Rubisco activity of scenedesmus ecornis: What is wrong with initial activity determination?

Abstract

A procedure was devised to measure the initial and total Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activities for the green microalga, Scenedesmus ecornis. Total Rubisco activities corresponded well with photosynthetic carbon assimilation rates. Initial activities ranged from 10 to 40% of the total activities and did not correlate with photosynthetic rates. Investigations into potential causes of the reduced initial activities yielded modest increases in percentage of the total activity. Values of K m for ribulose-1,5-bisphosphate (RuBP) were similar for both initial and CO 2-Mg 2+ activated enzyme. Total activities increased with increasing concentrations of RuBP to 400 μ m, the assay concentration. However, concentrations above the K m , 25 μ m RuBP, were inhibitory for the initial Rubisco form. Inhibition increased with increasing RuBP concentration. The addition of Mg 2+ in the extraction solution did not prevent RuBP inhibition. The results suggest that the low initial Rubisco activities are principally due to decarbamylation of the active sites of the enzyme during extraction.