Crystal structure of a RuBisCO-like protein from the green sulfur bacterium Chlorobium tepidum.

Ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO) catalyzes the incorporation of atmospheric CO(2) into ribulose 1,5-bisphosphate (RuBP). RuBisCOs are classified into four forms based on sequence similarity: forms I, II and III are bona fide RuBisCOs; form IV, also called the RuBisCO-like protein (RLP), lacks several of the substrate binding and catalytic residues and does not catalyze RuBP-dependent CO(2) fixation in vitro. To contribute to understanding the function of RLPs, we determined the crystal structure of the RLP from Chlorobium tepidum. The overall structure of the RLP is similar to the structures of the three other forms of RuBisCO; however, the active site is distinct from those of bona fide RuBisCOs and suggests that the RLP is possibly capable of catalyzing enolization but not carboxylation. Bioinformatic analysis of the protein functional linkages suggests that this RLP coevolved with enzymes of the bacteriochlorophyll biosynthesis pathway and may be involved in processes related to photosynthesis.