Fixing Carbon: the Structure of Rubisco

 

Rubisco (ribulose 1,5-bisphosphate carboxylase–oxygenase) is the key enzyme of the Calvin cycle. It catalyzes the fixation of atmospheric CO₂ into carbon compounds. This reaction involves the carboxylation of the five-carbon sugar ribulose 1,5 -bisphosphate by CO₂ with the eventual release of two three-carbon molecules of 3-phosphoglycerate. The reaction mechanism of Rubisco is shown in [Fixing Carbon: the Rubisco Reaction].

Rubisco makes up about 50% of the soluble protein in plant leaves making it one of the most abundant enzymes on Earth. Interestingly, its status as an abundant enzyme is due partly to the fact that it is not very efficient—the low turnover number of ~3 s^-1 (three reactions per second) means that large amounts of the enzyme are required to support CO₂ fixation!

The Rubisco of plants, algae, and cyanobacteria is composed of eight large (L) subunits and eight small (S) subunits as shown above in top (a) and side (b) views of the enzyme from spinach (Spinacia oleracea). The large subunits are shown as alternately yellow and blue and the small subunits are purple.

There are eight active sites located in the eight large subunits. Four additional small subunits are located at each end of the core formed by the large subunits. The Rubisco molecules in other photosynthetic bacteria have only the large subunits containing the active sites. For example, in the purple bacterium Rhodospirillum rubrum, Rubisco consists of a simple dimer of large subunits.

The purple bacterium version of Rubisco has a much lower affinity for than the more complex multisubunit enzymes in other species but it catalyzes the same reaction. In a spectacular demonstration of this functional similarity, tobacco plants were genetically engineered by replacing the normal plant gene with the one from the purple bacterium Rhodospirillum rubrum. The modified plants contained only the dimeric bacterial form of the enzyme. The plants grew normally and reproduced as long as they were kept in an atmosphere of high CO₂ concentration.

©Laurence A. Moran and Pearson Prentice Hall 2007