The response to ribulose bisphosphate4? (RuBP4?) and RuBP-Mg2? in catalysis by structurally divergent RuBP carboxylase/oxygenases

Abstract

Free ribulose bisphosphate (RuBP(4-)) rather than its magnesium complex (RuBP-Mg(2-)) was the apparent substrate for spinach ribulose bisphosphate carboxylase/oxygenase. The apparent Km for total RuBP (pH 8.0 at 30° C) increased with increasing Mg(2+) concentrations from 11.6 μM at 13.33 mM Mg(2+) to 32.6 μM at 40.33 mM Mg(2+). Similarly the apparent Km for RuBP-Mg(2-) complex increased with increasing Mg(2+) from 9.4 μM at 13.33 mM Mg(2+) to 29.7 μM at 40.33 mM Mg(2+). However, the Km values for uncomplexed RuBP(4-) were independent of the (saturating) concentration of Mg(2+) (Km=2.2 μM). The Vmax did not vary with the changing concentrations of Mg(2+).In contrast, the Km for total RuBP remained constant with varying Mg(2+) concentrations (Km=59.5 μM) for the enzyme from R. rubrum. The apparent Km for the RuBP-Mg(2-) complex decreased with increasing Mg(2+) concentrations from 16.0 μM at 7.5 mM Mg(2+) to 5.9 μM at 27.5 mM Mg(2+). The initial velocity for the C. vinosum enzyme was also found to be independent of the (saturating) concentration of Mg(2+) when total RuBP was varied in the assay. Thus the response to total RuBP by these two bacterial enzymes, which markedly differ in structure, was closely similar.

Reductions of Rubisco Activase by Antisense RNA in the C4 Plant Flaveria bidentis Reduces Rubisco Carbamylation and Leaf Photosynthesis

“…To simulate the very low Rubisco carbamylations observed at high pCO 2 in transgenic F. bidentis with low activase content, the model predicts that RuBP should bind very tightly (100 times more tightly; K f /K r# 5 0.01) to uncarbamylated compared to carbamylated Rubisco sites. This fits with in vitro measurements of K f for spinach (Spinacia oleracea; 20 nM) by , which shows it to be at least 2 orders of magnitude smaller than K r# (20 mM or 2 mM if chelation of RuBP by magnesium is taken into account; Yeoh et al, 1981;Roach and McFadden, 1983). Alternatively, the low carbamylation levels could be explained with a larger Michaelis Menten value, K e , for carbamylation (see equation in Fig.…”