Isolation of a stable enzyme.14CO2.Mg2+.carboxyarabinitol bisphosphate complex with ribulosebisphosphate carboxylase from Chromatium vinosum

Ribulose 1,5-bisphosphate carboxylase. Effect on the catalytic properties of changing methionine-330 to leucine in the Rhodospirillum rubrum enzyme

Oligonucleotide-directed mutagenesis of cloned Rhodospirillum rubrum ribulose bisphosphate carboxylase/oxygenase with a synthetic 13mer oligonucleotide primer was used to effect a change at Met-330 to Leu-330. The resultant enzyme was kinetically examined in some detail and the following changes were found. The Km(CO2) increased from 0.16 to 2.35 mM, the Km(ribulose bisphosphate) increased from 0.05 to 1.40 mM for the carboxylase reaction and by a similar amount for the oxygenase reaction. The Ki(O2) increased from 0.17 to 6.00 mM, but the ratio of carboxylase activity to oxygenase activity was scarcely affected by the change in amino acid. The binding of the transition state analogue 2-carboxyribitol 1,5-bisphosphate was reversible in the mutant and essentially irreversible in the wild type enzyme. Inhibition by fructose bisphosphate, competitive with ribulose bisphosphate, was slightly increased in the mutant enzyme. These data suggest that the change of the residue from methionine to leucine decreases the stability of the enediol reaction intermediate.

Subunit dissociation and reconstitution of ribulose-1,5-bisphosphate carboxylase from Chromatium vinosum

The large and small subunits of ribulose bisphosphate carboxylase from Chromatium vinosum were dissociated and separated at pH 9.6 by sucrose density gradient centrifugation. After further purification by gel filtration, the small subunit fraction contained no carboxylase activity. The large subunit fraction was highly depleted of small subunit based on analysis by denaturing polyacrylamide gel electrophoresis. Carboxylase activity of the large subunit fraction was approximately 1% of the untreated native enzyme. Addition of purified small subunit to the large subunit fraction yielded increases of up to 67-fold in carboxylase activity, further indicating that both subunit types are required for catalysis by this enzyme. The isolated large subunit was fully capable of high-affinity activator 14CO2 binding in the presence of Mg2+ and 2-carboxyarabinitol bisphosphate, indicating that the activator and catalytic sites were not grossly denatured by the depletion of small subunit. Kinetic constants of the native C. vinosum enzyme defined a new class of ribulose bisphosphate carboxylase, which permits the detection of possible kinetic differences if the large and small subunits can be favorably reassembled with those of another kinetic class. From experiments with the enzymes from tobacco and spinach leaves it is concluded that the enzyme from higher plant sources is not suitable for such dissociation/reconstitution-type experiments.

Essentiality of the small subunit (B) in the catalysis of RuBP carboxylase/oxygenase is not related to substrate-binding in the large subunit (A)

The small subunit (B) of ribulose 1,5-bisphosphate (RuBP) carboxylase/oxygenase from Aphanothece halophytica is absolutely required for the catalysis, but depletion of subunit B does not significantly affect the formation of the quaternary complex-[enzyme.activator CO2.Mg.carboxyarabinitol bisphosphate] in the catalytic core. The inhibition of RuBP carboxylase activity by the reaction of the epsilon-amino group of a lysine in the RuBP-binding site with pyridoxal 5-P is the same whether subunit B is added to the catalytic core before or after the inactivating reaction. The function of subunit B is not related to the substrate binding.

The response to ribulose bisphosphate(4-) (RuBP (4-)) and RuBP-Mg (2-) in catalysis by structurally divergent RuBP carboxylase/oxygenases

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.

The response to ribulose bisphosphate(4-) (RuBP (4-)) and RuBP-Mg (2-) in catalysis by structurally divergent RuBP carboxylase/oxygenases

Free ribulose hisphosphate (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.