RUBISCO (ribulose bisphosphate carboxylase-oxygenase) is the enzyme in the stroma that fixes carbon dioxide in the Calvin cycle. It catalyses the reaction between CO2 and RuBP to form two molecules of GP. Almost all the carbon in living things has passed through this reaction.RUBISCO is thought to be the most abundant protein on Earth. There is so much of it because it is a very slow enzyme: each molecule fixes only a few CO2 molecules per second, compared with thousands of reactions per second for many enzymes. To fix enough carbon, leaves contain large amounts of it, up to about half of their soluble protein.RUBISCO has a second weakness. Its active site can bind oxygen as well as carbon dioxide, and the two gases compete. When it reacts with oxygen (its oxygenase activity), RuBP is converted into one GP and one 2-carbon compound. Recycling the 2-carbon compound uses ATP and releases CO2, so fixed carbon is lost. This is photorespiration.Photorespiration increases when:
• the temperature is high, because RUBISCO's preference for CO2 over oxygen falls;
• the CO2 concentration is low and the oxygen concentration is high, for example when stomata close.This helps explain why raising the CO2 concentration in glasshouses increases crop yields: more CO2 outcompetes oxygen, which reduces photorespiration and increases the rate of fixation. Like other enzymes, RUBISCO also works faster as temperature rises, up to its optimum.
• the temperature is high, because RUBISCO's preference for CO2 over oxygen falls;
• the CO2 concentration is low and the oxygen concentration is high, for example when stomata close.This helps explain why raising the CO2 concentration in glasshouses increases crop yields: more CO2 outcompetes oxygen, which reduces photorespiration and increases the rate of fixation. Like other enzymes, RUBISCO also works faster as temperature rises, up to its optimum.