Assessment of structural and functional divergence far from the large subunit active site of ribulose-1,5-bisphosphate carboxylase/oxygenase

Despite conservation of three-dimensional structure and active-site residues, ribulose-1,5-bisphosphate carboxylase/ oxygenase ( Rubisco, EC 4.1.1.39) enzymes from divergent species differ with respect to catalytic efficiency and CO2/O-2 specificity. A deeper understanding of the structural basis for these differences may provide a rationale for engineering an improved enzyme, thereby leading to an increase in photosynthetic CO2 fixation and agricultural productivity. By comparing 500 active-site large subunit sequences from flowering plants with that of the green alga Chlamydomonas reinhardtii, a small number of residues were found to differ in regions previously shown by mutant screening to influence CO2/O-2 specificity. When directed mutagenesis and chloroplast transformation were used to change Chlamydomonas Met-42 and Cys-53 to land plant Val-42 and Ala-53 in the large subunit N-terminal domain, little or no change in Rubisco catalytic properties was observed. However, changing Chlamydomonas methyl-Cys- 256, Lys-258, and Ile-265 to land plant Phe-256, Arg-258, and Val-265 at the bottom of the alpha/beta-barrel active site caused a 10% decrease in CO2/O-2 specificity, largely due to an 85% decrease in carboxylation catalytic efficiency (V-max/K-m). Because land plant Rubisco enzymes have greater CO2/O-2 specificity than the Chlamydomonas enzyme, this group of residues must be complemented by other residues that differ between Chlamydomonas and land plants. The Rubisco x-ray crystal structures indicate that these residues may reside in a variable loop of the nuclear-encoded small subunit, more than 20 Angstrom away from the active site.