Nonessentiality of histidine 291 of Rhodospirillum rubrum ribulose-bisphosphate carboxylase/oxygenase as determined by site-directed mutagenesis

Chemical modification of spinach ribulosebisphosphate carboxylase/oxygenase by diethyl pyrocarbonate led to the conclusion that His-298 is an essential active-site residue (Igarashi, Y., McFadden, B.A., and El-Gul, T., (1985) Biochemistry 24, 3957-3962). From the pH dependence of inactivation, the pK(a) of His-298 was observed to be ~6.8, and it was suggested that this histidine might be the essential base that inititates catalysis (Paech, C. (1985) Biochemistry 24, 3194-3199). To explore further the possible function of His-298, we have used site-directed mutagenesis to replace the coresponding residue of the Rhodospirillum rubrum carboxylase (His-291) with alanine. Assays of extracts of Escherichia coli JM107, harboring either the wild-type or mutant gene in an expression vector, revealed that the mutant protein is ~40% as active catalytically as the normal carboxylase. After purification to near homogeneity by immunoaffinity chromatography, the mutant protein was partially characterized with respect to subunit structure, kinetic parameters, and interaction with a transition-state analogue. The purified mutant carboxylase had a k(cat) of 1.5 s^-1 and a k(cat)/K(m) of 1.7·10⁴ M^-1 s^-1 in contrast to values of 3.6 s^-1 and 6·10⁵ M^-1 s^-1 for the normal enzyme. The high level of enzyme activity exhibited by the Ala-291 mutant excludes His-291 in the R. rubrum carboxylase (and by inference His-298 in the spinach carboxylase) as a catalytically essential residue.