Rubisco (Ribulose Bisphosphate Carboxylase) – Role in Photorespiration

The enzyme Ribulose Bisphosphate Carboxylase/Oxygenase (Rubisco) fixes the oxygen. But in the Calvin cycle, rubisco fixes CO2, So photorespiration lowers the overall rate of carbon dioxide fixation and plant growth.  Ribulose 1,5 Bisphosphate (RuBP) reacts with oxygen in photorespiration.

Role Of Rubisco In Photorespiration:

The following steps take place during photorespiration:

The RuBP reacts with oxygen. This step is controlled by the enzyme Rubisco. The rubisco is the most abundant protein in the world.

The rubisco is carboxylase as well as oxygenase:

Rubisco As Carboxylase:

When rubisco acts as carboxylase, it adds carbon dioxide to RuBP. The RuBP acts as an acceptor molecule.

b)     Rubisco As Oxygenase:

1. When Rubisco acts as an oxygenase, it adds oxygen to RuBP.

Both these reactions compete with each other. The RuBP reacts with oxygen to form two carbon compounds called Glycolate.

2. This Glycolate diffuses into the peroxisome. It is a membranous bounded organelle. The glycolate is converted into Glycine in the Peroxisome through a series of reactions.

3. Glycine is a simple amino acid. It soon diffuses into mitochondria, ln mitochondria two molecules of Glycine combine to form serine. A molecule of carbon dioxide is also formed.

The process of photorespiration uses ATP and NADPH just like the Calvin Benson cycle.

These ATP and NADPH are produced during light reactions. In fact, photorespiration is the reverse of the Calvin cycle. During photorespiration, CO is produced and does not fix carbohydrates. Photorespiration reduces the number of carbohydrates by 25% in most plants.

Functions of Rubisco

The rubisco can act both as oxygenase and carboxylase. The relative concentration of the carbon dioxide and oxygen in the leaf determines whether rubisco has to act as carboxylase or oxygenase.

a) When more oxygen is present, it acts as an oxygenase, and photorespiration starts. So these stomata are closed during hot and dry days to prevent the loss of water. stomata do not allow oxygen to go outside and the level of the oxygen rises. The is consumed. So the level of CO2 falls. In this way, the rubisco acts as an oxygenase

b) When more CO2 is present, it acts as a carboxylase, and the Calvin cycle starts.