Enzymatic and quantitative properties of Rubisco in some conifers and lycopods

Information on the kinetic properties of Rubisco, a key enzyme for photosynthesis, is scarce in land plants that emerged early during the evolutionary process. This study examined the carboxylase activity and abundance of Rubisco in five conifers, two lycopods, and three control C₃ crops. The turnover rates of Rubisco carboxylation (k_cat^c) under saturated-CO₂ conditions in conifers and lycopods were comparable to those in the control C₃ crops. Rubisco carboxylase activity under CO₂-unsaturated conditions (v_cu) was also measured using reaction mixtures saturated with a N₂ gas containing CO₂ and O₂ at present atmospheric levels to predict the Rubisco CO₂ affinity from the percentage of v_cu in k_cat^c. The predicted CO₂ affinity in conifers and lycopods tended to be lower than that in the control C₃ crops. When the control C₃ crops and two previously examined C₄ crops were analyzed together, the k_cat^c of Rubisco with a low CO₂ affinity tended to be high. N allocation to Rubisco with a low k_cat^c tended to be high in these plants. In conifers and lycopods, the k_cat^c was lower than that expected on the basis of predicted Rubisco CO₂ affinity, unlike in the control crops. N allocation to Rubisco also tended to be lower than that expected on the basis of k_cat^c. These results indicate that Rubisco in the examined conifers and lycopods is not superior in terms of both k_cat^c and CO₂ affinity and that the abundance of Rubisco is not necessarily closely related to its kinetic properties. The reason for these phenomena is discussed in terms of the molecular evolution of Rubisco.