The biochemistry of all terrestrial life depends upon energy from oxidation processes. The end product of metabolic pathways from carbon-based life is carbon dioxide, which is released into the atmosphere. To complete the carbon cycle, that carbon dioxide must be consumed in the food chain. The only enzyme known to break down carbon dioxide is Ribulose-1,5-Bisphosphate carboxylase, or rubisco. Rubisco represents the beginning of the food chain.

In some bacteria, rubisco is composed of two identical subunits. In higher photosynthetic organisms, the enzyme is a 16-mer composed of two different subunits, a small subunit and the catalytically active large subunit.

Structures of rubisco from Synechococcus, tobacco and spinach have been elucidated. Shown here is the enzyme complex from spinach: Large (dark) and small (light) subunits symmetrically interlock to form rings (red and green). A side view is shown at right. Below is a view along the four-fold symmetry axis.

The spinach enzyme has also been co-crystallized with the substrate, ribulose bisphosphate (blue). From the X-ray diffraction data of the substrate-bound and unbound complexes, conclusions can be drawn regarding the stereochemistry and the reaction mechanism of rubisco.

View a movie of the structure of the enzyme, or the reaction mechanism.

Click here to read Belorussian translation