Comments on the structure and function of the large subunit of the enzyme ribulose bisphosphate carboxylase-oxygenase

Abstract

The complete amino acid sequence of the large subunit (LS) of ribulose bisphosphate carboxylase has been determined for maize through nucleotide sequencing of the chloroplast gene encoding this polypeptide (34). Previously published amino acid sequences from the large subunits of barley and spinach as well as new evidence for the amino acid sequence of these two polypeptides are aligned with the complete maize sequence. The complete spinach amino acid sequence is also presented according to the sequence of the spinach gene (63). The number of amino acids in the 475-residue LS primary structure for which there is direct or circumstantial evidence from barley, is 278. Amino acid replacements are encountered in seven positions in comparison with the maize sequence and in 20 positions when compared to the spinach LS. From affinity labelling studies (13, 26) active site residues, nominally lysines and cysteines, have been deduced and located in the total maize and spinach LS sequences, thus being available for further study of structure-function relationships. The complete amino acid sequence and knowledge about catalytic site residues also allows direct comparison with primary structures of other enzymes of the same class, the lyases. Particular attention is devoted to aldolase, an enzyme also functioning in carbohydrate metabolism. Comparison with the 361-residue aldolase of glycolysis in rabbit muscle cells suggests homology in 67 positions centered around catalytic site lysine-175 of RuBPCase LS and catalytic site lysine-227 of aldolase. It is discussed how this relationship may throw light on the reaction mechanism of the RuBP carboxylation.