Enzymatic and quantitative properties of Rubisco in some conifers and lycopods.

Abstract

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 (kcatc) under saturated-CO2 conditions in conifers and lycopods were comparable to those in the control C3 crops. Rubisco carboxylase activity under CO2-unsaturated conditions (vcu) 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 vcu in kcatc. 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 kcatc of Rubisco with a low CO2 affinity tended to be high. N allocation to Rubisco with a low kcatc tended to be high in these plants. In conifers and lycopods, the kcatc 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 kcatc. These results indicate that Rubisco in the examined conifers and lycopods is not superior in terms of both kcatc 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.