Abstract
Ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO) catalyzes the initial carbon fixation reaction in the Calvin-Benson-Bassham cycle. Among the many forms of RuBisCOs, form-I-a protein complex containing 8 large and 8 small subunits-is the most common, representing over 90% of all known RuBisCOs. Although many form-I RuBisCO structures have been determined, no structure has been reported for a form-IAq RuBisCO. Here, we detail the structure of the heat-stable form-IAq RuBisCO from the thermophilic and anaerobic purple bacterium Thermochromatium (Tch.) tepidum at 1.55 Å resolution. The overall structure of the Tch. tepidum form-IAq RuBisCO resembles both a form-IAc RuBisCO from a chemolithotrophic sulfur bacterium and a synthetic form-I RuBisCO reconstructed from ancestral sequences. However, the Tch. tepidum enzyme shows significantly greater interactions between adjacent small subunits through their extended N-terminal domains that contain a characteristic six-residue insertion unique to form-IAq RuBisCOs. Structural differences of Tch. tepidum RuBisCO from its mesophilic relative Allochromatium vinosum, and key substitutions on the hydrophilic surface of the small subunits suggests the mechanisms of its enhanced thermostability. Our structure represents the first structure of a form-IAq RuBisCO, providing fresh clues for unraveling the evolutionary history of RuBisCO and new details for how this key enzyme remains active at elevated temperatures.