Active Site Histidine in Spinach Ribulosebisphosphate Carboxylase/oxygenase Modified by Diethyl Pyrocarbonate

[3H] Diethyl pyrocarbonate was synthesized [Melchior, W. B., & Fahrney, D. (1970) Biochemistry 9, 251-258] from [3H] ethanol prepared by the reduction of acetaldehyde by NaB3H4. Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) from spinach was inactivated with this reagent at pH 7.0 the presence of 20 mM Mg2+, and tryptic peptides that contained modified histidine residues were isolated by reverse-phase high-performance liquid chromatography. Labeling of the enzyme was conducted in the presence and absence of the competitive inhibitor sedoheptulose 1,7-bisphosphate. The amount of one peptide that was heavily labeled in the absence of this compound was reduced 10-fold in its presence. The labeled residue was histidine-298. This result, in combination with our earlier experiments [Saluja, A. K., & McFadden, B. A. (1982) Biochemistry 21, 89-95], suggests that His-298 in spinach RuBisCO is located in the active site domain and is essential to enzyme activity. This region of the primary structure is strongly conserved in seven other ribulosebisphosphate carboxylases from divergent sources.