ENHANCED CO2 O2 SPECIFICITY OF A SITE-DIRECTED MUTANT OF RIBULOSE-BISPHOSPHATE CARBOXYLASE OXYGENASE/

The CO2/O2 specificity factor of ribulose-bisphosphate carboxylase/oxy genase partially determines the efficiency of photosynthetic carbon as similation. Heretofore, engineered alterations of the enzyme have only decreased the selectivity for CO2 utilization. We show that alanyl re placement of active-site Ser-368 of the Rhodospirillum rubrum carboxyl ase enhances the carboxylation selectivity approximately 1.6-fold over the wild-type level. This enhancement reflects a greater relative dec line in oxygenase efficiency than in carboxylase efficiency. In contra st to wild-type enzyme, the carboxylase activity of the Ser-368 mutant protein is not perceptibly inhibited by O2, perhaps indicative of a c hange in rate-limiting steps in the overall reaction pathway.