Crystal structure of carboxylase reaction-oriented ribulose 1,5-bisphosphate carboxylase oxygenase from a thermophilic red alga, Galdieria partita

Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) obta ined from a thermophilic red alga Galdieria partita has the highest specifi city factor of 238 among the Rubiscos hitherto reported. Crystal structure of activated Rubisco from G. partita complexed with the reaction intermedia te analogue, 2-carboxyarabinitol 1,5-bisphosphate (P-CABP) has been determi ned at 2.4-Angstrom resolution. Compared with other Rubiscos, different ami no residues bring the structural differences in active site, which are mark ed around the binding sites of P-2 phosphate of 2-CABP. Especially, side ch ains of His-327 and Arg-295 show the significant differences from those of spinach Rubisco. Moreover, the side chains of Asn-123 and His-294 which are reported to bind the substrate, ribulose 1,5-bisphosphate, form hydrogen b onds characteristic of Galdieria Rubisco. Small subunits of Galdieria Rubis co have more than 30 extra amino acid residues on the C terminus, which mak e up a hairpin-loop structure to form many interactions with the neighborin g small subunits. When the structures of Galdieria and spinach Rubiscos are superimposed, the hairpin region of the neighboring small subunit in Galdi eria enzyme and apical portion of insertion residues 52-63 characteristic o f small subunits in higher plant enzymes are almost overlapped to each othe r.