The response to free ribulose bis phosphate and ribulose bis phosphate magnesium in catalysis by structurally divergent ribulose bis phosphate carboxylase oxygenases ec 18.104.22.168
Roach, D.J.W.; Mcfadden, B.A.
Photosynthesis Research 4(2): 111-118
Free ribulose bisphosphate (RuBP4-) rather than its Mg complex (RuBP-Mg2-) was the apparent substrate for spinach ribulose bisphosphate carboxylase/oxygenase. The apparent Km for total RuBP (pH 8.0 at 30.degree. C) increased with increasing Mg2+ concentrations from 11.6 .mu.M at 13.33 mM Mg2+ to 32.6 .mu.M at 40.33 mM Mg2+. Similarly, the apparent Km for RuBP-Mg2- complex increased with increasing Mg2+ from 9.4 .mu.M at 13.33 mM Mg2+ to 29.7 .mu.M at 40.33 mM Mg2+. However, the Km values for uncomplexed RuBP4- were independent of the (saturating) concentration of Mg2+ (Km = 2.2 .mu.M). The Vmax did not vary with the changing concentrations of Mg2+. In contrast, the Km for total RuBP remained constant with varying Mg2+ concentrations (Km = 59.5 .mu.M) for the enzyme from Rhodospirillum rubrum. The apparent Km for the RuBP-Mg2- complex decreased with increasing Mg2+ concentrations from 16.0 .mu.M at 7.5 mM Mg2+ to 5.9 .mu.M at 27.5 mM Mg2+. The initial velocity for the Chromatium vinosum enzyme was also independent of the (saturating) concentration of Mg2+ when total RuBP was varied in the assay. Thus, the response to total RuBP by these 2 bacterial enzymes, which markedly differ in structure, was closely similar.