Species variation in the specificity of ribulose biphosphate carboxylase/oxygenase

Abstract

The balance between photosynthesis and photorespiration in many species, including most crop plants, is determined by the kinetic properties of ribulose-l,5-bisphosphate (RuBP) carboxylase/oxygenase^1,2. Photosynthesis is initiated by the carboxylase activity³ while the oxygenase activity catalyses the first reaction in the photorespiratory pathway^1,2,4. In these reactions, CO₂ and O₂ are competitive substrates². Because O₂ inhibits carboxylation, and photorespiration oxidizes reduced carbon to CO₂ with no known benefit to the plant, it has been suggested that photosynthetic efficiency and thus productivity might be increased by chemical or genetic alterations of the enzyme which increase carboxylation or decrease oxygenation^5–7. On the other hand, it has been argued that RuBP carboxylase/oxygenase cannot completely discriminate between CO₂ and O₂, so that photorespiration is unavoidable⁸. From analyses of RuBP carboxylase/oxygenase enzymes purified from several different species, we report here large differences in specificity towards the substrates CO₂ and O₂. Evolutionary pressures seem to have directed the enzyme towards more efficient utilization of CO₂.