Stoichiometry in the assay of ribulose bisphosphate oxygenase and carboxylase

Active-site histidines in recombinant cyanobacterial ribulose-1,5-bisphosphate carboxylase/oxygenase examined by site-directed mutagenesis

The functions of His(291), His(295) and His(324) at the active-site of recombinant A. nidulans ribulose-1,5-bisphosphate carboxylase/ oxygenase have been explored by site-directed mutagenesis. Replacement of His(291) by K or R resulted in unassembled proteins, while its replacement by E, Q or N resulted in assembled but inactive proteins. These results are in accord with a metal ion-binding role of this residue in the activated ternary complex by analogy to x-ray crystallographic analyses of tobacco and spinach enzymes.His(324) (H327 in spinach), which is located within bonding distance of the 5-phosphate of bound bi-substrate analog 2-carboxyarabinitol 1,5-bisphosphate in the crystal structures, has been substituted by A, K, R, Q and N. Again with the exception of the H324K and R variants, these changes resulted in detectable assembled protein. The mutant H324A protein exhibited no detectable carboxylase activity, whereas the H324Q and H324N changes resulted in purifiable holoenzyme with 2.0 and 0.1% of the recombinant wild-type specific carboxylase activity, respectively. These results are consistent with a phosphate binding role for this residue.The replacement of His(295), which has been suggested to aid in phosphate binding, with Ala in the A. nidulans enzyme leads to a mutant with 5.8% of the recombinant wild-type carboxylase activity. All other mutations at this position resulted in unassembled proteins. Purified H295A and H324Q enzymes had elevated Km(RuBP) values and unchanged CO2/O2 specificity factors compared to recombinant wild-type.

A facile method to determine the CO2/O 2 specificity factor for ribulose bisphosphate carboxylase/oxygenase

A rapid method to determine the CO2/O2 specificity factor of ribulose 1,5-bisphosphate carboxylase/oxygenase is presented. The assay measures the amount of CO2 and O2 fixation at varying CO2/O2 ratios to determine the relative rates of each reaction. CO2 fixation is measured by the incorporation of the moles of(14)CO2 into 3-phosphoglycerate, while O2 fixation is determined by subtraction of the moles of CO2 fixed from the moles of RuBP consumed in each reaction. By analyzing the inorganic phosphate specifically hydrolyzed from RuBP under alkaline conditions, the amount of RuBP present before and after catalysis by rubisco can be determined.