Crystallographic analysis of wild-type and mutant rubiscos from Chlamydomonas reinhardtii

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is the enzyme responsible for photosynthetic fixation of CO2. As indicated by its name, this enzyme is bifunctional. Atmospheric O2 competes with CO2 for substrate at the same active site and initiates a set of reactions that result in net loss of cellular carbon. Because of its economic importance, there is much interest in understanding the factors that control the ratio of the two reactions catalyzed by Rubisco. To improve knowledge of Rubisco selectivity, a systematic study was initiated to examine the structure of a wild-type and several mutant Rubiscos from Chlamydomonas reinhardtii. The structures of wild-type and one mutant enzyme from this organism are reported here at 2.7Å resolution. The mutant enzyme differs from the wild-type by the substitution of Valine for Alanine at position 331 and Serine for Glycine at 344 of the large subunit. These mutations are responsible for a 27% decrease in CO2/O 2-specificity relative to the wild-type enzyme, but a 25% increase relative to the V331A single mutant. Amino acids 331 and 344 lie on opposite sides of Loop 6, a flexible flap that covers the active site. Both structures are nearly identical and differ most along the C-terminal side of Loop 6. Except for a short insertion of amino acids in the small subunit hairpin loop region, and to a lesser extent in Loop 6, these structures superimpose well with the high-resolution spinach Rubisco structure. The disrupted structure of Loop 6 may provide a structural basis for the reduction in CO2/O 2 specificity for C. reinhardtii Rubisco relative to the higher plant enzymes