Background: Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) is the key enzyme of the Calvin-Benson cycle and catalyzes the primary reaction of CO 2 fixation in plants, algae, and bacteria. Rubiscos have been so far classified into two types. Type I is composed of eight large subunits (L subunits) and eight small subunits (S subunits) with tetragonal symmetry (L 8 S 8 ), but type II is usually composed only of two L subunits (L 2 ). Recently, some genuinely active Rubiscos of unknown physiological function have been reported from archaea.Results: The crystal structure of Rubisco from the hyperthermophilic archaeon Thermococcus kodakaraensis KOD1 (Tk-Rubisco) was determined at 2.8 A resolution. The enzyme is composed only of L subunits and showed a novel (L 2 ) 5 decameric structure. Compared to previously known type I enzymes, each L 2 dimer is inclined approximately 16 o to form a toroid-shaped decamer with its unique L 2 -L 2 interfaces. Differential scanning calorimetry (DSC), circular dichroism (CD), and gel permeation chromatography (GPC) showed that Tk-Rubisco maintains its secondary structure and decameric assembly even at high temperatures.Conclusions: The present study provides the first structure of an archaeal Rubisco, an unprecedented (L 2 ) 5 decamer. Biochemical studies indicate that Tk-Rubisco maintains its decameric structure at high temperatures. The structure is distinct from type I and type II Rubiscos and strongly supports that Tk-Rubisco should be classified as a novel type III Rubisco.
