The enzyme ribulose bisphosphate carboxylase (Rubisco) normally adds carbon dioxide to ribulose 1,5-bisphosphate.However, it will also catalyze a competing reaction in which O₂ is added to ribulose 1,5-bisphosphate to form 3-phosphoglycerate and phosphoglycolate.Assume that phosphoglycolate is a compound that cannot be used in any further reactions.If O₂ and CO₂ have the same affinity for Rubisco, which of the following is the lowest ratio of CO₂ to O₂ at which a net synthesis of sugar can occur?

Question

Quizplus · Accepted Answer

Since phosphoglycolate cannot be used in further reactions, it is essentially a waste product. Therefore, we want to minimize the formation of phosphoglycolate. The competing reaction that forms phosphoglycolate occurs when O₂ is added to ribulose 1,5-bisphosphate. Thus, we want to minimize the amount of O₂ that is present relative to carbon dioxide. Since the affinity of Rubisco for CO₂ and O₂ is the same, the ratio of their concentrations will determine the likelihood of their binding to Rubisco. Therefore, the lowest ratio of CO₂ to O₂ that will reduce the formation of phosphoglycolate is 3:1. This means that for every 3 molecules of CO₂, there is 1 molecule of O₂.

The Enzyme Ribulose Bisphosphate Carboxylase (Rubisco) Normally Adds Carbon Dioxide