Rubisco is the probably the most abundant protein on the planet Earth. It contains two important activities.

1. Rubisco is the enzyme catalyzing the following reaction:

Ribulose-1,5-Bisphosphate + CO2 + H2O <=> 2 3-Phosphoglycerate + 2 H⁺ 

The reaction above is the first step of the Calvin cycle and is reponsible for a vast amount of the carbon dioxide fixation that occurs as a result of photosynthesis. The enzyme also has an unusual oxygenase actvity, shown below:

2. Ribulose-1,5-Bisphosphate + O2<=> 3-Phosphoglycerate + Phosphoglycolate + H2O + 2H⁺ 

Phosphoglycolate is subsequently dephosphorylated and passed into organelles called the peroxisomes where it is further oxidized, the glyoxylate is amidated, and glycine is produced. This process is referred to as photorespiration and it occurs under conditions where the oxygen concentration is high. The C4 cycle, which occurs in so-called C4 plants, bypasses some of the inefficiency of photosynthesis arising from photorespiration.