In: Biology
Rubisco is A) the enzyme that fixes CO2 to RuBP B) a slow enzyme C) an enzyme that can catalyze two different reactions D) All of the above
Answer : (D). All of the above.
Option (A). RuBisCo is an important enzyme in the dark reactions of C3 plants. C3 plants are those plants whose first stable product of dark reaction is a 3 Carbon compound called 3- Phosphoglyceric acid. This reaction is actually the carboxylation of Ribulose-1,5-bisphosphate (commonly called RuBP) to produce 2 molecules of 3-Phosphoglyceric acid. RuBP is a 5 carbon compound. This reaction is catalysed by RuBP Carboxylase /oxygenase enzyme by making use of its carboxylase activity.
Option (B). RuBisCo is a slow enzyme. This is because, most of the enzymes are capable of initiating reactions with thousands or lakhs of molecule per second, but RuBisCo can catalyse only the reaction of 3 molecules of carbon di oxide with 3 molecules of RuBP per second. (RuBisCo provide active site for CO2)
Option (C). RuBisCo is an enzyme that can catalyze two different reactions. One of the two reactions is discussed in explanation for option (A).
As the name of the enzyme RuBP Carboxylase/Oxygenase suggests, it has catalytic role in oxygenation reaction as well. The reaction involving oxygenation reaction by RuBisCo is called Photorespiration. In photorespiration, RuBP reacts with Oxygen to form 2-Phosphoglycolate in the presence of RuBisCo due to the oxygenase activity of the enzyme. This happens only in light. Since it involves the uptake of oxygen and happens in the presence of light, it is called photorespiration. Photorespiration consumes ATP and NADPH, but produce no ATP or NADPH. Hence it is an energy wasteful process.