What is the most abundant enzyme on Earth?

The most abundant enzyme on Earth is Rubisco, short for ribulose-1,5-bisphosphate carboxylase/oxygenase.

Rubisco performs the key carbon-fixation step of photosynthesis. It attaches atmospheric carbon dioxide to ribulose-1,5-bisphosphate, helping begin the chain of reactions that produces sugars. Because plants, algae, and many photosynthetic microorganisms make enormous quantities of it, Rubisco is generally regarded as Earth's most abundant enzyme.

Its abundance compensates for a major weakness: Rubisco is relatively slow and can bind oxygen instead of carbon dioxide. That competing reaction leads to photorespiration, which reduces photosynthetic efficiency. Plants therefore produce large amounts of the enzyme to keep carbon fixation moving.

Rubisco belongs to the Calvin cycle, which takes place in the chloroplast stroma. A common mix-up is to call it simply a carbon-fixing protein; it is specifically an enzyme, and its oxygenase activity explains why photosynthesis is not perfectly efficient.