RuBisCO is the most abundant enzyme in the world because:

Concept:

• RuBisCO stands for Ribulose-1,5-biphosphate carboxylase-oxygenase.
• It is a chief enzyme of the Calvin cycle in photosynthesis. It is present in the stroma.

Explanation: 

• RuBisCO is called the most abundant enzyme in the world because:

1. Its active site can bind with both CO2 and O2 i.e. RuBisCO can act as both carboxylase as well as oxygenase.

• It acts as carboxylase at a lower concentration of oxygen and a higher concentration of carbon dioxide. It catalyzes the process of carboxylation of 5C compound Ribulose-1,5-bisphosphate into a 6C compound 3-phosphoglycerate in the Calvin cycle
• It acts as an oxygenase at a higher concentration of oxygen and a low concentration of carbon dioxide. This causes photorespiration in C3 plants which is a wasteful process.

      2. RuBisCO comprises about 15-50% of total chloroplast proteins.

Thus it can be said that RuBisCO is the most abundant enzyme in the world.