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Metabolism and Enzymology

Protection of Tryptic-Sensitive Sites in the Large Subunit of Ribulosebisphosphate Carboxylase/Oxygenase by Catalysis ¹

Department of Horticulture and Landscape Architecture, Plant Physiology/Biochemistry/Molecular Biology Program, University of Kentucky, Lexington, Kentucky 40546, Department of Developmental and Cell Biology, University of California, Irvine, California 92717

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.

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