Protection of tryptic-sensitive sites in the large subunit of ribulosebisphosphate carboxylase/oxygenase by catalysis

Limited tryptic proteolysis of spinach (Spinacia oleracea) ribulose bisphosphate carboxylase/oxygenase (ribulose-P₂ carboxylase) resulted in the ordered release of two adjacent N-terminal peptides from the large subunit, and an irreversible, partial inactivation of catalysis. The two peptides were identified as the N-terminal tryptic peptide (acetylated Pro-3 to Lys-8) and the penultimate tryptic peptide (Ala-9 to Lys-14). Kinetic comparison of hydrolysis at Lys-8 and Lys-14, enzyme inactivation, and changes in the molecular weight of the large subunit, indicated that proteolysis at Lys-14 correlated with inactivation, while proteolysis at Lys-8 occurred much more rapidly. Thus, enzyme inactivation is primarily the result of proteolysis at Lys-14. Proteolysis of ribulose-P₂ carboxylase under catalytic conditions (in the presence of CO₂, Mg²⁺, and ribulose-P₂) also resulted in ordered release of these tryptic peptides; however, the rate of proteolysis at lysyl residues 8 and 14 was reduced to approximately one-third of the rate of proteolysis of these lysyl residues under noncatalytic conditions (in the presence of CO₂ and Mg²⁺ only). The protection of these lysyl residues from proteolysis under catalytic conditions could reflect conformational changes in the N-terminal domain of the large subunit which occur during the catalytic cycle.