Modification of Rhodospirillum rubrum ribulose bisphosphate carboxylase with pyridoxal phosphate. 2. Stoichiometry and kinetics of inactivation.

Rhodospirillum rubrum ribulose bisphosphate carboxylase contains two high affinity binding sites for pyridoxal phosphate and two catalytic sites per dimer. However, pyridoxal phosphate binding at only one site is sufficient for inactivation of both catalytic sites. In the presence of 20 mM bicarbonate, 10 mM magnesium, and pyridoxal phosphate, the rates of inactivation and Schiff base formation are pseudo-first-order and show saturation kinetics. These observations provide additional evidence that pyridoxal phosphate binds at the active site of the R. rubrum carboxylase. It is also proposed that the large subunit may contain regulatory as well as catalytic properties.