Trapping of a reaction intermediate by tetranitromethane during catalysis by ribulose biphosphate carboxylase

Intermediates formed by the Co2+-activated ribulose-1,5-bisphosphate carboxylase/oxygenase from spinach studied by electron paramagnetic resonance spectroscopy

Two enzyme-metal-bound intermediates formed by the Co2+-activated ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) have been studied by electron paramagnetic resonance (EPR) spectroscopy. Their rates of approach to a stationary state are different and their relative amounts at steady state are dependent on the concentration of ribulose 1,5-bisphosphate. It is therefore proposed that enzyme-metal-coordinated ribulose 1,5-bisphosphate and an enzyme-metal-coordinated enediolate anion of it, where bound ribulose 1,5-bisphosphate appears first, constitute the two EPR-detectable intermediates, respectively.

The lability of an intermediate of the ribulose bisphosphate carboxylase reaction

Interruption of the catalytic cycle of ribulose-bisphosphate carboxylase by acid denaturation liberated an intermediate with a labile phosphate ester. Addition of fresh, buffered carboxylase enzyme to the acidified carboxylase reaction after 5 s inhibited phosphate release from the intermediate. Therefore, the species with a labile phosphate ester was stable for 5 s in acid and was apparently a substrate for the enzymatic reaction, since the labile intermediate was converted to a stable form by the protein. After acid denaturation, the carboxylated intermediate could be stabilized by reduction after 5 s in acid, but after 1 h no carboxylated intermediate remained. The stoichiometries of phosphate released to enzyme active sites and the carboxylated intermediate trapped to enzyme active sites were approximately 0.04. It was concluded that the labile phosphate species is probably the carboxylated intermediate rather than the enediol(ate) intermediate. The carboxylase and oxygenase reactions were probed for intermediates by the ability of the enzymatic reaction to reduce hexacyanoferrate(III), dichlorophenolindophenol, or nitroblue tetrazolium. Reduction of these reagents and hexacyanoferrate(III)-dependent paracatalytic inactivation were not observed. The copper chelate of lysine, a superoxide dismutase active species, did not selectively inhibit ribulose-bisphosphate oxygenase.

Evidence for a carbanion intermediate in catalysis by spinach ribulose-1,5-bisphosphate carboxylase/oxygenase

Spinach ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (EC 4.1.1.39) showed marked reactivity towards tetranitromethane in the presence of RuBP, when the enzyme was in the fully activated state. The inactivated enzyme did not catalyze substrate-dependent nitroform production. Nitroform production was proportional to the concentration of the enzyme and tetranitromethane at optimal substrate concentrations. The pH optimum for nitroform production was similar to that for carboxylation. The effects of bicarbonate, ribulose-1,5-bisphosphate, and temperature on partition of the intermediate between production of phosphoglycerate and nitroform indicate that both routes share a common intermediate. Data on stoichiometry of the reaction imply that the intermediate is not oxidized by tetranitromethane but instead is nitrated. The data are compatible with a carbanion intermediate in the reaction catalyzed by ribulose-1,5-bisphosphate carboxylase.

Pyridoxal phosphate as a probe in the active site of ribulose bisphosphate carboxylase/oxygenase

Ribulose bisphosphate (RuBP) carboxylase is rapidly and irreversibly inactivated by photooxidation sensitized by pyridoxal phosphate. Both pyridoxal and pyridoxamine phosphate were much less effective in sensitizing the photooxidation even when used at twice the concentration of pyridoxal phosphate. These results imply that pyridoxal phosphate binds at the active site not only through a Schiff base, but also through ionic interaction with the phosphate binding region. Spectral analysis of the photooxidized enzyme showed a new absorption maximum at 325 nm due to reduction of the Schiff base between pyridoxal phosphate and a lysyl residue with concomitant oxidation of a histidine residue. The stoichiometry of photooxidative [3H]pyridoxal phosphate incorporation was 0.87 mol/mol of a 70,000-dalton large subunit-small subunit combination. Studies with 3H-labeled diethyl pyrocarbonate showed that both photooxidation and carbethoxylation occur at the same histidine residue. However, photooxidation by pyridoxal phosphate is very specific for an active site histidine residue due to the high specificity of this affinity label. Several competitive inhibitors with respect to ribulose bisphosphate offered appreciable protection against pyridoxal phosphate-induced photooxidation of the enzyme. The photooxidized enzyme showed an increase in the net negative charge on the protein which was evident from the higher mobility of the photooxidized enzyme toward the anode in polyacrylamide gel electrophoresis.