Accepted Answer

The correct option is 4: C2 cycle

The enzyme Ribulose-1,5-bisphosphate carboxylase/oxygenase, universally known as RuBisCO, is arguably the most abundant enzyme on Earth, yet it possesses a crucial and often wasteful dual nature. The "oxygenase" activity of RuBisCO initiates the C2 cycle, which is more commonly referred to as photorespiration. This process occurs when RuBisCO, instead of binding to carbon dioxide (CO2), binds to oxygen (O2) because of high O2 concentrations or high temperatures. When oxygen is the substrate, RuBisCO catalyzes the oxygenation of Ribulose-1,5-bisphosphate (RuBP), a five-carbon sugar. This reaction yields two products: one molecule of the usable three-carbon compound 3-phosphoglycerate (3-PGA) and one molecule of the two-carbon compound 2-phosphoglycolate. The 2-phosphoglycolate, a toxic compound that inhibits photosynthesis, cannot be used directly in the Calvin cycle. The plant must expend energy in a complex, multi-organelle pathway involving the chloroplast, peroxisome, and mitochondrion to salvage the carbon and nitrogen locked within this two-carbon molecule, hence the name C2 cycle. The entire C2 cycle, or photorespiration, is a critical metabolic burden for C3 plants, as it results in the release of previously fixed CO2 and consumes ATP and NADPH without producing sugars.

The incorrect option is 1: C3 cycle

The C3 cycle, also known as the Calvin-Benson cycle, is the fundamental pathway for carbon fixation in all photosynthetic organisms. However, it is initiated by the *carboxylation* activity of the RuBisCO enzyme, not its oxygenase activity. In the C3 cycle, RuBisCO binds CO2 to RuBP, leading to the formation of two molecules of 3-phosphoglycerate (3-PGA), a three-carbon compound. Therefore, while RuBisCO is central to both the C3 and C2 cycles, the specific trigger for the C3 cycle is the enzyme's carboxylase function, whereas the C2 cycle is initiated by its oxygenase function.

The incorrect option is 2: C4 cycle

The C4 cycle represents an advanced evolutionary adaptation specifically designed to *minimize* the photorespiration (C2 cycle) caused by RuBisCO's oxygenase activity. In C4 plants, the primary carbon fixation occurs in the mesophyll cells and is catalyzed by a different enzyme, Phosphoenolpyruvate carboxylase (PEPcase), which has no affinity for oxygen. This enzyme fixes CO2 into a four-carbon compound, which is then transported to the bundle sheath cells. The purpose of this entire mechanism is to concentrate CO2 around the RuBisCO enzyme in the bundle sheath cells, thereby promoting its *carboxylase* activity and practically eliminating the undesirable *oxygenase* activity.

The incorrect option is 3: CAM cycle

The CAM (Crassulacean Acid Metabolism) cycle is another carbon-concentrating mechanism primarily found in desert plants to reduce water loss. Like the C4 pathway, the initial fixation of carbon is carried out by the enzyme PEP carboxylase, not RuBisCO, and results in a four-carbon acid. The CAM cycle separates the initial CO2 fixation (which occurs at night when stomata are open) from the Calvin cycle (which occurs during the day when stomata are closed). The purpose of this temporal separation is to deliver a high concentration of CO2 to RuBisCO during the day, which, just like in C4 plants, ensures that RuBisCO preferentially uses its *carboxylase* activity, thereby preventing the initiation of the C2 cycle.