The most abundant enzyme on the Earth is:

Correct answer: (d) RuBisCO

Explanation

RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) is the most abundant enzyme, and probably the most abundant protein, on Earth. It fixes CO₂ in the Calvin cycle of every photosynthetic plant and alga.

It works slowly and also reacts with oxygen (photorespiration), so plants make large amounts of it, up to about half of the soluble protein in a leaf.

• Cytochrome oxidase is the last enzyme of the electron transport chain in mitochondria.
• PEP carboxylase fixes CO₂ in C4 and CAM plants.