Why is the most common enzyme on Earth so bad at its job?

RuBisCO is the enzyme that grabs carbon dioxide from the air to build sugar, and it is the most abundant enzyme on Earth. It is also oddly bad at its job in two ways: it is slow, and it gets confused.

Slow first. Some enzymes carry out thousands of reactions each second, but RuBisCO fixes only 3 to 10 carbon dioxide molecules per second. Because its reaction is the primary rate-limiting factor of the Calvin cycle during the day, plants compensate with quantity: RuBisCO accounts for 50% of soluble leaf protein in C3 plants.

The confusion is worse. Oxygen and carbon dioxide compete for the same active site, and about 25% of the time RuBisCO adds oxygen instead. The product cannot be used by the Calvin cycle, and the plant must spend energy cleaning up in a process called photorespiration, which can lower photosynthetic output by 25% in C3 plants. Hot days make it worse: high temperature lowers the CO2 concentration in leaf tissue relative to oxygen, and RuBisCO also discriminates less well, so healthy leaves in bright light may fix no net carbon at all.

Why has evolution not fixed this? One suggestion is that RuBisCO is an evolutionary relic: it emerged about four billion years ago, before oxygen was present on Earth, so it was not shaped by the need to tell the two gases apart. Another view is that it is a good compromise between speed and accuracy.