Question

1)What mechanisms have evolved to prevent Rubisco from binding O2?

2)Explain the Calvin cycvolved to prevent Rubisco from binding O2? with reference to CO2, ATP, G3P, 3PG, NADPH, RuBP, and sugar.

Answer 1

The enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase, most commonly known by the shorter name RuBisCO or just rubisco is used in the Calvin cycle to catalyze the first major step of carbon fixation. The desired reaction is the addition of carbon dioxide to RuBP (carboxylation), a key step in the Calvin–Benson cycle, but approximately 25% of reactions by RuBisCO instead add oxygen to RuBP (oxygenation), creating a product that cannot be used within the Calvin–Benson cycle. This process reduces 2the efficiency of photosynthesis, potentially reducing photosynthetic output by 25% in C3 plants.

In rubisco, an oxygen molecule can bind comfortably in the site designed to bind to carbon dioxide. Rubisco then attaches the oxygen to the sugar chain, forming a faulty oxygenated product.

Biochemical CCMs concentrate carbon dioxide in one temporal or spatial region, through metabolite exchange. C4 and CAM photosynthesis both use the enzyme Phosphoenolpyruvate carboxylase (PEPC) to add CO2 to a 3-Carbon sugar. PEPC is faster than RuBisCO, and more selective for CO2.

C4 plants capture carbon dioxide in their mesophyll cells (using an enzyme called phosphoenolpyruvate carboxylase which catalyzes the combination of carbon dioxide with a compound called phosphoenolpyruvate (PEP)), forming oxaloacetate. This oxaloacetate is then converted to malate and is transported into the bundle sheath cells (site of carbon dioxide fixation by RuBisCO) where oxygen concentration is low to avoid photorespiration. Here, carbon dioxide is removed from the malate and combined with RuBP by RuBisCO in the usual way, and the Calvin Cycle proceeds as normal.

Photorespiration increases availability of NADH, which is required for the conversion of nitrate to nitrite. Certain nitrite transporters also transport bicarbonate, and elevated CO2 has been shown to suppress nitrite transport into chloroplasts.The oxygenase reaction of RuBisCO may prevent CO2 depletion near its active sites and contributes to the regulation of CO2 concentration in the atmosphere.

ATP and NADPH produced by the light reactions are used in the Calvin cycle to reduce carbon dioxide to sugar. ATP is the energy source, while NADPH is the reducing agent that adds high-energy electrons to form sugar. The Calvin cycle actually produces a three-carbon sugar glyceraldehyde 3-phosphate (G3P). The molecules of ADP and NAD+, resulting from the reduction reaction, return to the light-dependent reactions to be re-energized. One of the G3P molecules leaves the Calvin cycle to contribute to the formation of the carbohydrate molecule, which is commonly glucose.