Active site studies of ribulose-1,5-bisphosphate carboxylase/oxygenase with pyridoxal 5'-phosphate.

There are 16 epsilon-amino groups of lysyl residues which are essential for the activity of ribulose-1,5-bisphosphate carboxylase/oxygenase. These lysyl residues formed a Schiff base with pyridoxal 5'-phosphate which was stabilized by NaBH4 reduction. The stoichiometry of covalently bound pyridoxal 5'-phosphate after NaBH4 reduction was determined spectrophotometrically with a derived molar extinction coefficient of 4800 M-1 cm-1. The incorporation of pyridoxal 5'-phosphate into the protein was accompanied by loss of the carboxylase and oxygenase activities, but the ratio of their activities remained constant. Vmax, but not Km, values were changed by this modification of the amino acid groups. Half of the epsilon-amino groups of lysine appeared to be at the 8 catalytic sites and half at the 8 activator sites for CO2, as indicated by kinetics of reactivation of the enzyme activity during dissociation of the Schiff base between pyridoxal 5'-phosphate and the protein. Reduction with NaB3H4 revealed that all 16 of the lysyl residues were on the large subunit. Ribulose-1,5-bis-phosphate alone protected 16 amino groups from Schiff base formation with pyridoxal 5'-phosphate, and the enzyme activity was fully conserved. NaHCO3 increased and MgCl2 lowered slightly the protective effect of ribulose-1,5-bisphosphate. Modification of sulfhydryl groups by p-chloromercuribenzoic acid inhibited the enzyme and excluded binding of 8 equivalents of pyridoxal 5'-phosphate. Upon removal of the mercuribenzoate groups with excess dithiothreitol, the loss of enzyme activity was exponentially correlated with the binding of 8 mol of pyridoxal 5'-phosphate/mol of enzyme. In contrast to p-chloromercuribenzoic acid, iodoacetamide, which inhibited the catalysis, had no influence on the binding of the 16 pyridoxal 5'-phosphate/mol of enzyme. It is suggested that the CO2 activator site with one epsilon-amino group for binding of CO2 and the catalytic site with one epsilon-amino group of a yet unknown function are located closely together on the large subunits of the enzyme. The results are consistent with a sulfhydryl group as the proton acceptor opposite carbon 3 of ribulose-1,5-bisphosphate, and when this sulfhydryl group is blocked by the bulky mercuribenzoate group, but not by the small carboxyamidomethyl group, pyridoxal 5'-phosphate binding is modified.