The lability of an intermediate of the ribulose bisphosphate carboxylase reaction

Studies of the Co2+-activated ribulose-1,5-bisphosphate carboxylase/oxygenase by the use of spectrophotometry

Transient optical absorption bands are formed upon addition of ribulose-1,5-bisphosphate to the Co2+-activated ribulose-1,5-bisphosphate carboxylase/oxygenase from spinach and parsley. In the visible region, the prominent absorption band during steady state has a maximum at 610 nm. Stopped-flow technique was used to study the increase in absorbance at this wavelength, and two distinct phases in the progress curve for the approach to steady-state absorbance were observed. The rates for these two phases, respectively, were similar to those found earlier for the two enzyme-Co2+-bound intermediates using EPR technique (Brändén et al. (1987) Biochim. Biophys. Acta 916, 298-303). It is therefore proposed that most of the transient optical absorption originates from an enzyme-Co2+-coordinated ribulose-1,5-bisphosphate molecule and an enzyme-Co2+-coordinated enediolate anion of it, where bound ribulose-1,5-bisphosphate appears first. Furthermore, the most rapid phase in the progress curve is a first-order reaction, independent of the ribulose-1,5-bisphosphate concentration. This indicates that the formation of enzyme-Co2+-coordinated ribulose-1,5-bisphosphate is preceeded by another reaction in which ribulose-1,5-bisphosphate binds to the enzyme, probably without metal coordination.

Ribulose 1,5-bisphosphate carboxylase-oxygenase. I. Structural, immunochemical and catalytic properties

Some structural, immunochemical and catalytic properties are examined for ribulose 1,5-bisphosphate carboxylase-oxygenase from various cellular organisms including bacteria, cyanobacteria, algae and higher plants. The native enzyme molecular masses and the subunit polypeptide compositions vary according to enzyme sources. The molecular masses of the large and small subunits from different cellular organisms, on the other hand, show a relatively high homology due to their well-conserved primary amino acid sequence, especially that of the large subunit. In higher plants, the native enzyme and the large subunit are recognized by the antibodies raised against either the native or large subunit, whereas the small subunit apparently cross-reacts only with the antibodies directed against itself. A wide diversity exists, however, in the serological response of the native enzyme and its subunits with antibodies directed against the native enzyme or its subunits from different cellular organisms. According to numerous kinetic studies, the carboxylase and oxygenase reactions of the enzyme with ribulose 1,5-bisphosphate and carbon dioxide or oxygen require activation by carbon dioxide and magnesium prior to catalysis with ribulose 1,5-bisphosphate and carbon dioxide or oxygen. The activation and catalysis are also under the regulation of other metal ions and a number of chloroplastic metabolites. Recent double-labeling experiments using radioactive ribulose 1,5-bisphosphate and 14CO2 have elucidated the carboxylase/oxygenase ratios of the enzymes from different organisms. Another approach, i.e., genetic experiments, has also been used to examine the modification of the carboxylase/oxygenase ratio.